WWW.ElectronicsCircuits.TK

This compact ballast is intended for driving a 20-watt ‘bare’ Compact Fluorescent Lamps (CFL) tube or bulb, that is, one without a driver circuit built into its socket that makes it ready to screw into an existing lamp socket. Pin-base CFLs are designed to be used with a separate ballast. As with a linear fluorescent system, the lamp and ballast must be compatible. Pin-base CFLs are available in low-power versions to replace incandescent light bulbs and in medium- and high-power versions to take over from linear fluorescent lamps or even high-intensity discharge (HID) lamps.

Making a pin-base CFL light

The circuit shown in Inductor picture uses a dedicated integrated circuit type FAN7710 from our friends at Fairchild. As illustrated in Figure 4, this device combines one high-side 625-V gate driver circuit, two 550-V MOSFETs, afrequency control circuit and a shunt regulator –– plus active ZVS control and an open lamp detection function, all crammed into one ultra-compact 8-way DIP package. Its high functionality and built-in protection features save board space, reduce power dissipation and guarantee enhanced reliability in end systems. Good!

The AC line input voltage (here, 230 VAC 50 Hz) is rectified to provide a bus voltage of approximately 320 volts DC. Startup resistor R1 supplies initial (micro-) power to the FAN7710 IC. The IC begins to oscillate and the charge pump circuit consisting of C2, D2 and D7 supplies the current to the VDD pin, which gets regulated through the internal 15-V shunt regulator.

The FAN7710’s oscillator circuitry employs three discrete frequencies: one to pre-heat the CFL gas; one to ignite it and one for the on state — see the inset for the associated (simple) maths. In addition to this, it protects the ballast circuitry from low AC as well as lamp removal conditions.

Making the inductor
The bare PCB, FAN7710N IC and the 2.5-millihenry inductor used in the circuit come as a set from the Elektor Shop. However we would not discourage anyone from purchasing the inductor parts and making it yourself.

Let’s first carefully write down the specifications:

Inductance: 2.5 mH
Core material: Epcos N19 or equivalent
Core size: 20 / 10 / 6
Bobbin: E19
Gap: 1.5 mm
Wire gauge: 0.2 mm (SWG #32)
Number of turns: 280

Now look at the construction details.
First, wind the 280 turns of enamelled copper wire (ECW) on the E19 bobbin. Bare the wire ends for about 5 mm by scratching with a scalpel, then pre-tin. Check continuity of the coil. Put the Ecore halves over the bobbin as shown, then insert and adjust the spacers to get the required air gap of 1.5 mm which is essential to achieve the required inductance. The final step is to wrap electrical isolation tape around the core frame.

Ballast for Energy-Saving Lamps Printed Circuit Board )PCB)

Elektor labs have designed a circut board for the project; the component mounting plan is shown in PCB. Thecopper track layout is available as a free .pdf file from our website at Elektor for those wishing to etch their own circuit board. Reflected and non-reflected artwork is included in the .pdf file for your convenience. Component stuffing is a breeze as only normal size leaded components are used on a spacious board. The wiring to the mains and the lamp, and all connections and connectors in between, should comply with electrical safety guidelines. (Author: T. A. Babu, Elektor Magazine, 2008)

Veroboard and How to Construct an Electronics Circuit

2012-02-12T13:38:00.000-08:00

Electronics Circuit

In electronics, one has to know about circuit building. Circuit building is an essential skill if one is interested to build robots. Robots, need a brain or rather an intelligent brain that tells them what to do. In this case, electronic circuits are the controller for the robots.

There are the bread board, ‘vero board’ and ‘PCB’ (printed circuit board). These three are the types of electronics circuits. They are widely used all over the world. The cheapest one, is the ‘vero board’ and also one of the most popular technique.

A ‘vero board’ will have many holes lined up horizontally and vertically. What you need to know, is that each rows (horizontal) can be thought of as a horizontal wires. So, a ‘vero board’ contains many horizontal wires, row by row. If you have ten horizontal rows of holes, then you have ten horizontal wires. Note that you still need to have basic common sense of how to connect a complete, closed circuit.

With this new knowledge, now you can start building an electronics circuit. To insert the components into the board, you have to solder. Soldering is whereby you heat up the soldering iron tip until it is hot enough. Then you heat the particular spot you want to solder, for about 3 seconds maximum. Immediately apply a strand of flux material. This has to be done only once, ideally. If you have to re-solder the same part, it is no good.

However, take into account that soldering is a skill. The more you solder, the better you will be at it. Be wary of cold solder joints. A good soldering is when you see somewhat a shine on the joint. But, if you see dull looking texture, that appears greyish, then you have cold solder joint. A cold joint contains holes and cracks and voids inside. Therefore, it is not desirable and will produce joint failures in long term.

By Alfred Chai Wei Liang

WiTricity notes

2012-02-08T02:25:00.000-08:00

WiTricity, a portmanteau for "wireless electricity", is a trademark of WiTricity corporation[1] referring to their devices and processes which use a form of wireless energy transfer including resonant energy transfer etc., the ability to provide electrical energy to remote objects without wires using oscillating magnetic fields. The term WiTricity was used for a project that took place at MIT, led by Prof. Marin Soljačić in 2007.
In April 27, 2011, car maker Toyota made an investment in WiTricity.

Technical background

Overview
WiTricity is based on strong coupling between electromagnetic resonant objects to transfer energy wirelessly between them. This differs from other methods like simple induction, microwaves, or air ionization. The system consists of transmitters and receivers that contain magnetic loop antennas critically tuned to the same frequency. Due to operating in the electromagnetic near field, the receiving devices must be no more than about a quarter wavelength from the transmitter (which is a few meters at the frequency used by the example system). In their first paper, the group also simulated GHz dielectric resonators. The WiTricity devices are coupled almost entirely with magnetic fields (the electric fields are largely confined within capacitors inside the devices), which is argued to make them safer than resonant energy transfer using electric fields (most famously in Tesla coils, whose high electric fields allow them to be used as lightning generators), since most materials couple weakly to magnetic fields (Kurs, 2007). The WiTricity devices are also claimed to be unusual in that they support efficient energy transfer for "mid-range" distances several times larger than the diameter of the resonant objects (Karalis, 2007).
Unlike the far field wireless power transmission systems based on traveling electro-magnetic waves, WiTricity employs near field resonant inductive coupling through magnetic fields similar to those found in transformers except that the primary coil and secondary winding are physically separated, and tuned to resonate to increase their magnetic coupling. These tuned magnetic fields generated by the primary coil can be arranged to interact vigorously with matched secondary windings in distant equipment but far more weakly with any surrounding objects or materials such as radio signals or biological tissue.
In particular, WiTricity is based on using 'strongly-coupled' resonances to achieve a high power-transmission efficiency. Aristeidis Karalis, referring to the team's experimental demonstration, says that "the usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular systemThe researchers suggest that the exposure levels will be below the threshold for FCC safety regulations, and the radiated-power levels will also comply with the FCC radio interference regulations.
Researchers attribute the delay in developing wireless-power technology to limitations of well-known physical laws and a simple lack of need. Only recently have modern consumers obtained a high number of portable electronic devices which currently require batteries and plug-in chargers.[3]
[edit]Experimental demonstration
The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb wirelessly, using two 5-turn copper coils of 60 cm (24 in) diameter, that were 2 m (7 ft) away, at roughly 45% efficiency.[6] The coils were designed to resonate together at 9.9 MHz (≈ wavelength 30 m) and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. Currently, researchers have been able to power a 60 watt light bulb at roughly 90% efficiency at a distance of 3 feet[citation needed].
The emerging technology was demonstrated by Eric Giler, CEO of the US firm WiTricity, at the TED Global Conference held at Oxford in July 2009.[7][8] In this demonstration, Giler shows a WiTricity power unit powering a television as well as three different cell phones, the initial problem which inspired Soljacic to get involved with the project.
[edit]Radiation levels
See also: Electromagnetic radiation and health
The company's FAQ [1] claims that it uses a "non-radiative mode of energy transfer, relying instead on the magnetic near field. Magnetic fields interact very weakly with biological organisms—people and animals—and are scientifically regarded to be safe." No actual studies or reports are claimed of the specific technology, power levels and use in home environments but it does claim that "WiTricity products are being designed to comply with applicable safety standards and regulations." No clinical field study radiation levels from in-home tests are reported on the WiTricity web site as of November 2010.

Who Invented Touch Screen Technology?

2012-02-04T23:19:00.000-08:00

Touch Screen Technology - How It Works

There are three components used in touch screen technology:

The touch sensor is a panel with a touch responsive surface. Systems are built based on different types of sensors: resistive (most common), surface acoustic wave, and capacitive (most smart phones). However, in general sensors have an electrical current running through them and touching the screen causes a voltage change. The voltage change signals the location of the touching.
The controller, is the hardware that converts the voltage changes on the sensor into signals the computer or other device can receive.
Software tells the computer, smartphone, game device, etc, what's happening on the sensor and the information coming from the controller. Who's touching what where; and allows the computer or smart phone to react accordingly.

Of course, the technology works in combination with a computer, smart phone, or other type of device.

Resistive & Capacitive Explained

According to Malik Sharrieff, an eHow Contributor, "the resistive system is comprised of five components, including the CRT (cathode ray tube) or screen base, the glass panel, the resistive coating, a separator dot, a conductive cover sheet and a durable top coating."

When a finger or stylus presses down on the top surface, the two metallic layers become connected (they touch), the surface acts as a pair of voltage dividers with connected outputs. This causes a change in the electrical current. The pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance, which registers as a touch screen event that is sent to the computer controller for processing.

apacitive touch screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact.

History of Touch Screen Technology

1960s

Historians consider the first touch screen to be a capacitive touch screen invented by E.A. Johnson at the Royal Radar Establishment, Malvern, UK, around 1965 - 1967. The inventor published a full description of touch screen technology for air traffic control in an article published in 1968.

1970s

In 1971, a "touch sensor" was developed by Doctor Sam Hurst (founder of Elographics) while he was an instructor at the University of Kentucky. This sensor called the "Elograph" was patented by The University of Kentucky Research Foundation. The "Elograph" was not transparent like modern touch screens, however, it was a significant milestone in touch screen technology. The Elograph was selected by Industrial Research as one of the 100 Most Significant New Technical Products of the Year 1973.

In 1974, the first true touch screen incorporating a transparent surface came on the scene developed by Sam Hurst and Elographics. In 1977, Elographics developed and patented a resistive touch screen technology, the most popular touch screen technology in use today.

In 1977, Siemens Corporation financed an effort by Elographics to produce the first curved glass touch sensor interface, which became the first device to have the name "touch screen" attached to it. On February 24, 1994, the company officially changed its name from Elographics to Elo TouchSystems.

Elographics Patents
US3662105: Electrical Sensor Of Plane Coordinates
Inventor(s)Hurst; George S., Lexington, KY - Parks; James E., Lexington, KY
Issued/Filed Dates:May 9, 1972 / May 21, 1970
US3798370: Electrographic Sensor For Determining Planar Coordinates
Inventor(s)Hurst; George S. , Oak Ridge, TN
Issued/Filed Dates:March 19, 1974 / April 17, 1972

1980s

In 1983, the computer manufacturing company, Hewlett-Packard introduced the HP-150, a home computer with touch screen technology. The HP-150 had a built in grid of infrared beams across the front of the monitor which detected finger movements. However, the infrared sensors would collect dust and require frequent cleanings.

1990s

The nineties introduced smart phones and handhelds with touch screen technology. In 1993, Apple released the Newton PDA, equipped with handwriting recognition; and IBM released the first smart phone called Simon, which featured a calendar, note pad, and fax function, and a touch screen interface that allowed users to dial phone numbers. In 1996, Palm entered the PDA market and advanced touch screen technology with its Pilot series.

2000s

In 2002, Microsoft introduced the Windows XP Tablet edition and started its entry into touch technology. However, you could say that the increase in the popularity of touch screen smart phones defined the 2000s. In 2007, Apple introduced the king of smart phones, the iPhone, with nothing but touch screen technology

X-Ray Defender

2012-02-04T23:10:00.000-08:00

Backscatter X-Ray Detector

Backscatter X-ray imaging devices have been in the news of late, primarily due to their use in U.S. airports and other public facilities. But, not everyone is aware that the technology is also being deployed in ordinary vans that can image the insides of passing cars and trucks, and even peer into the interior of homes and businesses. In order to penetrate the exteriors of vehicles and buildings, the x-ray beam is substantially more intense. In the wrong hands, the beam could even serve as an undetectable weapon. That is, until now. the X-Ray Defender is designed to detect the short, powerful X-ray beam from such scanners, giving the owner time to hightail it out of there, before his hair catches on fire. Remember, the X-rays will zip right through aluminum foil hats, too!

The X-ray Defender employs an ordinary PIN photodiode to detect x-rays, instead of light. The photodiode must be kept in the dark, hence the heatshrink tubing sealed with "liquid tape" cap. The uncovered diode can be seen in the insert on top of the battery. It isn't clear how powerful the scanning beam is, so it's possible that the photodiode could directly drive the SCR, without any amplification. But, it was decided to enhance the sensitivity, stretch the response to short pulses, and make the device respond only to sudden field changes. The prototype also has a small block of scintillation plastic sitting on top of the photodiode to further enhance the sensitivity, but the beam will almost certainly trigger this circuit without it. The amplifier is intentionally "starved" for current so that it doesn't respond to smaller signals, causing false triggering and the standby current is virtually zero.

Here's how it works: The powerful pencil beam of x-rays sweeps across the photodiode, causing a sudden current flow from the diode into the base of the MPSW45. The collector pulls low and current flows up through the 1N5711, clamping the gate of the SCR near zero volts. The dwell time of the pulse isn't known, but this circuit will respond rather quickly. When the x-ray beam moves away, the transistor turns off and the voltage on the collector swings up. The other side of the coupling capacitor also swings up and current flows into the gate of the SCR, triggering it. This slower waveform has plenty of width and energy to trigger a sensitive-gate SCR. The buzzer is simply reset by cycling power, a fairly easy operation to perform while running. When power is first applied, the circuit will trigger. Let it buzz for a few seconds then turn the power off and back on quickly. Try again if it continues to buzz. When the power is on and the unit is not triggered and buzzing, the power consumption is practically zero, so the unit may be left on. The tendency to trigger when power is first applied serves as a circuit/battery check.

Component notes: The PIN photodiode in the prototype is an Advanced Photonics, SD200-11-31-241 which has a 0.2" active area. Most other types should work. The high-value 62 megohm resistor provides a path to ground for any leakage and it could probably be left out for even more sensitivity, if there is no significant leakage from the photodiode. Or, a 22 megohm could be substituted. The NPN darlington isn't critical, but it should be a modern small-signal type. The 1N5711 is a small-signal schottky diode but an ordinary silicon diode will also work. The SCR should be a sensitive-gate type, but substitutions are fine. The buzzer is a low-current type designed to operate on 9 volts. The capacitor across it keeps the intermittent current it consumes from resetting the SCR.

Keep the unit nearby or build several.

Continental USA Breakfast

Class-A Audio Amplifiers

2012-02-04T22:59:00.001-08:00

A class-A audio amplifier is pretty wasteful of power but when plenty of power is available the simplicity is attractive. Here is a simple darlington transistor example intended for use with a 5 volt power supply:

This circuit and the following aren't for beginners; they are of limited usefulness and require an understanding of the underlying principles and potential applications. They all pass DC through the speaker which is wasteful and can cause problems for the inexperienced builder. If built without variation, they should perform as described but make sure to read the text.

The 5 volts should be provided by a regulated power supply. The efficiency is below 25% and significant DC current flows in the speaker and that additional power should be figured in to the power rating of the speaker. But look how simple it is! The voltage gain is only about 20 and the input impedance is about 12k. The schematic shows two values of bias resistor to be used with the corresponding speaker impedance. With the 150k bias resistor and 8 ohm speaker, the circuit draws about 210mA (1 watt) and can deliver about 250 mW to the speaker which is plenty of volume for most small projects. The speaker should be rated at 500 mW or more and should exhibit a DC resistance near 8 ohms (perhaps 7 ohms). Check the candidate speaker with an ohmmeter; much below 7 ohms will cause excessive current draw. With the 220k resistor and 16 ohm speaker, the circuit draws about 100 mA (500 mW) and delivers about 125 mW to the speaker. The 16 ohms speaker should be rated at 200 mW or more and exhibit nearly 16 ohms of DC resistance. (Most small speakers have a DC resistance near the rated impedance and that resistance is used to set the quiescent current level in this circuit.) Other NPN darlington transistors will work but choose one that can dissipate 1 watt minimum. Most power types don't need a heatsink but tiny TO92's might overheat.

If the inefficiency of the class-A hasn't dissuaded you yet, here is a 4-transistor amplifier suitable for small signals:

The input impedance is about 5000 ohms and the frequency response is flat from 30 Hz to over 20,000 Hz. With the 8 ohm speaker the current drain is about 215 mA and the gain is about 1700 (64 dB). With the 16 ohm speaker the current gain is about 110 mA and the gain is about 2500 (68 dB). A volume control may be added by connecting one end of a 5k potentiometer to ground, the wiper to the amplifier input. The other end of the pot becomes the input.

Lets face it; just about any of the various IC audio amplifiers make more sense than this inefficient design. But, this circuit uses parts with only 3 legs. Umm, it doesn't use large capacitors except for the power supply bypassing. Lets see, its more fun-ariffic. Well, lets see if we can come up with a project that takes advantage of the inefficiency:

So, what is it?

It is a modulated light sender! Connect the input to an audio source or microphone (a speaker will work) and the audio will amplitude modulate the light intensity. The inefficiency of the class-A works in our favor now, lighting the lamp to mid-brightness with no audio present. Actually, with a 4.7 volt bulb, the lamp will be near full brightness and will be "overdriven" on sound peaks. A higher voltage bulb will last longer but will be dimmer. Try a 6.8 volt bulb as a compromise. With a sensitive detector like a phototransistor, this communicator will work several hundred feet (at night). Best range is realized if the bulb is mounted in a typical flashlight reflector and the detector is similarly mounted. The input capacitor is reduced to .01 uF to give the amplifier a high-pass character to compensate for the slow response of the bulb. The audio will sound a bit muffled, anyway. The clever designer could use this amplifier for the receiver, too, switching the speaker to the input for transmitting and to the output for listening. If you choose a detector with good infrared response, like a pin photo diode, you can add plastic IR filters to block out ambient light and make the communicator harder to see at night.

Increasing the voltage to 12 VDC, replacing the bulb with a 3 watt, 16 ohm speaker and replacing the .01uF with a 1uF gives an audio amp that will deliver nearly 1 watt of audio power. The speaker will get warm, however! (Due to the nearly 2 watts of DC power in the speaker coil.)

how to make Crystal Radio (and other purpose) Audio Amplifier

2012-02-04T22:58:00.001-08:00

Crystal Radio (and other purpose) Audio Amplifier

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior (with the exception of the amazingone-transistor reflex ). The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

Here is the amplifier used to boost the output from a simple crystal radio. The volume control is at the bottom left and the other components are on the terminal strip at the bottom of the picture. This is a really quick and easy audio amplifier!

how to make Op-Amp Audio Amplifier

2012-02-04T22:57:00.000-08:00

Op-Amp Audio Amplifier

The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. For example, an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground, destabilizing feedback via the power supply is greatly reduced.

My utility amplifier was built into an aluminum Bud box and eventually ended up bolted to the bottom of a shelf as shown. The well-behaved and ready-to-go amplifier is really handy.

As is often the case, the circuit values are not critical. Other op-amps will usually work but a bit of experience may be necessary if problems develop. The two 4.7 ohm resistors in the emitters may be replaced with a single 10 ohm resistor in either position - I just like the symmetry!

how to make 4-Transistor Amplifier for Small Speaker Applications

2012-02-04T22:56:00.001-08:00

4-Transistor Amplifier for Small Speaker Applications

The circuit above shows a 4-transistor utility amplifier suitable for a variety of projects including receivers, intercoms, microphones, telephone pick-up coils, and general audio monitoring. The amplifier has a power isolation circuit and bandwidth limiting to reduce oscillations and "motorboating". The values are not particularly critical and modest deviations from the indicated values will not significantly degrade the performance.

Three cell battery packs giving about 4.5 volts are recommended for most transformerless audio amplifiers driving small 8 ohm speakers. The battery life will be considerably longer than a 9 volt rectangular battery and the cell resistance will remain lower over the life of the battery resulting in less distortion and stability problems.

The amplifier may be modified to work with a 9 volt battery if desired by moving the output transistors' bias point. Lowering the 33k resistor connected from the second transistor's base to ground to about 10k will move the voltage on the output electrolytic capacitor to about 1/2 the supply voltage. This bias change gives more signal swing before clipping occurs and this change is not necessary if the volume is adequate.

As before, the two 4.7 ohm resistors may be replaced with a single 10 ohm resistor in series with either emitter.

how to makeComputer Audio Booster

2012-02-04T22:55:00.001-08:00

Here is a simple amplifier for boosting the audio level from low-power sound cards or other audio sources driving small speakers like toys or small transistor radios. The circuit will deliver about 2 watts as shown. The parts are not critical and substitutions will usually work. The two 2.2 ohm resistors may be replaced with one 3.9 ohm resistor in either emitter.

Curiously Low Noise Amplifier

2012-02-04T22:54:00.001-08:00

Curiously Low Noise Amplifier

The Curiously Low Noise Amplifier takes advantage of the wonderful noise characteristics of the 2SK170 JFET that boasts a noise voltage below 1 nV/root-Hz and virtually no noise current. The noise voltage of the amplifier is only 1.4 nV/root-Hz at 1 kHz, increasing to only 2.7 nV/root-Hz at 10 Hz. The noise current is difficult to measure, so this simple utility amplifier can see the noise from a 50 ohm resistor and a 100k resistor, too. (The 1.4 nV input-referred noise will increase to about 1.7 nV with a 50 ohm resistor, instead of a short, and a 100k resistor will give an input-referred noise near 40 nV, with very little contribution from the amplifier.)

This amplifier is a "utility" amplifier with a gain of 100, that would typically be used in a lab setting to boost tiny signals for measurement or further processing. It isn't intended to drive a speaker or headphones directly. (It could drive the LM386 quite nicely.) The circuit is a simple discrete transistor feedback circuit with two gain stages and a unique class-A output buffer:

The 2sk117 is from the "BL" Idss current range and is selected for an Idss near 7 mA. The drain resistor is adjusted to achieve about 4 volts on the drain and the value depends on the Idss of the JFET.
Most of the resistors aren't critical, but precision values are shown because the resistors should be metal film types for best noise performance. Approximate DC voltages are shown for helping with resistor selection. Deviating from the shown voltages will reduce the available output voltage swing, but the amplifier might work fine for smaller signals. Unloaded swing should be about 6 volts, p-p with about 60 mV p-p input, before distortion is observed.
The MPSA18 acts as a noise filter. High gain is desirable here to keep the value of the base filter capacitor reasonable, but a 2N4401 could be substituted by reducing the 10k and 120k by a factor of 5. The filter will still be rolling off the noise voltage from the 15 volts supply above about 0.2 Hz. But some power supplies can be really noisy!
The 0.1 uF capacitors serve as bypass capacitors but mainly as terminals for holding the components. These are the white rectangles seen in the photo.
The feedback resistor is selected for a gain of exactly 100 and the value is well above the expected 1k, due to the limited open-loop gain of the simple circuit.
A small resistor is included in series with the output for stability and that resistor can reduce the gain a bit when driving a lower resistance load. The designer may choose to set the gain for that particular load, say 75 ohms, or for a high impedance load. The circuit can drive a lower resistance than 100 ohms, but the swing will be somewhat limited. It may be possible to leave out the 33 ohm resistor without stability issues. (Usually, such a utility amplifier is driving a much higher resistance load, typically 600 ohms or above.) Note: To give you an idea of how you can play with the output resistance, I just changed my unit's series output resistor to 55 ohms and adjusted the gain for 55 dB, when driving 75 ohm loads. Unloaded, the gain is exactly 5 dB higher at 60 dB. This way, I have even number gains whether driving a 75 ohm instrument or a high-Z device. The output buffer has no trouble driving the total 125 ohm load, with a swing limit of about 3.5 volts, p-p.
The output stage is an unusual self-biasing arrangement where the PNP holds the gate-source voltage near 0.6 volts, running the JFET somewhat below its Idss. The 2N5486 was chosen to not waste too much current, but a higher Idss JFET will give more drive capability, if desired.

Input Impedance: 47 megohm (set by bias resistor), shunted by 20 pF
Output Impedance: 36 ohms, set by series resistor plus about 3 ohms from the circuit. My 55 ohm resistor mentioned above gives an output Z of about 58 ohms and exactly 5 dB of gain loss from no load to 75 ohms.
Output voltage swing: 6 volts p-p into a high impedance load.
Gain: 100 (40 dB) set by feedback resistor. Lower gain could be selected for wider bandwidth.
Frequency Response: flat from below 1 Hz to above 2 MHz.
Input Noise: 1.4 nV, rising to 2.7 nV at 10 Hz. Noise current has eluded measurement so far, but it's really low. With a 97.3 k resistor (100k in parallel with 3.6 meg) connected across the input, the noise voltage measures within a tiny fraction of a dB of 40 nV, so little to no noise current is seen. In fact, this amp and a selected resistor make an inherently accurate noise source. Connect a 152k across the input (in a shielded box), and you have a precise 5 uV/root-Hz noise source throughout the audio spectrum (50 nV times 100). A quick measurement at 40 Hz gives 770 nV/root-Hz with nothing connected; the 47 megohm is expected to contribute 867 nV. That's pretty close and still little noise current from the FET.

For even better performance, the bipolar stages could be replaced with a low noise op-amp. The input noise would drop a little, perhaps to 1 nV, as would the input capacitance, perhaps below 10 pf. Compensating the op-amp might be a bit of a challenge.

Simple LM386 Audio Amplifier

2012-02-04T22:53:00.000-08:00

This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum

How Cell Phone Jammers Work

2012-01-14T23:48:00.000-08:00

Introduction to How Cell Phone Jammers Work

Cell phones are everywhere these days. According to the Cellular Telecommunications and Internet Association, almost 195 million people in the United States had cell-phone service in October 2005. And cell phones are even more ubiquitous in Europe.
It's great to be able to call anyone at anytime. Unfortunately, restaurants, movie theaters, concerts, shopping malls and churches all suffer from the spread of cell phones because not all cell-phone users know when to stop talking. Who hasn't seethed through one side of a conversation about an incredibly personal situation as the talker shares intimate details with his friend as well as everyone else in the area?
While most of us just grumble and move on, some people are actually going to extremes to retaliate. Cell phones are basically handheld two-way radios. And like any radio, the signal can be disrupted, or jammed.
In this article, you'll see how cell-phone jammers work and learn about the legality of their use.

Cell Phone Jamming Basics

Disrupting a cell phone is the same as jamming any other type of radio communication. A cell phone works by communicating with its service network through a cell tower or base station. Cell towers divide a city into small areas, or cells. As a cell-phone user drives down the street, the signal is handed from tower to tower.A jamming device transmits on the same radio frequencies as the cell phone, disrupting the communication between the phone and the cell-phone base station in the tower.

It's a called a denial-of-service attack. The jammer denies service of the radio spectrum to the cell-phone users within range of the jamming device.

Cell Phone Jamming Device

Jamming devices overpower the cell phone by transmitting a signal on the same frequency and at a high enough power that the two signals collide and cancel each other out. Cell phones are designed to add power if they experience low-level interference, so the jammer must recognize and match the power increase from the phone.
Cell phones are full-duplex devices, which means they use two separate frequencies, one for talking and one for listening simultaneously. Some jammers block only one of the frequencies used by cell phones, which has the effect of blocking both. The phone is tricked into thinking there is no service because it can receive only one of the frequencies.
Less complex devices block only one group of frequencies, while sophisticated jammers can block several types of networks at once to head off dual-mode or tri-mode phones that automatically switch among different network types to find an open signal. Some of the high-end devices block all frequencies at once, and others can be tuned to specific frequencies.
To jam a cell phone, all you need is a device that broadcasts on the correct frequencies. Although different cellular systems process signals differently, all cell-phone networks use radio signals that can be interrupted. GSM, used in digital cellular and PCS-based systems, operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. Jammers can broadcast on any frequency and are effective against AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems. Old-fashioned analog cell phones and today's digital devices are equally susceptible to jamming.
The actual range of the jammer depends on its power and the local environment, which may include hills or walls of a building that block the jamming signal. Low-powered jammers block calls in a range of about 30 feet (9 m). Higher-powered units create a cell-free zone as large as a football field. Units used by law enforcement can shut down service up to 1 mile (1.6 km) from the device.

Inside Cell Phone Jammers

Electronically speaking, cell-phone jammers are very basic devices. The simplest just have an on/off switch and a light that indicates it's on. More complex devices have switches to activate jamming at different frequencies. Components of a jammer include:

Antenna

Every jamming device has an antenna to send the signal. Some are contained within an electrical cabinet. On stronger devices, antennas are external to provide longer range and may be tuned for individual frequencies.

Circuitry

The main electronic components of a jammer are:

Voltage-controlled oscillator - Generates the radio signal that will interfere with the cell phone signal
Tuning circuit - Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
Noise generator - Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
RF amplification (gain stage) - Boosts the power of the radio frequency output to high enough levels to jam a signal

Power supply

Smaller jamming devices are battery operated. Some look like cell phone and use cell-phone batteries. Stronger devices can be plugged into a standard outlet or wired into a vehicle's electrical system.

 Check your phone - If the battery on your phone is okay, and you'd like to continue your conversation, try walking away from the area. You may be able to get out of the jammer's range with just a few steps.

Cell Phone Jammer Applications

Cell phone jamming devices were originally developed for law enforcement and the military to interrupt communications by criminals and terrorists. The bombs that blew up commuter trains in Spain in March 2004, as well as blasts in Bali in October 2002 and Jakarta in August 2003, all relied on cell phones to trigger explosives. It has been widely reported that a cell-phone jammer thwarted an assassination attempt on Pakistani President Musharraf in December 2003. When President Bush visited London in November 2004, it was reported that British police considered using jammers to protect the president's motorcade through London.
During a hostage situation, police can control when and where a captor can make a phone call. Police can block phone calls during a drug raid so suspects can't communicate outside the area. Cell-phone jammers can be used in areas where radio transmissions are dangerous, (areas with a potentially explosive atmosphere), such as chemical storage facilities or grain elevators. The TRJ-89 jammer from Antenna System & Supplies Inc. carries its own electrical generator and can block cellular communications in a 5-mile (8-km) radius.
Corporations use jammers to stop corporate espionage by blocking voice transmissions and photo transmissions from camera phones. On the more questionable end of the legitimacy spectrum, there are rumors that hotel chains install jammers to block guests' cell-phone usage and force them to use in-room phones at high rates.

Photo courtesy Netline Communications Technologies (NCT) Ltd.

Cell Phone Jamming Legal Issues

In the United States, United Kingdom, Australia and many other countries, blocking cell-phone services (as well as any other electronic transmissions) is against the law. In the United States, cell-phone jamming is covered under the Communications Act of 1934, which prohibits people from "willfully or maliciously interfering with the radio communications of any station licensed or authorized" to operate. In fact, the "manufacture, importation, sale or offer for sale, including advertising, of devices designed to block or jam wireless transmissions is prohibited" as well.
Jamming is seen as property theft, because a private company has purchased the rights to the radio spectrum, and jamming the spectrum is akin to stealing the property the company has purchased. It also represents a safety hazard because jamming blocks all calls in the area, not just the annoying ones. Jamming a signal could block the call of a babysitter frantically trying to contact a parent or a someone trying to call for an ambulance.
The Federal Communications Commission is charged with enforcing jamming laws. However, the agency has not yet prosecuted anyone for cell-phone jamming. Under the U.S. rules, fines for a first offense can range as high as $11,000 for each violation or imprisonment for up to one year, and the device used may also be seized and forfeited to the government.
In most countries, it is illegal for private citizens to jam cell-phone transmission, but some countries are allowing businesses and government organizations to install jammers in areas where cell-phone use is seen as a public nuisance. In December 2004, France legalized cell-phone jammers in movie theaters, concert halls and other places with performances. France is finalizing technology that will let calls to emergency services go through. India has installed jammers in parliament and some prisons. It has been reported that universities in Italy have adopted the technology to prevent cheating. Students were taking photos of tests with their camera phones and sending them to classmates.
With phones ringing in movies, weddings and classrooms, it's no wonder people want to tone down the intrusion. So what legally can be done to stop annoying cell-phone use?

Alternatives to Cell Phone Jamming

While the law clearly prohibits using a device to actively disrupt a cell-phone signal, there are no rules against passive cell-phone blocking. That means using things like wallpaper or building materials embedded with metal fragments to prevent cell-phone signals from reaching inside or outside the room. Some buildings have designs that block radio signals by accident due to thick concrete walls or a steel skeleton.
Companies are working on devices that control a cell phone but do not "jam the signal." One device sends incoming calls to voicemail and blocks outgoing calls. The argument is that the phone still works, so it is technically not being jammed. It is a legal gray area that has not been ruled on by the FCC as of April 2005.
Cell-phone alerters are available that indicate the presence of a cell-phone signal. These have been used in hospitals where cell-phone signals could interfere with sensitive medical equipment. When a signal is detected, users are asked to turn off their phones.
For a less technical solution, Coudal Partners, a design firm in Chicago, has launched the SHHH, the Society for HandHeld Hushing. At its Web site, you can download a note to hand to people conducting annoying cell-phone conversations, expressing your lack of interest in what they're talking about.
For more information on cell-phone jamming and related topics, check out the links on the next page.

learm about Mobile phone jammer

2012-01-14T23:37:00.000-08:00

A mobile phone jammer is an instrument used to prevent cellular phones from receiving signals from base stations. When used, the jammer effectively disables cellular phones. These devices can be used in practically any location, but are found primarily in places where a phone call would be particularly disruptive because silence is expected.

History

The rapid proliferation of cell phones at the beginning of the 21st century to near ubiquitous status eventually raised problems such as their potential use to invade privacy or contribute to rampant and egregious academic cheating. In addition public backlash was growing against the intrusive disruption cell phones introduced in daily life. While older analog cell phones often suffered from chronically poor reception and could even be disconnected by simple interference such as high frequency noise, increasingly sophisticated digital phones have led to more elaborate counters. Cell phone jamming devices are an alternative to more expensive measures against cell phones, such as Faraday cages, which are mostly suitable as built in protection for structures. They were originally developed for law enforcement and the military to interrupt communications by criminals and terrorists. Some were also designed to foil the use of certain remotely detonated explosives. The civilian applications were apparent, so over time many companies originally contracted to design jammers for government use switched over to sell these devices to private entities. Since then, there has been a slow but steady increase in their purchase and use, especially in major metropolitan areas.

Operation

As with other radio jamming, cell phone jammers block cell phone use by sending out radio waves along the same frequencies that cellular phones use. This causes enough interference with the communication between cell phones and towers to render the phones unusable. On most retail phones, the network would simply appear out of range. Most cell phones use different bands to send and receive communications from towers (called frequency division duplexing, FDD). Jammers can work by either disrupting phone to tower frequencies or tower to phone frequencies. Smaller handheld models block all bands from 800MHz to 1900MHz within a 30-foot range (9 meters). Small devices tend to use the former method, while larger more expensive models may interfere directly with the tower. The radius of cell phone jammers can range from a dozen feet for pocket models to kilometers for more dedicated units. The TRJ-89 jammer can block cellular communications for a 5-mile (8 km) radius.
Less energy is required to disrupt signal from tower to mobile phone, than the signal from mobile phone to the tower (also called base station), because the base station is located at larger distance from the jammer than the mobile phone and that is why the signal from the tower is not as strong.
Older jammers sometimes were limited to working on phones using only analog or older digital mobile phone standards. Newer models such as the double and triple band jammers can block all widely used systems (CDMA, iDEN, GSM, et al.) and are even very effective against newer phones which hop to different frequencies and systems when interfered with. As the dominant network technology and frequencies used for mobile phones vary worldwide, some work only in specific regions such as Europe or North America.
Components of a jammer include:

Antenna

Circuitry

The main electronic components of a jammer are:

Voltage-controlled oscillator — Generates the radio signal that will interfere with the cell phone signal
Tuning circuit — Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
Noise generator — Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
RF amplification (gain stage) — Boosts the power of the radio frequency output to high enough levels to jam a signal

Power supply

Smaller jamming devices are battery operated. Some look like cellphones and use cellphone batteries. Stronger devices can be plugged into a standard outlet or wired into a vehicle's electrical system.
The jammer's effect can vary widely based on factors such as proximity to towers, indoor & outdoor settings, presence of buildings and landscape, even temperature and humidity play a role.
There are concerns that crudely designed jammers may disrupt the functioning of medical devices such as pacemakers.^{[citation needed]}However, like cellphones, most of the devices in common use operate at low enough power output (<1W) to avoid causing any problems.^{[citation needed]}

Resistors

2012-01-13T10:41:00.000-08:00

Question 1:

Shown here is the schematic symbol for a resistor:

What is the purpose of a resistor? What function does it perform? Also, draw an illustration of what a real resistor looks like.

The purpose of a resistor is to provide a precise amount of electrical resistance in a circuit. Here is an illustration of a small (1/8 or 1/4 watt) resistor:

It is also good to know that the zig-zag symbol shown in the question is not the only symbol used to represent resistors. Another common resistor symbol is shown here:

Notes:
Students may (properly) ask, "Why is there such a thing as a component whose sole purpose is to impede the flow of electrons?" While resistors may seem rather pointless at first, they end up being extremely valuable electrical/electronic components. If asked, you may cite several uses of resistors in circuits:

•: To limit maximum circuit current to a safe value.
•: To ßplit" a voltage into proportions.
•: To ßcale" meter movements, for precise measurement of current and voltage.
•: To provide a non-shorting path to discharge static electricity.

Question 2:

Resistors are sometimes represented in electrical and electronic schematic diagrams by a symbol other than this:

Draw this other symbol next to the one shown above.

Notes:
It might be a good idea to occasionally draw schematic diagrams for your students using the öther" resistor symbol, just so they are not taken by surprise when they see this symbol in real schematics. Just be sure to remain consistent in your symbolism within each diagram: never mix the two different symbols within the same schematic!

Question 3:

A primitive resistor may be formed by sketching a thick line on a piece of paper, using a pencil (not an ink pen!):

How may the end-to-end electrical resistance of this pencil mark be increased? How may it be decreased? Explain your answers.

The electrical resistance of a pencil mark may be increased by increasing its length. It may be decreased by increasing its width.

Notes:
Creating a resistor on paper using a pencil is a very easy experiment to perform, the resistance of which may be measured with an ohmmeter. I strongly recommend this as a classroom exercise!

Question 4:

When a resistor conducts electric current, its temperature increases. Explain how this phenomenon is significant to the application of resistors in electric circuits. In other words, why would we care about a resistor's temperature increasing?
Also, what does this indicate about the technical ratings of resistors? Aside from having a specific resistance rating (i.e. a certain number ofohms), what other rating is important for proper selection of resistors in electric circuits?

The heating effect of electricity through a resistance is significant because that resistance may be damaged by excessive temperature. To avoid damage, resistors must be selected to be able to withstand a certain amount of heating.

Notes:
Students need to understand that resistance alone does not fully dictate the selection of a resistor for electrical service. Failure to heed the dissipation ratings of a resistor can result in catastrophic failure!
A good follow-up question to this is to ask what the unit of measurement is for this kind of thermal rating.

Question 5:

Many resistors have their electrical resistance shown by a set of color codes, or "bands," imprinted around their circumference. A standard color code associates each color with a specific decimal digit (0 through 9). Associate each of the following digits with its respective color:

0: =
1: =
2: =
3: =
4: =
5: =
6: =
7: =
8: =
9: =

0: = Black
1: = Brown
2: = Red
3: = Orange
4: = Yellow
5: = Green
6: = Blue
7: = Violet
8: = Grey
9: = White

Notes:
Several limericks have been invented to remember this color code, most of them "politically incorrect." I often challenge students to invent their own limericks for remembering this color code, and screen the inappropriate creations from general class discussion.

Hide Answer

Question 6:

Observe the following "4-band" resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value):

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality), the resistance values, and the physical sizes of the resistors?

Question 7:

There is more than one answer to this question! On some resistors, the last band represents the tolerance (also known as precision) for that resistor, expressed as a percentage. On other resistors, the last band represents a reliability rating for that resistor.

Notes:
This question is worded simply and directly enough that students might think there is only one right answer. However, upon doing some research they should find that there is more involved than one simple answer can encompass! Discuss with your students the different color code types, and what applications one might find resistors with "reliability" color codes in.
Regarding precision, nothing in life is perfectly accurate. However, the absence of perfect accuracy does not necessarily imply total uncertainty. In science, especially, it is important that all data be qualified by a statement of precision (or tolerance). Your students may be familiar with "margins of error" stated for public opinion polls. With resistors, this "margin of error" (expression of uncertainty) is explicitly given in the form of a separate color band.

Question 8:

Determine the nominal resistance values of these resistors, given their band colors, and also express the allowable tolerance in ohms.
For example, a 25 kΩ resistor with a 10% tolerance rating would have an allowable tolerance of +/- 2.5 kΩ.

•: Red, Org, Blu, Gld =
•: Brn, Blk, Grn, Sil =
•: Blu, Blk, Brn, Gld =
•: Yel, Vio, Red, Sil =
•: Grn, Brn, Yel =
•: Wht, Blu, Blk, Sil =
•: Gry, Grn, Org, Gld =
•: Org, Org, Gld =
•: Vio, Red, Sil, Gld =
•: Brn, Red, Blk, Sil =

Question 9:

Observe the following "5-band" precision resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value):

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality) and the resistance values of each resistor? Why do precision resistors use a "5-band" color code instead of a "4-band" color code?

The first three color "bands" for precision five-band resistors denote three digits and a "multiplier" value, respectively. A five-band color code is necessary to express resistance with a greater number of significant digits than a four-band code.

Notes:
The normal way to teach students the resistor color code is to show them the code first, then show them some resistors. Here, the sequence is reversed: show the students some resistors, and have them figure out the code. An important cognitive skill is the ability to detect and apply patterns in sets of data. Exercises such as this help build that skill.
It should be noted that there is a 5-band color code for non-precision resistors as well, with the first four bands serving the same purpose as in a 4-band code, the extra band indicating resistor reliability. This scheme was developed for military purposes and is seldom seen in civilian circuitry.

Question 10:

Determine whether or not the following resistors measure within the resistance range specified by their color codes:

•: (Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω
•: (Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω
•: (Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ
•: (Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ
•: (Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ
•: (Gry, Red, Brn, Sil) Measured resistance = 905 Ω
•: (Grn, Blu, Gld) Measured resistance = 6.73 Ω
•: (Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω
•: (Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ
•: (Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ

Assume that all five-band resistors listed here use the precision color code as opposed to the military 5-band code where the fifth band indicates resistor reliability.

•: (Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω Within tolerance
•: (Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω Out of tolerance!
•: (Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ Within tolerance
•: (Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ Within tolerance
•: (Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ Within tolerance
•: (Gry, Red, Brn, Sil) Measured resistance = 905 Ω Out of tolerance!
•: (Grn, Blu, Gld) Measured resistance = 6.73 Ω Out of tolerance!
•: (Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω Within tolerance
•: (Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ Within tolerance
•: (Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ Out of tolerance!

Notes:
This question serves as a great review for the mathematical concepts of scientific notation and percentages. They will have to calculate the allowable range of resistance values for each resistor in order to determine whether or not the measured value falls within that range.

Question 11:

Find one or two real resistors and bring them with you to class for discussion. Identify as much information as you can about your resistors prior to discussion:

•: Resistance (ideal)
•: Resistance (actual)
•: Power rating
•: Type (carbon composition, metal film, wire-wound, etc.)

If possible, find a manufacturer's datasheet for your components (or at least a datasheet for a similar component) to discuss with your classmates.
Be prepared to prove the actual resistance of your resistors in class, by using a multimeter!

Notes:
The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a ßhow and tell" exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they're learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting color codes and/or component markings, use a multimeter, access datasheets, etc.

Sources of electricity

2012-01-13T10:31:00.000-08:00

Sources of electricity

Question 1:

What is the difference between DC and AC electricity? Identify some common sources of each type of electricity.

DC is an acronym meaning Direct Current: that is, electrical current that moves in one direction only. AC is an acronym meaningAlternating Current: that is, electrical current that periodically reverses direction (älternates").
Electrochemical batteries generate DC, as do solar cells. Microphones generate AC when sensing sound waves (vibrations of air molecules). There are many, many other sources of DC and AC electricity than what I have mentioned here!

Notes:
Discuss a bit of the history of AC versus DC in early power systems. In the early days of electric power in the United States of America, there was a heated debate between the use of DC versus AC. Thomas Edison championed DC, while George Westinghouse and Nikola Tesla advocated AC.
It might be worthwhile to mention that almost all the electric power in the world is generated and distributed as AC (Alternating Current), and not as DC (in other words, Thomas Edison lost the AC/DC battle!). Depending on the level of the class you are teaching, this may or may not be a good time to explain why most power systems use AC. Either way, your students will probably ask why, so you should be prepared to address this question in some way (or have them report any findings of their own!).

Question 2:

Suppose you are building a cabin far away from electric power service, but you desire to have electricity available to energize light bulbs, a radio, a computer, and other useful devices. Determine at least three different ways you could generate electrical power to supply the electric power needs at this cabin.

There are several different devices capable of producing electrical power for this cabin of yours:

•: Engine-driven generator
•: Solar cell
•: Thermopile
•: Windmill

For each of these devices, what is its operating principle, and where does it obtain its energy from?

Notes:
For each of these electric power ßources," there is a more fundamental source of energy. People often mistakenly think of generator devices as magic sources of energy, where they are really nothing more than energy converters: transforming energy from one form to another.

Question 3:

Where does the electricity come from that powers your home, or your school, or the streetlights along roads, or the many business establishments in your city? You will find that there are many different sources and types of sources of electrical power. In each case, try to determine where the ultimate source of that energy is.
For example, in a hydroelectric dam, the electricity is generated when falling water spins a turbine, which turns an electromechanical generator. But what continually drives the water to its üphill" location so that the process is continuous? What is the ultimate source of energy that is being harnessed by the dam?

Some sources of electrical power:

•: Hydroelectric dams
•: Nuclear power plants
•: Coal and oil fired power plants
•: Natural gas fired power plants
•: Wood fired power plants
•: Geothermal power plants
•: Solar power plants
•: Tidal/wave power plants
•: Windmills

Notes:
A great point of conversation here is that almost all ßources" of energy have a common origin. The different ßources" are merely variant expressions of the same true source (with exceptions, of course!).

Question 4:

There is a fundamental Law in physics known as the Law of Energy Conservation. This law states that energy can neither be created nor destroyed, merely transformed from one form to another.
In regard to this Law, is it possible to make an electrical battery that lasts forever, and never becomes exhausted? Explain why or why not.

According to the Law of Energy Conservation, it is not possible to make a battery that lasts forever.

Notes:
This question may lead into a discussion on perpetual motion machines, a subject that seems to be perpetual itself in the popular interest.

Question 5:

The mathematical equivalence between watts and horsepower is approximately 746:1. Given this equivalency, how many watts of electrical energy may theoretically be produced by a generator, if turned by an engine rated at 50 horsepower?

37.3 kW of electrical power.

Notes:
Unit conversion problems are handled most easily by using ünity fractions" comprised of a quotient of two different numerical quantities having the same physical value. For instance:

Question 6:

Suppose a person decides to attach an electrical generator to their exercise bicycle, so as to do something useful with their "pedal power" while they exercise. The first time this person uses their bicycle generator, the electricity is used to power a single 60-watt light bulb. However, the next time this person uses their bicycle generator, a second 60-watt light bulb is connected to the generator, for a total load of 120 watts.
When pedaling with the additional load, the person notices the bicycle is much more difficult to pedal than before. It takes greater force on the pedals to maintain the same speed as before, when there was only a single 60-watt light bulb to power. What would you say to this person if they asked you, the electricity expert, to explain why the bicycle is more difficult to pedal with the additional light bulb connected?

I won't give you the answer directly, but here is a hint: the Law of Energy Conservation.

Notes:
This phenomenon is more easily understood when experienced directly. If you happen to have a hand-powered generator available for a classroom demonstration, help your students set it up to demonstrate this principle.
An excellent topic of discussion related to this question is the effect that using more electrical power has on the generators at power plants (hydroelectric, nuclear, coal-fired, etc.). What would happen at the power plants supplying electricity to the nation's electrical "grid" if everyone simultaneously turned on all their electrical loads at home?

Question 7:

Where does the energy come from that causes a battery to be a source of electricity for powering electrical devices? Ultimately, what is the energy source of a battery?

Electrical batteries derive their energy from chemical compounds stored within.

Notes:
Battery electrochemistry is quite complex, and not easily addressed in a class environment unless all students have a good background knowledge of chemistry. It is not the point of this question to address issues of electrochemistry, but merely to point out the nature of a battery's energy source.

Hide Answer

Question 8:

Describe what a fuel cell is, and what the practical importance of such a device might be.

"Fuel cells" are essentially batteries externally supplied with chemicals for their source of energy.

Notes:
Much attention has been directed toward fuel cells as energy conversion devices, for their high efficiency and environmentally "clean" operation. Your students should have no trouble finding current information on fuel cell technology.

Question 9:

What will happen, electrically speaking, when the shaft of the machine is turned?

A voltage will be ïnduced" in the wire coil as the magnet passes by.
Follow-up question: do you think the voltage generated by this machine would be DC or AC? Explain your answer.

Notes:
Although this machine would not be very efficient in the form shown in the illustration, it would work to generate electricity when turned. As such, it demonstrates the principle of electromagnetic induction.

Question 10:

Suppose there was something wrong in this electrical system. When the shaft of the generator is turned, the light bulb does not light up:

What are some of the possible causes of this failure? Please be specific. Also, what could you do to either confirm or deny these specific possibilities?

There may be an öpen" fault in the circuit somewhere, and/or the light bulb could be improperly sized for the generator's rated voltage output. I'll let you determine how certain diagnostic checks could be made in this system to determine the exact nature of the fault!

Notes:
This question poses an excellent opportunity to group discussion on troubleshooting theory and technique. There are several different ways in which the nature of the fault may be determined. Encourage your students to think creatively!

Soldering

2012-01-13T10:22:00.000-08:00

Question 1:

Both soldering irons and soldering guns are tools used in the process of electrical soldering. Describe what each of these tools looks like, and how they function.

A modern soldering ïron" is a pencil-shaped device with an electrical heating element in its end. Soldering "guns" have a pistol-shaped body, with a loop of thick copper wire at the end acting directly as a heating element. Soldering irons have no on/off switch, while soldering guns do, which should tell you something about the speed at which they heat.

Notes:
Antique soldering irons were really nothing more than iron wedges with wooden handles, which were set directly in a fire for heat, much like clothes irons used to be nothing more than an anvil-shaped mass of iron that was set on top of a hot stove for heat. I've used one of these old soldering irons to solder sheet metal pieces together, and the technique of use was exactly the same as it is for a modern (electric) soldering iron.
If you have soldering irons and guns available in your classroom, show them to your students during discussion.

Question 2:

What does it mean to tin a piece of wire prior to making a soldered connection with it?

"Tinning" is the act of pre-soldering the bare wire end, so that its appearance is like solder instead of copper.

Notes:
Some wire may be purchase "pre-tinned" to save you this step, but most electrical wire is not. Here is a helpful hint: tinning the ends of a piece of stranded copper wire really helps to prevent individual strands from bending and breaking away when the wire end is clamped in a terminal strip!

Question 3:

All electrical soldering, and most other forms of soldering as well, requires the addition of a substance known as flux. What is the purpose of adding "flux" to the soldering process, and what type of flux is typically used for electrical soldering?

"Flux" is a substance that chemically cleans the metal surfaces prior to soldering. For electrical work, the preferred flux is "rosin".

Notes:
This question, and the corresponding answer, point to one of the fundamental requirements for a good solder joint: clean metal surfaces. Flux is designed to chemically clean the joint prior to and during the soldering process, but it is no substitute for proper preparatory cleaning beforehand.
Discuss this point with your students. If possible, set up some wire connections to be soldered, and show the effects of a small amount of oil or grease on a solder joint (even when using flux).

Question 4:

As in all technician tasks, safety must be your first priority. Soldering harbors specific dangers of its own that you need to be aware of. Explain what precautions you need to take with regard to the following aspects of soldering:

•: Safe handling of soldering iron/gun
•: Safe handling of solder and flux
•: Personal protective equipment
•: What to do (and not do) after soldering

Safe handling of soldering iron/gun

•: Never hold it by the metal end - always by the handle!
•: Place iron on a stand when idle, and be careful that no flammable materials contact it.
•: Never leave the iron on, unattended.

Safe handling of solder and flux

•: Never hold solder in your mouth, as it contains heavy metals that are not healthy for your body.
•: Avoid breathing fumes - use a ventilation hood if available.
•: Do not eat food while soldering.
•: Wash your hands after soldering.

Personal protective equipment

•: Safety glasses or goggles - an absolute must!
•: If soldering large wires, use heat-resistant gloves.

What to do (and not do) after soldering

•: Clean up worksite.
•: Give the soldering iron enough time to cool down before putting it away.
•: Put soldering equipment away where it belongs, and where others can find it.
•: Never leave the iron on, unattended.

Notes:
Much of this is common sense, but some of it is not. Students must realize that even though soldering is a relatively low-temperature process, it is still hot enough to start fires and burn skin. Also, molten droplets of solder can cause major damage to the eyes, so safety glasses/goggles are imperative.

Question 5:

Semiconductor components such as diodes and transistors are easily damaged by the high temperatures of soldering, so care must be taken to protect these components during the soldering process. One way to do this is to use a heat sink to draw heat away from the component without cooling the connection point too much. Heat sinks made out of sheet metal may be temporarily clipped to the component leads, one at a time, to prevent the solder's high temperature from thermally conducting all the way to the component body:

In the absence of a formal heat sink, can you think of any ways to fashion your own ïmpromptu" heat sinks out of commonly available tools and/or objects?

Question 6:

When soldering a wire into a metal lug, which of these positions would be considered best for soldering iron and solder?

Explain why you think one of these positions is better than the others for producing a sound solder connection between the lug and the wire.

Question 7:

What is a cold solder joint? Identify its causes, its appearance, and the means of correction.

A "cold solder joint" is a poor solder connection resulting from motion of the pieces during the cooling process or insufficient temperature.

Notes:
Ask your students where they obtained their information regarding the causes, appearance, and remedy for cold solder joints. There is a lot of information available on soldering, and it is always interesting and informative to discover new sources of information for future reference.

Question 8:

After a period of use, the tip of a soldering iron (or a soldering gun) becomes dark and ëncrusted" with oxidation and flux residue. Describe why this accumulation impedes the soldering process, and how it may be removed.

Any oxidation or foreign material on the soldering tool tip will impede heat flow to the item being soldered. Routine cleaning is easy: just touch the hot tip to a water-soaked sponge or rag.

Notes:
Discuss with your students why a wet sponge or rag works to clean the end of a heated soldering tool. Also, discuss what to do to the tip of a soldering tool that has been abused by lack of periodic cleaning.

Question 9:

What may be done if you solder a component into a circuit board and then realize you soldered it in the wrong place? What, exactly, must you do to move this component to a different location on the board?

You must desolder the component from the board, using any number of desoldering tools:

•: De-soldering "wick"
•: De-soldering bulb
•: De-soldering "pump"
•: Vacuum station

Notes:
Ask your students to describe how some common desoldering tools (wick, bulb, plunger, vacuum system) work, and also ask them where they obtained their information.

Question 10:

Explain what surface-mount devices are, and how the soldering and desoldering processes for them differ from the processes used to solder and de-solder "through-hole" components on printed circuit boards. The following illustration contrasts "through-hole" component construction versus surface-mount construction for a voltage divider circuit (three resistors connected in series):

"Surface-mount" components solder to the surface of a printed circuit board, and have no mechanical means of attachment, unlike "through-hole" components. Because of their small size and lack of attachment prior to soldering, special soldering equipment must be used.

Notes:
Ask your students to describe some of the special soldering equipment that must be used for surface-mount devices? How does it differ in terms of power from regular soldering irons and guns? Why are surface-mount soldering tools rated at a different power level?
Ask your students to describe how surface-mount components may be held to the board during the soldering process. Are all surface-mount components identical, or are there different types? What special soldering tool attachments are available for working with surface-mount devices?

Question 11:

The equation for converting degrees Celsius (^oC) to degrees Fahrenheit (^oF) is as follows:

T_F =

T_C + 32

Use this equation to convert the following temperatures from Celsius to Fahrenheit:

•: 0^oC =
•: 100^oC =
•: -40^oC =
•: 250^oC =
•: 310^oC =

Now, algebraically manipulate the equation so that you may solve for degrees Celsius from a given figure in degrees Fahrenheit. Show all your work, then check your results by working backward through the five calculations above to arrive at the original figures in degrees Celsius.

•: 0^oC = 32^oF
•: 100^oC = 212^oF
•: -40^oC = -40^oF
•: 250^oC = 482^oF
•: 310^oC = 590^oF

T_C =

(T_F − 32)

Notes:
The last instruction given in the question - working backwards through the five calculations to see if you get the original (given) figures in degrees Celsius - is actually a very useful way for students to check their algebraic work. Be sure to make note of this in class!

Question 12:

What is soldering? What is solder?

"Soldering" is a low-temperature form of welding commonly used to join electrical conductors together for permanent connections.
"Solder" is a special alloy of metals designed to melt at a low temperature, to make permanent electrical connections.

Notes:
It should be noted that not all forms of soldering are "low temperature." Silver soldering, for example, takes place at much greater temperatures than regular electrical soldering. However, all soldering is a form of welding.

Question 13:

Find a printed circuit board with soldered components on it, and bring it with you to class for discussion. If you have access to a scrap bin from an electronics or computer repair shop, this would be an excellent place to find a printed circuit board. On your PCB, try to identify:

•: Number of layers on board
•: "Traces"
•: "Lands"
•: Cold solder joints (if any)
•: Solder bridges (if any)

Electrical connections

2012-01-13T10:19:00.000-08:00

Question 1:

What, exactly, is necessary to establish electrical continuity between two wires? If I want to have an electric current flow out of one wire and into another, what must be done with those two wires to make that flow path complete?
Conversely, what things might prevent continuity from being established between two wires when they are supposed to electrically connect with one another?

There must be metal-to-metal contact between the two wires in order to establish electrical continuity between the two. Anything preventing this clean contact between metal surfaces will inhibit continuity. This includes dirt, dust, oil, corrosion, misplaced insulation, and the like.

Notes:
It might be helpful to show students what real pieces of wire look like, in order for them to better understand the nature of the problem. Most electrical wire is insulated in one form or another, and this insulation must not be removed (or ßtripped") in order to establish bare metal-to-metal contact.

Question 2:

What type of electrical test would be the most direct means of checking the integrity of an electrical connection between two conductors? Explain your answer.

An electrical resistance test.

Notes:
Discuss with your students the ideal resistance of an electrical connection, regardless of type (soldered, terminal strip, wire wrap, wire nut, crimp connector, etc.), and under what conditions you would test the resistance of an electrical connection.

Question 3:

What is the difference between a plug and a jack? What are these two devices used for? Draw the schematic diagram symbols for single-conductor plugs and jacks.

A "plug" is the male half of a temporary electrical connector, while the "jack" is the female half. Electrical connectors are used to make convenient connections between individual wires or sets of wires.

Notes:
Be sure to inform your students that poor electrical connections, often with regard to plug-and-jack assemblies, are a very common source of failure in electrical and electronic systems.

Question 4:

What is a terminal block, or terminal strip? How are these devices used to make electrical connections between different conductors?

Terminal "blocks" or ßtrips" are rows of short, metal blocks with screws or other fastener devices used to make firm mechanical bonds to conductors:

Sometimes, terminal blocks also go by the name of "barrier strips."

Notes:
Be sure to have a variety of terminal blocks available for your students to see and touch during discussion time. Terminal blocks are widely used in industrial applications, for making permanent and semi-permanent connections.

Question 5:

What is generally considered to be the maximum number of individual wires that may be attached to a single terminal on a terminal block?

Two wires per fastener, maximum:

Notes:
I'm not sure if there is a specific code or regulation governing the number of terminals connected to a terminal block, but the "no more than two wires per lug" rule makes good sense. That is, of course, unless two conductors stuffed in the same hole would exceed the maximum cross-sectional area limit of the block, then "no more than one wire" is the rule.
I have had the misfortune of seeing egregious violations of both rules in my work experience!

Hide Answer

Question 6:

Wire nuts are special electrical connection devices commonly used in residential power wiring. Describe what a "wire nut" is and how they are used to make connections between wires.

"Wire nuts" are electrical fasteners which use a coiled spring to clasp bare metal wire ends together. The "nut" is screwed on to the ends of two or more wires to be joined, as though it were a nut being threaded onto a bolt. The coils of the spring inside the nut function as threads, which grip the wire ends tighter as the nut is turned clockwise over them.

Notes:
It would be good to have several sizes of wire nuts available in the classroom for your students to see and touch during discussion.

Question 7:

How are connections made between the various wire ends of components inserted into printed circuit boards ("PCBs")?

Component wire ends are typically soldered onto strips of copper metal (called "traces") affixed on the surface of the printed circuit board material, which itself is electrically insulating.

Notes:
Be sure to have a variety of printed circuit boards available for your students to see and touch during discussion time. PCBs are widely used in all varieties of electronic applications, and are well suited for mass production manufacturing techniques.

Question 8:

Suppose this circuit has a problem: the light bulb does not light up when the switch is turned ÖN". You suspect that a "bad" connection may exist in the circuit which is preventing current. How would you test the integrity of each connection using a voltmeter only?

If voltage is measured across any connection of two wires, that connection is definitely faulty.

Notes:
It may be more difficult that it first appears to test for voltage across any single connection, if the wires are covered with insulation! Ask your students this question: if you test for voltage between two screws on a terminal strip, how many electrical connections are you actually measuring voltage across?

Question 9:

Map the interconnections between holes in a typical solderless breadboard, also commonly known as a proto-board:

Notes:
"Mapping" the holes on a solderless breadboard is a fun exercise to do in class with ohmmeters (or any other form of continuity indicator).

Question 10:

Show how to build a simple circuit consisting of a battery, a lamp, and a switch, mounting the lamp and switch on a solderless breadboard (also known as a proto-board):

This is just one possibility - there are many others!

Notes:
Solderless breadboards are extremely useful tools for prototyping circuits in a classroom/lab environment. Familiarity with their use is a virtual necessity for any modern electronics curriculum. That being said, it is important not to over-emphasize breadboards, though. I have seen some electronics courses where breadboards are the only form of circuit construction students ever use! This does not prepare them for challenges of the job, where breadboards are (rightfully) used only for prototyping.
In summary, use breadboards in your students' labwork, but not all the time, or even most of the time!

Hide Answer

Question 11:

What is soldering? What is solder?

Question 12:

An alternative to soldering components into printed circuit boards is to use a technique called wire wrap. Describe what "wire wrap" is and what applications it might be suitable for.

"Wire wrap" is a connection technique for joining components together on a circuit board without need for pre-printed copper "traces" on the board. Instead, small-gauge wire is tightly wrapped around the pins of each component, with the wire making point-to-point connections between components.

Notes:
Though wire-wrapping is not as prevalent now as it was a few decades ago, it still has merit for certain applications, notably prototyping. If possible, have some wire-wrapped circuit boards available to show your students during discussion time.

Question 13:

Solderless breadboards provide convenient means for electronics hobbyists, students, technicians, and engineers to build circuits in a non-permanent form. The following illustration shows a three-resistor series circuit built on a breadboard:

The interconnections between the metal spring clips within the holes of the breadboard allow continuity between adjacent leads of the resistors, without the resistor leads having to be jammed into the same hole.
However, new students often get themselves into trouble when first learning how to use solderless breadboards. One common mistake is shown here, where a student has attempted to create a simple single-resistor circuit:

What the student has actually created here is a short circuit. Re-draw this circuit in schematic form, and explain why this circuit is faulty.

Follow-up question #1: explain what might happen if a large battery or high-current power supply were powering this short circuit.
Follow-up question #2: show how the single-resistor circuit should have been built on the breadboard so as to avoid a short circuit.

Notes:
Situations such as this are very common among new students! Be sure to discuss the significance of ßhort circuits" as well as how to avoid them.

Elementary circuits

2012-01-13T10:16:00.000-08:00

Elementary circuits

Question 1:

In the simplest terms you can think of, define what an electrical circuit is.

An electrical circuit is any continuous path for electrons to flow away from a source of electrical potential (voltage) and back again.

Notes:
Although definitions are easy enough to research and repeat, it is important that students learn to cast these concepts into their own words. Asking students to give practical examples of "circuits" and "non-circuits" is one way to ensure deeper investigation of the concepts than mere term memorization.
The word "circuit," in vernacular usage, often refers to anything electrical. Of course, this is not true in the technical sense of the term. Students will come to realize that many terms they learn and use in an electricity or electronics course are actually mis-used in common speech. The word ßhort" is another example: technically it refers to a specific type of circuit fault. Commonly, though, people use it to refer to any type of electrical problem.

Question 2:

Given a battery and a light bulb, show how you would connect these two devices together with wire so as to energize the light bulb:

This is the simplest option, but not the only one.

Notes:
This question gives students a good opportunity to discuss the basic concept of a circuit. It is very easy to build, safe, and should be assembled by each student individually in class. Also, emphasize how simple circuits like this may be assembled at home as part of the "research" portion of the worksheet. To research answers for worksheet questions does not necessarily mean the information has to come from a book! Encourage experimentation when the conditions are known to be safe.
Have students brainstorm all the important concepts learned in making this simple circuit. What general principles may be derived from this particular exercise?

Question 3:

Draw an electrical schematic diagram of a circuit where a battery provides electrical energy to a light bulb.

This schematic diagram is not the only valid way to show a battery powering a light bulb:

Other orientations of the components within the diagram are permissible. What matters, though, is for there to be a single, continuous path for electric current from the battery, to the light bulb, and back to the other terminal of the battery.

Notes:
Impress upon the students the importance of learning to "communicate" in the language of schematic diagrams. The symbols and conventions learned here are international, and not limited to use in the United States.

Question 4:

How could a battery, a light bulb, and some lengths of metal wire be used as a conductivity tester, to test the ability of different objects to conduct electricity?

The following circuit would function as a simple continuity tester. Simply place the open wire ends in contact with the object to be tested, and the light bulb will indicate whether or not the object conducts electricity to any substantial degree:

Notes:
Not only is this question an opportunity to solve a problem, but it lends itself well to simple and safe experimentation. Encourage students to build their own conductivity testers and test various substances with them.

Question 5:

Suppose we had a long length of electrical cable (flexible tubing containing multiple wires) that we suspected had some broken wires in it. Design a simple testing circuit that could be used to check each of the cable's wires individually.

Notes:
A significant portion of electrical/electronic circuit problems are caused by nothing more complex than broken wire connections, or faults along the length of wires. Testing cables for wire breaks is a very practical exercise.
The same technique may be used to "map" wires from one end of a cable to the other, in the event that the wires are not color-coded or otherwise made identifiable.

Question 6:

What do the symbols with the question marks next to them refer to? In the circuit shown, would the light bulb be energized?

These are ground symbols, and they can either refer to connections made to a common conductor (such as the metal chassis of an automobile or circuit enclosure), or the actual earth (usually via metal rods driven into the dirt).

Notes:
Ask the students about the relative conductivities of metal chassis versus dirt (earth ground). Is a current pathway formed by two metal chassis grounds equivalent to a current pathway formed by two earth grounds? Why or why not? What conditions may affect these relative conductivities?

Question 7:

Shown here is a simplified representation of an electrical power plant and a house, with the source of electricity shown as a battery, and the only electrical "load" in the house being a single light bulb:

Why would anyone use two wires to conduct electricity from a power plant to a house, as shown, when they could simply use one wire and a pair of ground connections, like this?

This is not a practical solution, even though it would only require half the number of wires to distribute electrical power from the power plant to each house! The reason this is not practical is because the earth (dirt) is not a good enough conductor of electricity. Wires made of metal conduct electricity far more efficiently, which results in more electrical power delivered to the end user.

Notes:
Discuss the fact that although the earth (dirt) is a poor conductor of electricity, it may still be able to conduct levels of current lethal to the human body! The amount of current necessary to light up a household light bulb is typically far in excess of values lethal for the human body.

Question 8:

What, exactly, is a short circuit? What does it mean if a circuit becomes shorted? How does this differ from an open circuit?

A short circuit is a circuit having very little resistance, permitting large amounts of current. If a circuit becomes shorted, it means that a path for current formerly possessing substantial resistance has been bypassed by a path having negligible (almost zero) resistance.
Conversely, an open circuit is one where there is a break preventing any current from going through at all.

Notes:
Discuss with your students some of the potential hazards of short circuits. It will then be apparent why a ßhort circuit" is a bad thing. Ask students if they can think of any realistic circumstance that could lead to a short-circuit developing.
I have noticed over several years of teaching electronics that the terms ßhort" or ßhort-circuit" are often used by new students as generic labels for any type of circuit fault, rather than the specific condition just described. This is a habit that must be corrected, if students are to communicate intelligently with others in the profession. To say that a component ïs shorted" means a very definite thing: it is not a generic term for any type of circuit fault.

Question 9:

What would have to happen in this circuit for it to become shorted? In other words, determine how to make a short circuit using the components shown here:

Notes:
In real life, of course, short circuits are usually things to be avoided. Discuss with your students why short circuits are generally undesirable, and what role wire insulation plays in preventing them.

Question 10:

Examine this schematic diagram:

Now, without moving the following components, show how they may be connected together with wires to form the same circuit depicted in the schematic diagram above:

Follow-up question: suppose the circuit were built like this but the light bulb did not turn on when the switch was closed. Identify at least five specific things that could be wrong with the circuit to cause the light not to turn on when it should.

Notes:
One of the more difficult skills for students to develop is the ability to translate a nice, neat schematic diagram into a messy, real-world circuit, and visa-versa. Developing this skill requires lots of practice.
In case students have not learned battery symbol convention yet, please point out to them the "+" and "-" polarity marks, and note which side of the battery is which.
One analogy to use for the switch's function that makes sense with the schematic is a drawbridge: when the bridge is down (closed), cars may cross; when the bridge is up (open), cars cannot.

Question 11:

This circuit has four "test points" labeled with the letters A, B, C, and D. Assuming the circuit is functioning (light bulb is energized), determine whether or not there will be substantial voltage between the following sets of points:

•: Between A and B: voltage or no voltage?
•: Between B and C: voltage or no voltage?
•: Between C and D: voltage or no voltage?
•: Between D and A: voltage or no voltage?
•: Between A and C: voltage or no voltage?
•: Between D and B: voltage or no voltage?

Based on these voltage determinations, what general statement(s) can you make about the presence or absence of voltage in a functioning circuit?

Rather than tell you whether or not there will be voltage, I'll let you build your own circuit and measure for yourself! I will, however, give you two "rules" that may be derived from these measurements:

•: Measuring between two terminals of a voltage source, or between two terminals of an energized load (a component having substantial resistance, with current going through it), there will be a voltage.
•: Measuring between two points that are electrically common to each other, there will be no voltage (or at least an insignificant amount of voltage).

Notes:
Electrically common points are defined as points within a circuit that are connected together by conductors of negligible resistance.
The concept of ëlectrically common" points is one that I have found very helpful in the analysis of circuits, because there are a some simple rules regarding voltage that relate to whether or not points in a circuit are "common" to each other.

Question 12:

In this electrical circuit, trace the direction of current through the wires:

Reveal Answer

This is a "trick" question, as there are two accepted ways of denoting the direction of electric current: conventional flow (sometimes called hole flow), and electron flow.

Notes:
This question breaches one of the more contentious subjects in electricity/electronics: which way do we denote the direction of current? While there is no debate as to which direction electrons move through a metal conductor carrying current, there are two different conventions for denoting current travel, one of which goes in the direction of electrons and the other which goes against the direction of electrons. The reason for having these two disparate conventions is embedded in the history of electrical science, and what your students find in their research will likely fuel an interesting conversation.

Conductors and Insulators

2012-01-13T10:13:00.000-08:00

Conductors and Insulators

Question 1:

Given two lengths of metal wire, which one will have the least electrical resistance: one that is short, or one that is long? Assume all other factors are equal (same metal type, same wire diameter, etc.).

The short wire will have less electrical resistance than the long wire.

Notes:
Many analogies exist to express this concept: water through a pipe, compressed air through a hose, etc. Which pipe or hose is less restrictive: the short one or the long one?

Question 2:

Given two lengths of solid metal wire with round cross-sections, which one will have the least electrical resistance: one that is small-diameter, or one that is large-diameter? Assume all other factors are equal (same metal type, same wire length, etc.).

The large-diameter wire will have less electrical resistance than the small-diameter wire.

Notes:
Many analogies exist to express this concept: water through a pipe, compressed air through a hose, etc. Which pipe or hose is less restrictive: the skinny one or the fat one?

Question 3:

What is the difference between materials classified as conductors versus those classified as insulators, in the electrical sense of these words?

Electrical "conductors" offer easy passage of electric current through them, while electrical ïnsulators" do not. The fundamental difference between an electrical "conductor" and an electrical ïnsulator" is how readily electrons may drift away from their respective atoms.
For an illustration of electron mobility within a metallic substance, research the terms electron gas and ßea of electrons" in a chemistry reference book.

Notes:
It is important to realize that electrical "conductors" and ïnsulators" are not the same as thermal "conductors" and ïnsulators." Materials that are insulators in the electrical sense may be fair conductors of heat (certain silicone gels used as heat-transfer fluids for heat sinks, for instance). Materials that are conductors in the electrical sense may be fair insulators in the thermal sense (conductive plastics, for example).

Question 4:

Identify several substances that are good conductors of electricity, and several substances that are good insulators of electricity.

It is very easy to research (and test!) whether or not various substances are either conductors or insulators of electricity. I leave this task in your very capable hands.

Notes:
If students have access to simple multimeters, they may perform conductivity tests on various substances with them. This is a fun and interesting classroom activity!

Question 5:

What naturally occurring substance is the best conductor of electricity?

Most people think the answer to this question is gold. However, there is another type of metal that conducts electricity better than any other type of metal, including gold! Document where you found your answer to this question.

Notes:
A few of your students may come across superconductors in their research, which of course conduct electricity perfectly (with no resistance to electric current whatsoever). However, the answer I'm looking for here is the non-superconducting metal with the lowest electrical resistance of all.

Question 6:

Electrical wire is often rated according to its cross-sectional diameter by a gauge scale. Which is the larger-diameter wire size, 14 gauge or 8 gauge?

8 gauge is the larger diameter.

Notes:
For students familiar with shotguns, the methodology of the wire gauge scale makes sense. For just about everyone else, the gauge scale seems "backward."

Question 7:

What disadvantages might there be to using undersized wire in an electrical power distribution system, where electricity is transmitted from generators (sources) to "loads" (devices requiring power) far away? What might happen if the wires we choose for this application were to have insufficient cross-sectional area?

An analogy: what would happen if the water pipes in a municipal water supply system were sized too small?

Notes:
The effects of undersized wiring are multiple: not only is there inefficiency in the system when conductors are too small, but there are safety hazards as well!

Question 8:

What disadvantages might there be to using oversized wire in an electrical power distribution system, where electricity is transmitted from generators (sources) to "loads" (devices requiring power) far away? What might be bad about using wires with more than sufficient cross-sectional area for the purpose?

An analogy: what would be bad about using oversized water pipes in a municipal water supply system? Nothing, perhaps, once the system was operational. However, what about the cost of building the system?

Notes:
The bad effects of oversized wiring are more difficult to discern than the bad effects of undersized wiring. With oversized wiring, there is little electrical inefficiency, but this doesn't mean the system is inefficient in other ways! The cost of the wiring itself, of course, is obvious, but there are other costs as well. Brainstorm ideas with your students about the cost-inefficiencies of oversized wiring in an electrical system.

Question 9:

What does the breakdown voltage (also known as dielectric strength) rating for an electrically insulating substance mean?

The breakdown voltage, or dielectric strength, rating of an insulator is the maximum amount of applied voltage it may withstand per unit thickness without conducting appreciable current.

Notes:
This question provides an opportunity for "qualitative" analysis: does the voltage rating of an insulator increase or decrease as its thickness is increased, given a constant dielectric strength?

Question 10:

How much voltage can a 50 mm thick piece of mica withstand before breaking down?

Approximately 1000 volts, or 1 kV.

Notes:
Be sure to ask your students where they obtained the dielectric strength rating for mica. Different sources of information may give different figures, which will lead to different answers to this question.

Question 11:

Design a circuit to test the breakdown voltage of a material.

I won't provide you with a ready-made schematic diagram for such a circuit, but I will give you a hint. If a substance were to be subjected to a voltage in excess of its dielectric strength, what effect(s) would indicate breakdown?

Notes:
Designs for testing dielectric strength may range from the crude to the sophisticated, but the real point of this question is to challenge students to reason from the effects of high-voltage breakdown to a means of detecting when that takes place, and measuring the voltage needed for it to happen.

Voltage, Current, and Resistance

2012-01-13T10:10:00.000-08:00

Voltage, Current, and Resistance

Question 1:

Voltage is commonly defined as ëlectrical pressure." The unit of the volt, however, may be defined in terms of more fundamental physical units. What are these units, and how do they relate to the unit of the volt?

1 volt is equal to 1 joule of energy imparted to 1 coulomb of charge (6.25 ×10¹⁸ electrons):

V =

Where,
V = Voltage (volts)
W = Work, or potential energy (joules)
Q = Charge (coulombs)

Notes:
Note that I use the letter "V" to denote voltage rather than Ë" as I usually do. This is because in general physics work, Ë" usually stands for either Ënergy" or Ëlectric field". Some electronics reference books use the letter Ë" for voltage, while others use the letter "V", or even use the two letters interchangeably.

Question 2:

Electric current is measured in the unit of the ampere, or amp. What is the physical definition for this unit? What fundamental quantities constitute 1 ampere of electric current?

1 ampere of electric current is the rate of electron motion equal to 1 coulomb per second:

I =

Where,
I = Electric current (amperes)
Q = Charge in motion (coulombs)
t = Time (seconds)

Notes:
It may be helpful at this point to review the number of electrons constituting one coulomb of charge: 6.25 ×10¹⁸ electrons.
Technically, current's mathematical definition involves calculus:

I =

However, students at this stage may not be ready to explore derivatives yet, and so the equation give in the answer for (average) current will suffice.

Question 3:

Explain what the electrical terms voltage, current, and resistance mean, using your own words.

Voltage: electrical "pressure" between two different points or locations.
Current: the flow of electrons.
Resistance: opposition, or "friction," to the flow of electrons.

Notes:
While it is easy enough for students to look up definitions for these words from any number of references, it is important that they be able to cast them into their own words. Remembering a definition is not the same as really understanding it, and if a student is unable to describe the meaning of a term using their own words then they definitely do not understand it! It is also helpful to encourage students to give real-life examples of these terms.

Question 4:

Describe what ëlectricity" is, in your own words.

If you're having difficulty formulating a definition for ëlectricity," a simple definition of ëlectric current" will suffice. What I'm looking for here is a description of how an electric current may exist within a solid material such as a metal wire.

Notes:
This question is not as easy to answer as it may first appear. Certainly, electric current is defined as the "flow" of electrons, but how do electrons "flow" through a solid material such as copper? How does anything flow through a solid material, for that matter?
Many scientific disciplines challenge our "common sense" ideas of reality, including the seemingly solid nature of certain substances. One of the liberating aspects of scientific investigation is that it frees us from the limitations of direct sense perception. Through structured experimentation and rigorous thinking, we are able to ßee" things that might otherwise be impossible to see. We certainly cannot see electrons with our eyes, but we can detect their presence with special equipment, measure their motion by inference from other effects, and prove empirically that they do in fact exist.
In this regard, scientific method is a tool for the expansion of human ability. Your students will begin to experience the thrill of "working with the invisible" as they explore electricity and electric circuits. It is your task as an instructor to foster and encourage this sense of wonder in your students' work.

Question 5:

What units of measurement are used to express quantities of voltage, current, and resistance?

•: Voltage is measured in the unit of the volt (V).
•: Current is measured in the unit of the ampere, or amp (A).
•: Resistance is measured in the unit of the ohm (Ω).

Notes:
Like all other physical quantities, there are units of measurement appropriate for specifying electrical quantities. In this sense, the "volt," ämp," or öhm" are no different from "gram," ÿard," or "bushel."

Question 6:

How many physical points must be referenced when speaking of the following electrical quantities?

•: Voltage
•: Current
•: Resistance

In other words, does it make sense to speak of voltage at a single point, or between two points, or between three points, etc.? Does it make sense to speak of current at a single point, between two points, between three points, etc.?

•: Voltage is always measured between two points.
•: Current may be measured at a single point (at a cross-section of a conductive path).
•: Resistance is always measured between two points.

Follow-up question: explain, if you can, the relevance of these facts to electrical safety. For example, why is it important to know that voltage is always a quantity existing between two points (rather than existing at a single point) when considering your personal safety?

Notes:
One of the more confusing aspects of electrical theory is that both voltage and resistance are quantities relative between two points. It is meaningless to speak of the amount of voltage ät this point" in a circuit, at least without implying a reference point such as ground. The same goes for resistance: it makes no sense to speak of how much electrical resistance there is ät this point" in a circuit either.
Conversely, it makes perfect sense to speak of either voltage or resistance lying "between this point and that point," because both these quantities are relative between exactly two points.
Current, on the other hand, is the coordinated motion of electric charge through a conductor. As such, it may be measured at any single point in a circuit, like measuring the flow of water in a river at any one particular cross-sectional sample.

Question 7:

Lightning is a natural, electrical phenomenon. It is caused by the accumulation of a large electrical charge over time resulting from air, dust, and water droplets transporting small electrical charges.
Explain how the terms voltage, current, and resistance relate to the process of lightning. In other words, use these three terms to explain the cycle of charge accumulation and lightning discharge.

As electric charges accumulate between clouds and earth, the voltage between these points increase. Air under normal conditions is a good insulator of electricity: that is, it possesses very high electrical resistance. So, at first there is no current resulting from the rise in voltage between clouds and earth. However, when the voltage exceeds the air's ïonization" potential, the air becomes a good conductor of electricity (its electrical resistance decreases dramatically), resulting in a transient current as the accumulated electric charge dissipates in the form of a lightning bolt.

Notes:
I usually avoid spending a lot of time on technical definitions, because undue emphasis on the definitions of words tends to reinforce rote memorization rather than true comprehension. If students must master certain definitions, though, it is best to develop that mastery in the context of application: ask the students to use their new vocabulary, not just recite it.

Question 8:

Is it possible to have a condition where an electrical voltage exists, but no electric current exists? Conversely, is it possible to have a condition where an electric current exists without an accompanying voltage? Explain your answers, and give practical examples where the stated conditions are indeed possible.

It is not only possible, but quite common in fact, to have a condition of voltage with no current. However, the existence of an electric current must normally be accompanied by a voltage. Only in very unique conditions (in ßuperconducting" circuits) may an electric current exist in the absence of a voltage.

Notes:
This question challenges students' comprehension of voltage and current by asking them to explain the relationship between the two quantities in practical contexts. Do not allow students to simply give a ÿes" or a "no" answer to either of the stated conditions. Encourage them to think of examples illustrating a possible condition.
The term ßuperconducting" may spur some additional questions. As usual, do not simply tell students what superconductivity is, but let them research this on their own. Your more inquisitive students will probably have already researched this topic in response to the answer!

Question 10:

Suppose a battery outputs a voltage of 9 volts. Using algebra, calculate how many joules of energy are imparted to every individual electron moving through this battery.

For a 9 volt battery, there will be 1.44 ×10⁻¹⁸ joules of energy imparted to each electron moving through it.

Notes:
This problem is another exercise in using scientific notation, and requires that students put concepts together that were learned previously.

Question 11:

What is the difference between DC and AC electricity?

DC is an acronym meaning Direct Current: that is, electrical current that moves in one direction only. AC is an acronym meaningAlternating Current: that is, electrical current that periodically reverses direction (älternates").
It is important to realize that "DC" and ÄC" may also be used to represent voltage as well as current. That is, it is possible to have a voltage that alternates back and forth in polarity (ÄC" voltage) just as it is possible to have a current that alternates back and forth in direction of motion.

Notes:
Though it may seem oxymoronic to refer to either "DC" or ÄC" voltage (Direct Current voltage, or Alternating Current voltage - "current voltage"?), the usage of these acronyms as such is commonplace.

Static electricity

2012-01-13T10:07:00.000-08:00

Static electricity

Question 1:

What does it mean for an object to have an electric charge? Give an example of an object receiving an electric charge, and describe how that charged object might behave.

For an object to be electrically charged, it must have either a surplus or a deficit of electrons among its atoms.
A common example of electrically charging objects is rubbing latex balloons against wool clothing, or brushing your hair with a plastic comb. The consequences of these electric charges are very easy to perceive!

Notes:
This question naturally leads to a discussion on atomic theory. Encourage your students to discuss and explore simple models of the atom, and how they serve to explain electricity in terms of electron placement and motion.

Question 2:

How many electrons are contained in one coulomb of charge?

There are 6.25 ×10¹⁸ electrons in one coulomb of charge. What would this appear as without the use of scientific notation? Write this same figure using the most appropriate metric prefix.

Notes:
A little math review here: using scientific notation to denote very large (or very small) numbers.

Hide Answer

Question 3:

What is happening when two objects are rubbed together and static electricity results?

When certain combinations of materials are rubbed together, the rubbing action transfer electrons from the atoms of one material to the atoms of the other. This imbalance of electrons leaves the former material with a positive charge and the latter with a negative charge.

Notes:
The terms "positive" and "negative" seem backward in relation to the modern concept of electrons as charge carriers. Be sure to discuss the historical aspect of this terminology (Benjamin Franklin's conjecture), and the subsequent designation of an electron's individual charge as "negative."

Question 4:

How many electrons are there in 2 ×10⁻⁵ coulombs of charge? Determine the answer without using a calculator, and express the answer in scientific notation!

There are approximately 1.25 ×10¹⁴ electrons in this quantity of charge.

Notes:
One of the benefits of using scientific notation is that it allows us to easily perform multiplication and division using very large and very small numbers.

Question 5:

If a positive charge is forcibly moved closer to another positive charge, mechanical work must be performed. This is analogous to compressing a spring: work must be done to cause motion against the natural repulsion of the system:

This mechanical work becomes ßtored" in the electric field between the charges, and is a form of potential energy. This, again, is similar to a mechanical spring, where the work done in compressing a spring is ßtored" as potential energy in its compressed state.
Define voltage in terms of the change in potential energy resulting from this forceful motion of the charge, the way a physicist would.

V_ab =

U_b − U_a

Where,
V_ab = Voltage between points A and B
U_b − U_a = Difference in potential energy between points A and B
q = Quantity of charge (moved), in Coulombs
Follow-up question: explain why, if a charge of twice the magnitude (2q) were moved from the same point A to the same point B, there would be twice as much work done and twice as much potential energy stored, but exactly the same voltage (potential) as gained by the smaller charge (q).

Notes:
If time permits, it is quite enlightening for students to discover that voltage may be defined in simple mechanical terms of repulsion and work. Of course, a basic familiarity with work in the scientific sense of the term is a prerequisite for understanding this scenario.

Question 6:

A co-worker of mine was once installing sheet-styrofoam insulation in the walls of his new garage. The sheets of insulation came wrapped together in bundles for easy transport and storage.
One side of each sheet was covered in aluminum foil. This acted as a reflective surface for infra-red (heat) radiation, and improved the effectiveness of the insulating panel. It also happened to function as a conductor of electricity, much to the dismay of my friend.
My friend found that the sheets were physically stuck together by the force of static electric charges when they were delivered to his house. Upon trying to pry two of them apart from each other, he received a surprisingly large electric shock from touching the aluminum foil surfaces of the respective panels! Explain how the physical work he did in separating the panels from each other became manifest as voltage, based on your knowledge of voltage, work, and electric charges.

Voltage is defined as work per unit charge, which tells us that this fellow's physical work of separating the attractive charges became translated into electrical voltage, which shocked him when the magnitude became great enough.

Notes:
By the way, this is an absolutely true story! This scenario could be easily replicated on a dry (low-humidity) day, with two large sheets of styrofoam insulation and a brave person pulling them apart!

Question 7:

Lightning is a spectacular example of naturally-generated static electricity. Explain how the vast static electric charges that cause lightning are formed by natural processes, and how those processes relate to the function of a device called a Van de Graaff generator.

The static electric charges that cause lightning are generated by the action of wind and rain transporting electrons between clouds and earth, or between clouds. Particles of dust may also transport electrons in the same manner, which accounts for the spectacular discharges of lightning often seen during massive uprisings of dust, such as during volcanic eruptions.
In a similar manner, a Van de Graaff generator moves electrons from one point to another on a moving loop of fabric: a sort of conveyor belt for electrons.

Notes:
Lighting provides a universal example of static electricity on a grand scale. The sheer awe of lightning is itself an attention-grabbing factor making it all the more appropriate for classroom discussion. When students' collective attention and imagination are enthralled by fascinating subjects, learning is enhanced!
It goes without saying that the presence of a Van de Graaff generator in the laboratory is an excellent enhancement for this topic, as well.

Question 8:

A special kind of electric light known as a neon bulb is an excellent tool for demonstrating the presence of static electricity, and is easily obtained at most electronics supply shops:

When a large enough static electric charge is applied between the two wires of the neon bulb, the neon gas inside will ïonize" and produce a colored flash of light. Experiment with generating your own static electricity and then making the bulb flash. Hint: it may be easier to see the bulb's flash if the room is dark.
What conditions produce the brightest flash from the bulb? What materials and techniques work best for producing strong static electric charges?

This is definitely a question best answered by direct experimentation!

Notes:
If it is difficult for students to obtain neon bulbs in your area, it might be a good idea to provide a few bulbs to experiment with when you give your students this worksheet. They are quite inexpensive and durable little devices.

Question 9:

If a neon bulb is used as a visual indicator of static electricity, it will be noticed that only one of the two metal electrodes inside the glass envelope glows upon discharge. Why is this? Why don't both electrodes glow?

Remember that static electricity is an imbalance of electric charge between two objects. This imbalance has a definite polarity: one object is positive while the other is negative. This means that electrons rush in one direction when the two objects discharge through the path created by the neon gas inside the lamp. This unidirectional rush of electrons makes only one of the electrodes glow.

Notes:
Of course, students will want to know: which type of charge makes the electrode glow, positive or negative? Don't just give them the answer, but challenge them to devise an experiment whereby they could tell for certain which pole of the charge creates a glow. The answer to this question may have to wait until they learn about DC voltage sources like batteries, where the polarity of the voltage is known. However, even if they are yet unaware of DC voltage sources, it is a good exercise to have them imagine how they might test the polarity indication of a neon bulb (perhaps by postulating a source of static charge where the polarity is already known).

Atomic structure

2012-01-13T09:58:00.000-08:00

Atomic structure

Question 1:

Shown here is a simplified representation of an atom: the smallest division of matter that may be isolated through physical or chemical methods.

Inside of each atom are several smaller bits of matter called particles. Identify the three different types of ëlementary" particles inside an atom, their electrical properties, and their respective locations within the atom.

Neutrons reside in the center ("nucleus") of the atom, as do protons. Neutrons are electrically neutral (no charge), while protons have a positive electrical charge. Electrons, which reside outside the nucleus, have negative electrical charges.

Notes:
Most, if not all, students will be familiar with the ßolar system" model of an atom, from primary and secondary science education. In reality, though, this model of atomic structure is not that accurate. As far as anyone knows, the actual physical layout of an atom is much, much weirder than this!
A question that might come up in discussion is the definition of "charge." I'm not sure if it is possible to fundamentally define what "charge" is. Of course, we may discuss "positive" and "negative" charges in operational terms: that like charges repel and opposite charges attract. However, this does not really tell us what charge actually is. This philosophical quandary is common in science: to be able to describe what something is in terms of its behavior but not its identity or nature.

Question 2:

Different types of atoms are distinguished by different numbers of elementary particles within them. Determine the numbers of elementary particles within each of these types of atoms:

•: Carbon
•: Hydrogen
•: Helium
•: Aluminum

Hint: look up each of these elements on a periodic table.

Each atom of carbon is guaranteed to contain 6 protons. Unless the atom is electrically charged, it will contain 6 electrons as well to balance the charge of the protons. Most carbon atoms contain 6 neutrons, but some may contain more or less than 6.
Each atom of hydrogen is guaranteed to contain 1 proton. Unless the atom is electrically charged, it will contain 1 electron as well to balance the charge of the one proton. Most hydrogen atoms contain no neutrons, but some contain either one or two neutrons.
Each atom of helium is guaranteed to contain 2 protons. Unless the atom is electrically charged, it will contain 2 electrons as well to balance the charge of the protons. Most helium atoms contain 2 neutrons, but some may contain more or less than 2.
Each atom of aluminum is guaranteed to contain 13 protons. Unless the atom is electrically charged, it will contain 13 electrons as well to balance the charge of the protons. Most aluminum atoms contain 14 neutrons, but some may contain more or less than 14.
While you're researching the numbers of particles inside each of these atom types, you may come across these terms: atomic numberand atomic mass (sometimes called atomic weight). Be prepared to discuss what these two terms mean.

Notes:
Be sure to ask your students what definitions they found for ätomic number" and ätomic mass".
It is highly recommended that students seek out periodic tables to help them with their research on this question. The ordering of elements on a periodic table may provoke a few additional questions such as, "Why are the different elements arranged like this?" This may build to a very interesting discussion on basic chemistry, so be prepared to engage in such an interaction on these subjects if necessary.

Question 3:

Of the three types of ëlementary particles" constituting atoms, determine which type(s) influence the following properties of an element:

•: The chemical identity of the atoms (whether it is an atom of nitrogen, iron, silver, or some other element).
•: The mass of the atom.
•: The electrical charge of the atom.
•: Whether or not it is radioactive (spontaneous disintegration of the nucleus).

•: The chemical identity of the atoms: protons.
•: The mass of the atom: neutrons and protons, and to a much lesser extent, electrons.
•: The electrical charge of the atom: electrons and protons (whether or not the numbers are equal).
•: Whether or not it is radioactive: neutrons, although one might also say protons in some cases, as there are no known ßtable" (non-radioactive) isotopes of certain elements, the identity of an element being determined strictly by the number of protons.

Notes:
It never ceases to fascinate me how many of the basic properties of elements is determined by a simple integer count of particles within each atom's nucleus.
In the answer, I introduce the word isotope. Let students research what this term means. Don't simply tell them!

Question 4:

The Greek word for amber (fossilized resin) is elektron. Explain how this came to be the word describing a certain type of subatomic particle (electron).

When a piece of amber is rubbed with a cloth, a static electric charge develops on both objects. Early experimenters postulated the existence of an invisible fluid that was transferred between the amber and the cloth. Later, it was discovered that tiny sub-atomic particles constituted this "fluid," and the name electron was given to them.

Notes:
This question provides a good opportunity to discuss the history of electricity, and how its understanding and mastery has dramatically changed peoples' lives. Be sure to ask questions about Benjamin Franklin and the modeling of electricity as a fluid. Scientific discovery is often assisted by models, but may also be hindered by them as well. Franklin's model of electricity as a fluid has done both (conventional versus electron flow notation)!

Question 5:

It is much easier to electrically "charge" an atom than it is to alter its chemical identity (say, from lead into gold). What does this fact indicate about the relative mobility of the elementary particles within an atom?

Electrons are much easier to remove from or add to an atom than protons are. The reason for this is also the solution to the paradox of why protons bind together tightly in the nucleus of an atom despite their identical electrical charges.

Notes:
Discuss with your students the importance of this fact: that electrons may be added to or taken from an atom rather easily, but that protons (and neutrons for that matter) are very tightly "bound" within an atom. What might atoms behave like if their protons were not so tightly bound as they are?
We know what happens to the electrons of some atoms when substances are rubbed together. What might happen to those substances if protons were not as tightly bound together as they are?

Question 6:

Early scientific researchers hypothesized that electricity was an invisible fluid that could move through certain substances. Those substances "porous" to this "fluid" were called conductors, while substances impervious to this "fluid" were called insulators.
We now know what electricity is composed of: tiny bits of matter, smaller than atoms. What name do we give these tiny bits of matter? How do these particles of matter relate to whole atoms?
In terms of these tiny particles, what is the difference between the atoms of conductive substances versus the atoms of insulating substances?

The tiny bits of matter that move through electrically conductive substances, comprising electricity, are called electrons. Electrons are the outermost components of atoms:

Although electrons are present in all atoms, and therefore in all normal substances, the outer electrons in conductive substances are freer to leave the parent atoms than the electrons of insulating substances. Such "free" electrons wander throughout the bulk of the substance randomly. If directed by a force to drift in a consistent direction, this motion of free electrons becomes what we callelectricity.

Notes:
It is worthy to note to your students that metallic substances - the best naturally-occurring conductors - are characterized by extremely mobile electrons. In fact, solid-state physicists often refer to the free electrons in metals as a "gas" or a ßea," ironically paying homage to the "fluid" hypothesis of those early experimenters.
The specific details of why some atoms have freer electrons than others are extremely complex. Suffice it to say, a knowledge of quantum physics is necessary to really grasp this basic phenomenon we call ëlectricity." The subject becomes even more complex when we turn to superconductivity and semiconducting substances.

Question 7:

Some of the particles comprising atoms react to each other in a way that scientists categorize as electric charge. There are two fundamental types of electric charge: positive and negative. Identify the respective charges of the following particles:

•: Electrons
•: Protons
•: Neutrons

What would happen if you placed two electrons near each other in free space? Would they repel each other or attract each other? How about two protons? How about an electron and a proton? How about a neutron and a proton?

I'll let you research which charge type (positive or negative) is characteristic of electrons, protons, and neutrons!
As for their respective physical reactions, particles of differing charge are physically attracted to each other while particles of identical charge repel each other.

Notes:
Many students will want to know "why?" in response to electrical charges. The technical answer has to do with electric fields extending through space, but this may be a philosophically impossible question to answer. The concept of charge was invented to explain the physical behavior of electrical attraction and repulsion, but coining a term to explain a phenomenon does nothing to explainwhy that phenomenon occurs.
Still, this is a worthwhile subject for discussion, especially if students have done their research well and know something about the history of electricity.

Question 8:

Atoms are very, very small pieces of matter. This fact should go without saying, but it behooves us to be reminded of just how small atoms are. Chemists and physicists use a unit of measurement to represent quantities of different materials based on how many atoms (or molecules) there are in a particular sample. This unit of measurement is called the mole.
1 mole of pure iron metal weighs about 56 grams. Based on the definition of a mole, how many atoms of iron are in this 56 gram sample?

There are approximately 6.022 ×10²³ atoms in this 56 gram sample of iron. What does this quantity look like when written in non-scientific notation?

Notes:
The purpose of this question is twofold: to reinforce the fact that atoms are really, really tiny, and to introduce students to the use of scientific notation.

Question 9:

How many atoms of iron are there in a sample measuring 2 ×10³ moles? Determine the answer without using a calculator!

There are 1.2044 ×10²⁷ atoms of iron in this sample.

Notes:
One of the benefits of using scientific notation is that it allows us to easily perform multiplication and division using very large and very small numbers.

Question 10:

Many students first learning about atomic structure and electricity notice a paradox with respect to the location of all the protons in an atom: despite their electrical charge, they are tightly bound together in a "core" called the nucleus. Based on what you know about electrical charges, explain why this is a paradox, and also what the solution to the paradox is.

Paradox: since similar charges tend to physically repel each other, why should protons (all positively charged) cling so tightly together in the nucleus of an atom? Why don't they fly apart from each other due to electrical repulsion?
I'll let you research the answer to this paradox yourself!

Notes:
Believe it or not, I once read a religious tract that used this paradox as proof of God's existence. The argument went like this: everyone knows that like charges repel, so it must be God who holds the protons together! Volumes could be written on the psychology behind this argument, but I digress . . .
The solution to this paradox is the subject of freshman-level college physics. For those of you who don't know the answer, I'll give you a hint: it has something to do with the power of nuclear fission.

Question 11:

Write two algebraic equations relating the number of protons and neutrons in an atom to that atom's atomic mass and atomic number:
Where,
P = Number of protons
N = Number of neutrons
A = Atomic mass
Z = Atomic number

A = P + N

Z = P

Where,
P = Number of protons
N = Number of neutrons
A = Atomic mass
Z = Atomic number

Notes:
Admittedly, this is not a challenging problem in algebra, but it is important to introduce practical algebra to students in a gentle way at first. You may be surprised how many students have a theoretical knowledge of mathematics only, and find it difficult to apply even simple algebraic principles to realistic situations.

how to make High impedance voltmeter

2012-01-13T09:53:00.001-08:00

PARTS AND MATERIALS

Operational amplifier, model TL082 recommended (Radio Shack catalog # 276-1715)
Operational amplifier, model LM1458 recommended (Radio Shack catalog # 276-038)
Four 6 volt batteries
One meter movement, 1 mA full-scale deflection (Radio Shack catalog #22-410)
15 kΩ precision resistor
Four 1 MΩ resistors

The 1 mA meter movement sold by Radio Shack is advertised as a 0-15 VDC meter, but is actually a 1 mA movement sold with a 15 kΩ +/- 1% tolerance multiplier resistor. If you get this Radio Shack meter movement, you can use the included 15 kΩ resistor for the resistor specified in the parts list.

This meter experiment is based on a JFET-input op-amp such as the TL082. The other op-amp (model 1458) is used in this experiment to demonstrate the absence of latch-up: a problem inherent to the TL082.

You don't need 1 MΩ resistors, exactly. Any very high resistance resistors will suffice.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Amplifiers"

LEARNING OBJECTIVES

Voltmeter loading: its causes and its solution
How to make a high-impedance voltmeter using an op-amp
What op-amp "latch-up" is and how to avoid it

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

An ideal voltmeter has infinite input impedance, meaning that it draws zero current from the circuit under test. This way, there will be no "impact" on the circuit as the voltage is being measured. The more current a voltmeter draws from the circuit under test, the more the measured voltage will "sag" under the loading effect of the meter, like a tire-pressure gauge releasing air out of the tire being measured: the more air released from the tire, the more the tire's pressure will be impacted in the act of measurement. This loading is more pronounced on circuits of high resistance, like the voltage divider made of 1 MΩ resistors, shown in the schematic diagram.

If you were to build a simple 0-15 volt range voltmeter by connecting the 1 mA meter movement in series with the 15 kΩ precision resistor, and try to use this voltmeter to measure the voltages at TP1, TP2, or TP3 (with respect to ground), you'd encounter severe measurement errors induced by meter "impact:"

Try using the meter movement and 15 kΩ resistor as shown to measure these three voltages. Does the meter read falsely high or falsely low? Why do you think this is?

If we were to increase the meter's input impedance, we would diminish its current draw or "load" on the circuit under test and consequently improve its measurement accuracy. An op-amp with high-impedance inputs (using a JFET transistor input stage rather than a BJT input stage) works well for this application.

Note that the meter movement is part of the op-amp's feedback loop from output to inverting input. This circuit drives the meter movement with a current proportional to the voltage impressed at the noninverting (+) input, the requisite current supplied directly from the batteries through the op-amp's power supply pins, not from the circuit under test through the test probe. The meter's range is set by the resistor connecting the inverting (-) input to ground.

Build the op-amp meter circuit as shown and re-take voltage measurements at TP1, TP2, and TP3. You should enjoy far better success this time, with the meter movement accurately measuring these voltages (approximately 3, 6, and 9 volts, respectively).

You may witness the extreme sensitivity of this voltmeter by touching the test probe with one hand and the most positive battery terminal with the other. Notice how you can drive the needle upward on the scale simply by measuring battery voltage through your body resistance: an impossible feat with the original, unamplified voltmeter circuit. If you touch the test probe to ground, the meter should read exactly 0 volts.

After you've proven this circuit to work, modify it by changing the power supply from dual to split. This entails removing the center-tap ground connection between the 2nd and 3rd batteries, and grounding the far negative battery terminal instead:

This alteration in the power supply increases the voltages at TP1, TP2, and TP3 to 6, 12, and 18 volts, respectively. With a 15 kΩ range resistor and a 1 mA meter movement, measuring 18 volts will gently "peg" the meter, but you should be able to measure the 6 and 12 volt test points just fine.

Try touching the meter's test probe to ground. This should drive the meter needle to exactly 0 volts as before, but it will not! What is happening here is an op-amp phenomenon called latch-up: where the op-amp output drives to a positive voltage when the input common-mode voltage exceeds the allowable limit. In this case, as with many JFET-input op-amps, neither input should be allowed to come close to either power supply rail voltage. With a single supply, the op-amp's negative power rail is at ground potential (0 volts), so grounding the test probe brings the noninverting (+) input exactly to that rail voltage. This is bad for a JFET op-amp, and drives the output strongly positive, even though it doesn't seem like it should, based on how op-amps are supposed to function.

When the op-amp ran on a "dual" supply (+12/-12 volts, rather than a "single" +24 volt supply), the negative power supply rail was 12 volts away from ground (0 volts), so grounding the test probe didn't violate the op-amp's common-mode voltage limit. However, with the "single" +24 volt supply, we have a problem. Note that some op-amps do not "latch-up" the way the model TL082 does. You may replace the TL082 with an LM1458 op-amp, which is pin-for-pin compatible (no breadboard wiring changes needed). The model 1458 will not "latch-up" when the test probe is grounded, although you may still get incorrect meter readings with the measured voltage exactly equal to the negative power supply rail. As a general rule, you should always be sure the op-amp's power supply rail voltages exceed the expected input voltages.