science and technology

Magnetic effects of electric current

2011-08-21T00:47:00.001-07:00

MAGNETIC EFFECTS OF ELECTRIC CURRENT

Compass needle

A compass needle is a small magnet. Its one end, which points towards north, is called a north pole, and the other end, which points towards south, is called a south pole.

Relation between Electricity and magnetism (Oersted’s Experiment)

The first evidence of any connection between Electricity and magnetism was established by Hans Christian Oersted. He accidentally discovered that as he laid a wire carrying an electric current near a magnetic compass needle, it got deflected as if acted upon by a magnet. This observation led to the discovery that when current passes through a conductor, magnetic field is produced around it.

Magnetic field

The space around a magnet or a current carrying conductor, in which the force of attraction or repulsion can be experienced, is called a magnetic field.

Demonstration of magnetic field lines (Iron-filings pattern)

1. Take a bar magnet and placed it on a cardboard.

2. Sprinkle some iron-fillings around the magnet.

3. Tap the cardboard gently.

4. Iron fillings arrange themselves in a pattern as shown in figure.

Conclusion: This pattern demonstrates that under the influence of magnetic field, the fillings align themselves along the magnetic field lines.

Tracing of magnetic field lines of a bar magnet

1. Place a bar magnet on a sheet of paper.

2. Bring the compass near the north pole of the magnet.

3. The needle will deflect such that its south pole points towards North Pole of the bar magnet.

4. Mark the position of two ends of needle.

5. Move the compass so that its south end occupies the position previously occupied by the north end.

6. Again mark the new position of ends of needle.

7. Repeat step 5 and 6 till you reach the south pole of the magnet.

8. Join the points marked to get a smooth curve, which represents a field line.

Magnetic field lines

Magnetic field lines are the imaginary lines used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction of themagnetic field at a point is given by the direction that a north pole placed at thatpoint would take.

Properties of Magnetic field lines

1. Outside the bar magnet, the magnetic field lines originate from the north pole of a magnet and end at its south pole.

2. Inside the bar magnet, field lines move from South Pole of the magnet to the North Pole.

3. Magnetic field lines always form closed curves.

4. The regions, where field lines are closer, the field is strong and the regions, where the field lines are farther apart, the field is weak.

5. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.

6. The magnetic field lines never cut each other. In case, two field lines intersect each other at a point, then it will mean that at the point of intersection, the magnetic needle would point in two different directions, which is not possible.

Magnetic field around a current carrying straight conductor

1. Take a straight wire AB and pierced it through a horizontal cardboard such that wire AB is vertical.

2. The ends of the wire AB are connected to a battery.

3. Place some iron fillings on the cardboard.

4. Switched the key on.

5. Gently tap the cardboard.

6. The iron fillings arrange themselves in concentric circles around the wire.

7. This shows that magnetic field lines are concentric circles. The circles become larger and larger as we move away from the wire.

Direction of field lines- Right Hand thumb rule

Imagine that you are holding a current carrying wire in your right hand such that the thumb is stretched along the direction of the current, then, the fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

Factors on which the magnetic field at a distance from the straight wire depends on

1. Directly proportional to the current flowing in the wire.

2. Inversely proportional to the distancefrom the wire.

MAGNETIC FIELD DUE TO A CURRENT CARRYING CIRCULAR WIRE

Using Right Hand thumb rule, the magnetic field lines at every point of the circular wire are in the form of concentric circles with wire as the center. These magnetic field lines become larger and larger as we move away from the wire. Just at the center of the coil, magnetic field lines are almost straight. By applying Right hand rule, it is easy to check that every section of the wire contributes to magnetic field lines in the same direction within the loop.

Factors on which the magnetic field at the centre of the circular loop depends upon

1. Directly proportional to the current flowing in the loop

2. Inversely proportional to the radius of the circular wire.

3. Directly proportional to the number of turns of the circular loop. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

SOLENOID

A coil of many circular turns of insulated copper wire wrapped closely in the form of a cylinder is called a solenoid.

MAGNETIC FIELD DUE TO CURRENT IN A SOLENOID

The magnetic field due to a solenoid is very much similar to that of a bar magnet. The patternof the magnetic field lines around a current-carrying solenoid is similar to that of bar magnet.Just like a bar magnet, one end of the solenoidbehaves as a magnetic north pole, while the other behaves as theSouth Pole. The field lines inside the solenoid are in the form ofparallel straight lines. This indicates that the magnetic field isthe same at all points inside the solenoid. That is, the field isuniform inside the solenoid.

PRACTICAL USE OF SOLENOID

A strong magnetic field produced in a solenoid can be used to magnetize a piece of magnetic material when it is placed within the coil, which is carrying electric current.

ACTIVITY

1. Take a bunch of iron nails.

2. Wrap a coil of insulated copper wire on it.

3. Connect the coil to a battery through a switch.

4. As the current is passed through the coil, the bunch of nails, which acts as a core inside the solenoid, gets magnetized.

The magnet so formed is called an electromagnet.

ELECTROMAGNET

An electromagnet consists of a long coil of insulated copper wire wound on a soft iron core.

TEMPORARY MAGNET

If the core of the solenoid is taken of soft iron and electric current is passed through the solenoid, the soft iron core is temporarily magnetized which means when the current is switched off soft iron loses its magnetic properties. An electromagnet is a temporary magnet.

PERMANENT MAGNET

If the core of the solenoid is taken of carbon steel, chromium steel, cobalt and tungsten steel and certain alloys like Nipermag (alloy of iron, nickel, aluminium and titanium) and ALNICO (alloy of Aluminium, nickel and cobalt) and a strong electric current is passed through the coil then these materials become permanently magnetized.

Uses: Such permanent magnets are used in microphones, loudspeakers, electric clocks, ammeter, voltmeter and speedometer, etc

FORCE ON A CURRENT CARRYING CONDUCTOR PLACED IN A MAGNETIC FIELD

Any current carrying conductor when kept in magnetic field experiences a force. The direction of force is given by FLEMING’S LEFT HAND RULE.

ACTIVITY

1. Take a small aluminium rod AB.

2. Suspend it horizontally with the help of connecting wires from a stand.

3. Place a strong horseshoe magnet in such a way that the rod is between the two poles with the field directed upwards.

4. When current is passed in the rod from B to A, the rod gets displaced towards left.

5. On reversing the direction of the current, the rod gets deflected towards right.

The deflection in the rod is caused by the force acting on the current carrying rod when placed in a magnetic field.

The displacement of the rod is largest (or the magnitudeof the force is the highest) when the direction of current is at right anglesto the direction of the magnetic field. In such a condition we can use asimple rule to find the direction of the force on the conductor.

FLEMING’S LEFT HAND RULE

Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger that of the current, then the thumb will point towards the force or direction of motion of the conductor.

FORCE ON A MOVING CHARGE PARTICLE IN A MAGNETIC FIELD

A current carrying conductor experiences a force when placed in a magnetic field. As current is simply flowing of charges, it implies that moving charged particles also experiences a force in a magnetic field.

The direction of the force on a moving positive charge is given by Fleming’s Left hand rule.

Application of force experienced when placed in a magnetic field

Devices that use current-carrying conductors and magnetic fieldsinclude electric motor, loudspeakers, microphones and measuring instruments.

ELECTRICITY

2007-09-28T11:02:00.000-07:00

ELECTRICITY

ELECTRIC CURRENT

If the electric charge flows through a conductor (for example, through a metallic wire), we say that there is an electric current in the conductor.

Definition:

Electric current is defined as the rate of flow of charge flowing through a cross-section of a wire/conductor.

Formula:

If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is

SI Unit

The SI unit of electric charge is coulomb (C) and time is second (s). Thus the SI unit of electric current is coulomb/second (C/s). This unit is given a special name called ampere (A), named after the French scientist, Andre-Marie Ampere (1775–1836).

Definition of 1ampere

One ampere is that amount of current when an electric charge of one coulomb flows through a cross section of wire in one second. That is,

Small units of electric current:

Small quantities of current are expressed in

(i) milliampere

(ii) microampere

Device used to measure current

An instrument called ammeter measures electric current in a circuit.

Symbol:

Connection in circuit: It is always connected in series in a circuit through which the current is to be measured.

The Red terminal (positive terminal) is connected to the positive terminal of the battery and black terminal (negative terminal) is connected to the negative of the battery.

Direction of electric current

Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.

In an electric circuit electric current flows from the positive terminal of the cell to the negative terminal of the cell through the bulb and ammeter.

Electric circuit

A continuous and closed path of an electric current is called an electric circuit.

Electric switch

A switch makes a conducting link between the cell and the bulb. If the switch of the circuit is turned off, the current stops flowing and the bulb does not glow.

Charge of an electron:

Number of electrons in 1C of charge:

CHECK YOUR PROGRESS!!!

1. What does an electric circuit mean?

2. Define the unit of current.

3. Calculate the number of electrons constituting one coulomb of charge.

4. A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of charge that flows through the circuit.

5. Name the instrument used to measure electric current in a circuit. How is this instrument connected in a circuit? Draw a simple circuit diagram to explain your answer.

6. Which particles constitute electric current in a metallic conductor?

7. Name two units for expressing the small values of current. Also write their symbols. How are these units related to ampere?

8. Write the use of following components in an electric circuit

(a) Cell/battery

(b) Ammeter

(d) Switch/plug key

ELECTRIC POTENTIAL

The electrons move only if there is a difference of electric pressure – called the potential difference – along the conductor. This difference of potential may be produced by a battery, consisting of one or more electric cells. The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it. When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current. In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it.

Definition:

We define the electric potential difference between two points in an electric circuit as the work done to move a unit charge from one point to the other.

Formula:

SI unit:

The SI unit of electric potential difference is volt (V), named after Alessandro Volta (1745–1827), an Italian physicist.

Definition of one volt

One volt is the potential difference between two points when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.

Therefore,

Device used to measure potential difference:

The potential difference is measured by means of an instrument called the voltmeter.

Symbol:

Connection:

The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

CHECK YOUR PROGRESS!!!

1. Name a device that helps to maintain potential difference across a conductor.

2. What is meant by saying that potential difference between two points is 1V?

3. Write the relation which states the relation between potential difference and work done.

4. How much energy is given to each coulomb of charge passing through a 6V battery?

5. How much work is done in moving a charge of 2 C across two points having a potential difference of 12 V?

6. Name the device that measures the potential difference across two points in an electric circuit. How it is connected in an electric circuit?

OHMS’ LAW

The potential difference, V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same.

Formula:

Where, R is a constant for the given metallic wire at a given temperature and is called its resistance.

Graph:

The V–I graph is a straight line that passes through the origin of the graph, as shown in Figure. Thus,

is a constant ratio.

Resistance

It is the property of a conductor to resist the flow of charges through it.

SI unit

Its SI unit is ohm, represented by the Greek letter Ω.

Definition of 1Ω

If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. That is,

Relation between current and resistance

According to Ohm’s Law,

the current through a resistor is inversely proportional to its resistance. If the resistance is doubled the current gets halved.

Application of varying resistance:

In many practical cases it is necessary to increase or decrease the current in an electric circuit like in case of an electric fan regulator and in electric iron for changing the amount of heat produced. The component used to regulate current without changing the voltage source is called variable resistance. In an electric circuit, a device called rheostat is often used to change the resistance in the circuit.

A Rheostat

Symbol of variable Resistance

Symbol of Rheostat

Resistance of different material is different

The motion of electrons in an electric circuit constitutes an electric current. The electrons, however, are not completely free to move within a conductor. They are restrained by the attraction of the atoms among which they move. Thus, motion of electrons through a conductor is retarded by its resistance.

A component of a given size that offers a low resistance is a good conductor.

A conductor having some appreciable resistance is called a resistor.

A component of identical size that offers a higher resistance is a poor conductor.

An insulator of the same size offers even higher resistance.

Factors on which the resistance of a conductor depends

Resistance of the conductor is

(i) Directly proportional to its length,

(ii) Inversely proportional to its area of cross-section, and

(iii) On the nature of its material.

That is,

Combining (1) and (2)

Where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor.

Resistivity of a given material is independent of the dimension (i.e. length and area of cross-section) of given material and it depends only on nature of material and temperature.

SI unit of resistivity

The SI unit of resistivity is Ω m. It is a characteristic property of the material.

Remarks:

(1) The metals and alloys have very low resistivity in the range of

They are good conductors of electricity.

(2) Insulators like rubber and glass have resistivity of the order of

(3) Both the resistance and resistivity of a material vary with temperature.

(4) Resistivity of a material does not change on changing the length and area of cross-section of the conductor.

(5) Resistivity of an alloy is generally higher than that of its constituent metals.

(6) Alloys are used in electrical heating devices, like electric iron, toasters etc because they do not oxidise (burn) readily at high temperatures.

(7) Tungsten is used almost exclusively for filaments of electric bulbs, because it does not melt at higher temperature, have high resistivity and can be easily drawn in to thin wires.

(8) Copper and aluminium are generally used for electrical transmission lines because of their low resistivity they behave as a good conductor of electricity.

CHECK YOYR PROGRESS!!!

1. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

2. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?

3. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?

4. A wire of given material having length l and area of cross-section A has a resistance of 4 Ω. What would be the resistance of another wire of the same material having length l/2 and area of cross-section 2A?

5. A wire of resistance 20 Ω is stretched to double its length. What will be its new (i) resistivity (ii) resistance?

6. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –

I (amperes) 0.5 1.0 2.0 3.0 4.0

V (volts) 1.6 3.4 6.7 10.2 13.2

Plot a graph between V and I and calculate the resistance of that resistor.

CHECK YOUR PROGRESS!!!

1. Name and state the law which relates the current in a conductor to the potential difference across a conductor and the current flowing through it.

2. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

3. On what factors does the resistance of a conductor depend?

4. Define the SI unit of resistance.

5. Why is the tungsten used almost exclusively for filament of electric lamps?

6. Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?

7. How does the resistance of a wire vary with its area of cross-section?

8. Why copper and aluminium wires are usually employed for electricity transmission?

9. Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?

10. Give two examples of materials which are (i) good conductor (ii) resistor (iii) insulator (iv) poor conductor.

11. Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?

12. Why do electricians wear rubber hand-gloves while working with electricity?

13. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

14. Name the device used to change resistance in a circuit to regulate current without changing the

voltage source.

15. What is nichrome? State its one property and one use.

16. On what factor does the resistivity of a material depends?

RESISTANCE OF A SYSTEM OF RESISTORS

There are two methods of joining the resistors together. Figure below shows an electric circuit in which

three resistors having resistances R1, R2 and R3, respectively, are joined end to end. Here the resistors are said to be connected in series. In series, there is only one path for flow of current.

Next Figure shows a combination of resistors in which three resistors are connected together between points X and Y. Here, the resistors are said to be connected in parallel. In parallel, there is separate path for flow of current in each device.

Resistors in Series

In a series combination of resistors

(1) Same current I flow through each resistor.

(2) Potential difference across each resistor is

different. V1 across R1, V2 across R2 and V3

across R3.

(3) Total potential difference across the combination is equal to the sum of potential difference across the individual resistors. That is,

Equivalent Resistance of Series Combination

Let I be the current through the circuit. The current through each resistor is also I. Applying the Ohm’s law to three resistors separately, we have

Since

We have

Where, RS is the equivalent resistance of the series combination.

We can conclude that when several resistors are joined in series, the resistance of the combination RS equals the sum of their individual resistances, R1, R2, R3, and is thus greater than any individual resistance.

It is possible to replace the three resistors joined in series by an equivalent single resistor of resistance RS such that the potential difference V across it, and the current I through the circuit remains the same.

Resistors in Parallel

In parallel combination of resistors

(1) Potential difference across each resistor is same.

(2) Current through each resistor is different. I1 across R1, I2 across R2 and I3 across R3.

(3) The total current I, is equal to the sum of the

separate currents through each resistor of the

combination.

Equivalent resistance in parallel combination

On applying Ohm’s law to each resistor of parallel combination, we have

Since

We have

Thus, we may conclude that the reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.

Preference of parallel combination over series

We prefer parallel combination over series combination in domestic circuit because of the following reasons:

(1) In a series circuit the current is constant throughout the electric circuit. Thus it is obviously impracticable to connect different appliances such as an electric bulb and an electric heater in series, because they need currents of widely different values to operate properly.

(2) Another major disadvantage of a series circuit is that when one component fails the circuit is broken and none of the components works.

On the other hand

(1) In a parallel circuit, different appliances are connected in different branches and each appliance gets its required amount of current in that branch.

(2) In a parallel circuit, if one component fails, the others are not affected.

CHECK YOUR PROGRESS!!!

1. A wire of resistance R is cut into five equal parts. These five parts are then connected in parallel. If the equivalent resistance of this combination is R', then calculate the ratio R/R'.

2. What is the (a) highest and (b) lowest, total resistance that can be obtained by combining four resistors of values 4Ω, 8Ω, 12Ω and 24Ω?

3. How can three resistors of resistances 2Ω, 3Ω and 6Ω be connected to give a total resistance of (a) 4Ω (b) 1Ω ?

4. Two resistors with resistances 5Ω and 10Ω respectively are to be connected to a battery of 6V so as to obtain (i) minimum current flowing (ii) maximum current flowing

(a) How would you connect the resistance in each case?

(b) Calculate the strength of the total current in the circuit in the two cases.

5. A battery of 9V is applied across resistors of 0.2Ω, 0.3Ω, 0.4Ω, 0.5Ω and 12Ω connected in series. How much current would flow through the 12Ω resistors?

6. How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?

7. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 Ω resistance, which may be used separately, in series, or in parallel. What are the currents

in the three cases?

8. In the given figure R1 = 10 Ω, R2 = 40 Ω, R3 = 30 Ω, R4 = 20 Ω, R5 = 60 Ω,

and a 12 V battery is connected to the arrangement. Calculate (a) the total resistance in the circuit, and (b) the total current flowing in the circuit.

9. An electric lamp, whose resistance is 20 Ω, and a conductor of 4 Ω resistance

are connected in series to a 6 V battery. Calculate (a) the total resistance of the circuit, (b) the current through the circuit, and (c) the potential difference

across the electric lamp and conductor.

10. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?

11. Three resistors of resistances 5Ω, 10Ω and 30Ω are connected in parallel across a 12V battery. Calculate:

(a) Total resistance in the circuit.

(b) Total current in the circuit.

HEATING EFFECT OF ELECTRIC CURRENT

Work is done by the battery in maintain an electric current in a circuit. A part of the battery’s energy in maintaining the current may be consumed into useful work (like in rotating the blades of an electric fan). Rest of the source energy may be expended in heat to raise the temperature of gadget.

If the electric circuit is purely resistive, that is, a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current. This effect is utilised in devices such as electric heater, electric iron etc.

Expression for heating effect of electric current

Consider a current I flowing through a resistor of resistance R. Let the potential difference across it be V. Let t be the time during which a charge Q flows across. The work done in moving the charge Q through a potential difference V is

For purely resistive circuit, the work done by the battery gets dissipated in the resistor as heat.

Joule’s Law of Heating

According to Joule’s law of heating, heat produced in a resistor is

(i) directly proportional to the square of current,

(ii) directly proportional to resistance, and

(iii) directly proportional to the time for which the current flows through the resistor.

Thus,

Undesirable effect of heating in electric circuit

(1) Heating effect of electric current convert useful electrical energy into heat.

(2) In electric circuits, the unavoidable heating increases the temperature of the gadget and alter their properties.

Practical Applications of Heating Effect of Electric Current

(1) The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater devices are based on Joule’s heating.

(2) The electric heating is used to produce light, as in an electric bulb.

(3) The functioning of fuse in electric circuit is based on joule’s heating.

Working of Electric iron, Electric toaster etc

Alloys such as nichrome ( an alloy of nickel, chromium, manganese and iron), Constantan ( alloy of copper and nickel) and manganin (alloy of copper, manganese and nickel ) are used as an element of heating devices. Two properties which make these alloys suitable for heating element are:

(a) High resistivity than metal.

(b) Do not oxidise at higher temperature.

When a large current is passed through these alloys then according to joule’s heating a large amount of heat is generated.

Working of Electric bulb

The filament of electric bulb is made up of tungsten. The melting point of tungsten is very high ( 3380°C) When electric current is passed through tungsten filament it gets very hot and emits light. The filament is thermally isolated from the surrounding. The bulbs are usually filled with chemically inactive nitrogen and argon gases to prolong the life of filament. Most of the power consumed by the filament appears as heat, but a small part of it is in the form of light radiated.

Two properties of tungsten which makes them suitable as filament of electric bulb

(a) Very high melting point ( 3380°C)

(b) Can be drawn into very thin wires

(c) High resistivity than metals

Electric Fuse

Electric fuse protects circuits and appliances by stopping the flow of any unduly high electric current. It consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc.

Working: If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

Capacity of fuse wire: The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.

Connection of fuse wire in circuit: The fuse is placed in series with the device.

Expression for work done:

ELECTRIC POWER

Electrical power is the rate at which electric energy is dissipated or consumed in an electric circuit.

The power P is given by

SI Unit: The SI unit of electric power is watt (W).

Definition of 1 watt:

It is the power consumed by a device that carries 1 A of current when operated at a potential difference

of 1 V. Thus,

1 W = 1 volt × 1 ampere = 1 V A

Bigger unit of power:

Watt-hour

Watt-hour (Wh) is the unit of energy.

One watt-hour is the energy consumed when 1 watt of power is used for 1 hour.

Kilowatt-hour(kWh): Commercial unit of energy

The commercial unit of electric energy is kilowatt-hour (kW h), commonly known as ‘unit’.

One kilowatt-hour is the energy consumed when 1 kilowatt of power is used for 1 hour.

Relation between kilowatt-hour and joule

CHECK YOUR PROGRESS!!!

1. An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.

2. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?

3. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?

4. Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.

5. Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?

6. An electric bulb is rated 220 V and 100 W. What will be the power consumed when it is operated at 110 V?

7. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. Calculate the ratio of heat produced in series and parallel combinations.

8. Two resistors each of resistance 6Ω are first connected in series and then parallel in a circuit across a battery of 10V. Calculate the ratio of heat produced in series and parallel combination.

9. An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.

10. An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kW h?

11. An electric bulb is connected to a 220 V generator. The current is 0.50 A. What is the power of the bulb?

12. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

13. Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.

14. 100 J of heat are produced each second in a 4 Ω resistance. Find the potential difference across the resistor.

15. An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate and 360 W when the heating is at the minimum. The voltage is 220 V. What are the current and the resistance in each case?