FREE PHYSICS NOTES FOR SECONDARY SCHOOL

Uses of Radioisotopes

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Uses of Radioisotopes

Radioisotopes

1. Radioisotopes are isotopes with radioactive properties.

2. Isotopes are elements whose atoms have the same number of proton but different number of neutrons.

3. isotopes have different nucleon numbers but the same proton number.

4. Radioisotopes is isotopes with unstable nuclei.

5. Synthetic isotopes are produced by unstable nuclei that decay.

6. There are many uses of radioisotopes in the field of medicine, agriculture, industry and research.

7. Radioisotopes as used as tracers in scientific research, medical diagnosis and industry.

Half Life

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Half-life

Concept of Half-life

1. The reactivity or activity of a radioactive material is the rate of decay of the material.

2. The rate of decay is the same as the number of atoms which decay or are emmited every second.

3. The rate of decay of a radioactive materials depends on the number of atoms that have not yet undergone decay. Thus, the reactivity of a radioactive material will decrease with time.

4. The half-life of a radioactive element is the time taken for half the number of atoms in a sample of radioactive atoms to decay.

Decay curve.

1. The half-life of the same radioactive element is the same but the half-lives of different radioactive elements are different.

2. The value of half-life is not influenced by factors such as temperatures, pressure and etc.

Usage of Half-life

Half-life in Archeology

1. Carbon-14 has a half-life of 5600 years.

2. Humus, animals and plants absorb carbon-14 through carbon dioxide gas in the atmosphere. A small amount in CO2 exists as carbon-14.

3. Living animals and vegetable have a constant amount of Carbon-14 because the c-14 decayed will always replaced.

4. However or dead beings the amount of C-14 in it will decrease because new C-14 will not be absorbed causing its reactivity to decrease.

5. When an antique or human skill are found, their age can be determined by

· Measuring the reactivity of C-14 in it.

· Determine the ratio of decay carbon-14 against intact carbon-14.

Radioactive Detectors

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Radioactive Detectors

Geiger-Muller Tube

The Geiger-Muller tube is an effective radioactive detector. It can trace alpha particles, beta particles and gamma rays.
The outer part of the G-M tube is made of aluminium which acts as the cathode.
The middle part of the G-M tube is a metal wire which acts as the anode.
The G-M tube is filled with argon gas at low pressure.
Initially, the G-M tube must be connected to a high voltage before being used.
This high voltage causes some ionization of argon gas.

Cloud Chamber

The cloud chamber is made by using a transparent plastic box. The space in it is divided into two parts by a metal.
The lower part is filled with solid carbon dioxide. Sponge is used to push the solid carbon dioxide towards the metal plate.
The upper part is filled with molecules of alcohol vapour released from the felt which is initially soaked in alcohol.
When the alcohol vapour diffuses downwards, it will become colder. Thus, a supersaturated condition will be produced in the space in the lower part of the chamber.
When the radioactive rays enter the upper part, the ionization of air will occur. Saturated alcohol vapour will move above the ions. Droplets of liquid alcohol on the ions will cause the formation of misty tracks.
Steps to ensure clear tracks:

The transparent Perspex cover is rubbed with a soft cloth to produce charges which will remove all ions in the chamber before any radioactive rays enter.
The cloud chamber must be placed horizontally to ensure smooth flow of particles in it.
If light is used, it must shine on the area superated with vapour and not on the black base of the chamber in order to avoid heating it.

Normally, the tracks produced are not uniform. This shows that the radioactive rays are produced randomly.
There are three types of tracks as shown in Table below.

Types of radioactive rays	Explanation
Tracks of alpha particles	The alpha tracks are thick and straight. This shows that alpha particles have the strongest ionizing power and the biggest mass.
Tracks of beta particles	The beta tracks are thin and curvy. This shows that beta particles have low ionizing power and small mass.
Tracks of gamma ray	Their tracks are short, curvy and spiky from the middle. It shows that it has the lowest ionizing power.

The number of radioactive tracks produced will decrease after a while. This is because after some time, the condensation of alcohol vapour on the radioactive source will block the emission of radioactive rays.

Spark counter

1. The wire gauze and thin wire are connected to a voltage of more than 2000 V.

2. The voltage is increased slowly until sparks are produced in between.

3. The sparks are formed due to ionisation of the air.

4. The voltage is then decreased until no sparks are formed.

5. The radioactive source is brought close to the wire gauze.

6. The radioactive rays will ionize the air molecules between the wire gauze and thin wire. Positively charged ions will be attracted to the negatively charged gauze and the negatively charged ions will be attracted to the positively charged thin ions.

7. Secondary ionization will occur due to the collision between the ions and the air molecules.

8. Therefore, sparks are formed.

9. The number of sparks measured the intensity of radioactive rays from its source randomly.

10. The spark counter can only trace alpha particles which have high ionizing power.

Electroscope

1. When charged plate of the electroscope is exposed to the source of alpha particles, the gold leaf will collapse slowly.

2. This is due to the ions and electron are produced by the alpha particles which will neutralize the charge in the electroscope.

3. The rate of collapse of the gold leaf indicates the strength of the radioactive source.

Photographic Plate

1. All types of radioactive rays will darken the photo film. The effect is like sunlight acting on it.

2. The ionization effect by the radioactive rays will decompose silver bromide crystals on the film.

3. Films which are exposed to sunlight will show white lines representing radioactive tracks.

4. Films are kept in the badges worn by workers as a tracer device of radioactive rays.

5. The main disadvantage of using a film as a radioactive tracer is that it needs to be processed in order to prove the presence of radioactive rays.

Radioactivity

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Radioactivity

1. Radioactivity is the spontaneous and random emission of radioactive rays from unstable radioactive materials after which they become more stable.

2. The process is said to be spontaneous because it is not influenced by any physical factors such as temperature, pressure, time, etc.

3. A nucleus is unstable if it is too big. All nuclei with z > 83 or A> 209 are unstable.

4. The emission of radioactive rays is random means that

Emission occurs at irregular intervals.
Emission does not occur at the same means.

5. There are three different types of radioactive emissions.

Alpha particle- a
Beta particle- B
Gamma ray-r

6. Table below shows the characteristics of alpha particle, beta particle, and gamma particle.

Characteristic	Alpha particle	Beta particle	Gamma ray
Nature	Positively charged helium nucleus, He	Negatively charged electron, e	Neutral electromagnet ray
In an electric field	Bends to the negative plate	Bends to the positive plate	Does not bend, showing that it is neutral.
In magnetic field	Bends a little showing that it has a big mass. Direction of the bend indicates that it is positively charged.	Bends a lot showing that it has a small mass. Direction of the bend indicates that it is negatively charged.	Does not bend showing that it is neutral.
Ionising power	Strongest	Intermediate	Weakest
Penetrating power	low	Intermediate	High
Stopped by	A thin sheet of paper	A few millimeters of aluminium	A few centimeters of lead or concrete
Range in air	A few centimeters	A few metres	A few hundred metres
Speed	1/20 X the speed of light, c	3%-99% of the speed of light, c	The speed of light,c

Understanding the Nucleus of an Atom

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The Composition of the Nucleus

1. Matter is made up of very small particles called atoms.

2. Each atom has a very small and very dense core known as the nucleus.

3. Most of the mass of the atom is contained in the nucleus.

4. The electrons move in orbits around the nucleus.

5. The diameter of the nucleus is about 100 000 times smaller than the diameter of the atom.

6. This means that there are lots of empty space within an atom.

7. The subatomic particles in a nucleus are called nucleons.

8. The two types of nucleons are protons and neutrons.

9. The proton is a positively charged particle. It carries a charge of +e, where e is equal to 1.6 × 10-19 C.

10. The neutron carries no charge. The neutrons has approximately the same mass as the proton.

11. The number of protons in the nucleus of an atom is known as the proton number, Z.

12. The total number of protons and neutrons in the nucleus of an atom is known as nucleon number, A or mass number.

13. Then number of neutrons, N = A – Z

Nuclide Notation

1. A nuclide is a type of atom with a particular nucleon number. This term is also used for a type of nucleus.

2. The nuclide notation of an atom gives the symbol of the elements, the proton number and the nucleon number of the atom.

Isotopes

1. Isotopes are atoms of the same elements with the same numbers of protons but different number of neutrons.

2. isotopes have the same proton number but different nucleon numbers.

3. All isotopes of an element have the same chemical properties because their electrons are arranged in exactly the same way.

4. Their physical properties such as densities, boiling points and melting points are different.

5. Some elements in nature such as oxygen,carbon, and bromine consist of a mixture of isotopes.

6. Some isotopes of an element are stable while some are unstable. The unstable isotopes or radioisotopes.

7. Radioisotopes will undergo spontaneous decay to emit radioactive rays such as alpha, beta and gamma rays. After radioactive decay, the proton number and nucleon number of the radioisotope may be changed.

Relationship between Energy, Voltage, Current and Time

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1. The potential difference or voltage, V across two points is defined as energy, E dissipated or transferred by coulomb of charge, Q that moves through the two points.
Therefore:

Potential difference = Electrical energy dissipated
Charge

V= E /Q

2. Current is the rate of charge flow. Therefore, the total charge flows through the two points is given as:
Q = It

3. Since the energy dissipated or transferred is given by:
E= VQ

Therefore, the relationship between E, V,I and t can be written as:
E = VIt

4. From Ohm’s law, V =IR, therefore,
E = IR × It
E = I²Rt

5. From I = V / R => E = V

Analyzing Electrical Energy and power

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1. Electrical energy is important because this form of energy can be converted easily into other forms of energy, such as heat, light, mechanical and electromagnet.

2. The energy carried by electrical charges can be transformed to other form of energy by using different electrical appliances. Table 2.15 shows the transformation of energy for a few household appliances.

Appliances	Energy transformed
Rice cooker	Electrical to heat
Radio	Electrical to sound
Light	Electrical to heat and light
Washing machine	Electrical to mechanical

3. Electrical energy can be defined as the energy carried by electrical charges which can be transformed to other forms of energy by using of an electrical device or appliances.

Electromotive force and Internal Resistance

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Electromotive force and Internal Resistance

1. A cell can be modeled as an e.m.f., E connected in series with an internal resistor, r.
2. When a high resistance voltmeter is connected across the terminals of the cell, the reading of the voltmeter gives the e.m.f., E of the cell.
3. If a resistor, R is then connected to the terminals of the cell, the voltmeter reading is the potential difference, V across the terminals of the cell.
4. The value of potential difference, V is less than e.m.f. of the cell. The difference between E and V is due to the potential difference needed to drive the current, I through the internal resistor, r of the cell.
Hence,

E-V = Ir ═> E = V + Ir

5. The internal resistance, r is given by:
r = E-V / I

Analyzing Electromotive Force and Internal Resistance

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1. The cell is the source of energy and

the light bulb is the energy consuming device.

2. The light bulb converts electrical energy into heat and light energy.

3. The electrical charges that flow round the circuit transfer energy from the source to the device.

4. In a cell, chemical reaction converts chemical energy into electrical energy. This energy pushes the free electrons to move them from the negative terminal to the positive terminal of the cell.

5. Work done by the source in driving the charges around a complete circuit. This work done is known as electromotive force.

6. The electromotive force(e.m.f.) is the work done by one source in driving a unit charge around a complete circuit.

Electromotive Force and Potential Difference

1. The definition for electromotive force (e.m.f.) is similar to that of potential difference (p.d.). However, there is a distinction between e.m.f. and p.d.

2. The e.m.f. of a cell is the energy supplied to a unit of charge within the cell.

3. The p.d. across a component in a circuit is the conversion of electrical energy into others forms of energy when a unit of charge passes through the components.

Internal Resistance

1. In an open circuit when there is no current flow, the potential difference, V across the cell is the electromotive force, E of the cell.

2. In a closed circuit when there is a current flow, the potential difference, V across the cell is smaller than the electromotive force, E of the cell.

3. This drop in potential difference across the cell is caused by the internal resistance of the cell.

4. The internal resistance of a source or a cell is the resistance against the moving charge due to the electrolyte in a source or the cell.

5. Work is needed to drive a charge against the internal resistance.

6. This causes a drop in potential difference across the cell as the charge flows through it.

Power, Energy and Efficiency

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POWER

1. The power,P, is the rate at which work is done or the rate of change of energy.

Power, P = Work done ,W / Time taken , T

Or

Power, P = Change of energy / Time taken , T

2. The SI unit of power is watt (w).
3. 1 watt is defined as the power required to perform 1 joule of work in 1 second.
4. Power depends on the time taken and the work done .
5. People or engine with high power rating can get the work done in short time.

6. For a force F which produces a constant velocity, V,or a stationary object , the power generated is:

P= Fv

Proof:

Power= Work / Time

= (Force x Displacement) / Time

=Force x ( Displacement/ Time)

= Force x velocity

P= Fv

POTENTIAL ENERGY

1. Potential energy is the energy possessed by an object due to its position or state.
2. Potential energy can be classified into gravitational potential energy and elastic potential energy.

GRAVITATIONAL POTENTIAL ENERGY

The gravitational potential energy of an object depends on:
a) its mass
b) its height
c) the gravitational field

RELATIONSHIP BETWEEN WORK AND GRAVITATIONAL POTENTIAL ENERGY

The work done against the force of gravity is known as the gravitational potential energy

Gravitational potential energy= mgh
Where, m= mass
g= Acceleration due to gravity
h= Change in the height of the object

ELASTIC POTENTIAL ENERGY

1.Energy is needed to compress and extend an elastic material such as a spring and rubber.
2.The spring obtains its energy when work is done on it by compressing or stretching it.
3. The energy which an object possesses when it is compressed or stretched is known as the elastic potential energy .

RELATIONSHIP BETWEEN WORK AND ELASTIC POTENTIAL ENERGY

Work done = mean force x displacement
W= 1/2 fx

• The extension of spring will increase if the force applied increases.
• Therefore, the elastic potential energy stored in the spring.
=Work done
= 1 / 2 Fx

KINETIC ENERGY

1. Kinetic energy is the energy possessed by an object due to its motion.
2. The kinetic energy of a moving object depends on its mass and speed.
Kinetic Energy= 1/2 mv^2 = (one over two multiply mass multiply velocity squared)

Where m is the mass of an object, v is speed of the object.

RELATIONSHIP BETWEEN WORK AND KINETIC ENERGY

1. Newton’s first Law of motion states that an object that moves with constant velocity will continue to move at this velocity if no external force acts on the object.
2. That mean an object which moves with constant velocity will conserve its kinetic energy.
3. Work is done when the kinetic energy increases or decreases. The change in kinetic energy of an object is equal to the work done on that object.
4. W= change in kinetic energy
= 1/2 (mv^2- mu^2)

PRINCIPLE OF CONSERVATION OF ENERGY

 The principle of conservation of energy states that energy can neither be destroyed nor created but it can change from one form to another.
 The changes of kinetic energy to gravitational potential energy also proves the equation of kinetic energy, v = u -2gh

EFFICIENCY

The efficiency of a device is defined as the percentage of the energy input that is transformed into useful energy.

Efficiency =( Useful energy output / energy input) x100 %

EFFICIENCY OF MACHINES

1.Machines are devices that make our work easier.
2.Machines require energy to work. This energy is called the input.
3.Machines transforms this input into other forms of energy to perform useful works.
4.However, the useful work obtained is not equal to the input as there is energy “loss” In this process. This loss is mainly due to work done against frictional forces and takes the forms of heat.
5.So, a machines is not perfect because the work done by the effort or input energy is not wholly used to overcome the load.

Work and Energy

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Work

1. Everyday we move or certain object to do work.
2. work is done when a force is exerted to move an object through a distance in the direction of the force.
3. Work W is defined as product of the force and the displacement of an object in the direction of the force.
Work=Fs
Where,

F= the force acting
S= the displacement (or distance traveled in the direction of the force)

4 .Work is a scalar quantity and its unit is joule (J) or N m. 1 joule =1nm
Example:
A block which is at rest is acted on by force of magnitude 3 N in different direction. Determine the wok done by the block in each case.
a) The force act from the left, the object move to the right for 2 m.
b) The force act from the right, the object to the left for 2 m.

Solution

a) F=3 N b) F= -3 N
moving to the right for 2 m moving to the left for2 m (negative
Work done,W= Fs sign indicates object move to the left)
=3 N x (-2 m) Work done W = Fs
=6 Nm = -3 N x (-2m)
=6 Nm

5 .1 joule is the work done when a force of 1newton moves of an object for 1 m in the direction of the force .
6 .Work is not done when a force is exerted on an object but the object does not move.
7 .In short, work is not done :

a) The direction of motion is perpendicular to the direction of the force exerted
b) Force is exerted on the object but the object does not move.

Energy

1. We need energy to do work.
2. Energy is defined as the Potential or the ability to do work.
3. Energy is scalar quantity and its unit is the joule (J) or N m.
4. Energy can exist in various form. Examples potential energy, kinetic energy, heat energy, electrical energy and sound energy.
5. Energy cannot be created or destroyed. The work done related to the change of the form of the energy.
Example
A student of mass 50 kg walks up a flight of stairs 1.5 m high. What is…

a) the work done by the student?
Work = Fx s
=mg x s
=(50 x 10) N x 1.5 m
=750 J
b) energy needed = work done
=750 J

Analysing Forces in Equilibrium

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Analysing forces in equilibrium

Vector Addition of Forces

1. A resultant force is a single force that represents the combined effect of two or more
forces in magnitude and direction. The direction of the forces have to be taken into
consideration when forces are added.

2. If the forces act in the same straight line, the resultant is found by simple addition or
subtraction as shown in figure 2.1

Resultant force, F = F1 – F2

Figure 2.1

3. The resultant of forces that do not act in the same straight line can be determined by
using the parallelogram law.

4. The parallelogram law states that if two forces acting at a point are represented in size
and direction by the sides of a parallelogram drawn from the point, their resultant is
represented in size and direction by the diagonal of the parallelogram drawn from the
point.

Forces in Equilibrium

1. An object is said to be in equilibrium if the object is at rest or is moving with a constant velocity in a straight line.

2. The resultant force that acts on an object is zero if it is in equilibrium. In other words, the forces that act on the object are balanced in all directions.

3. If object is in equilibrium, the resultant force that acts is zero.

4. For two forces acting in the same direction or opposite direction, if the force is not zero, then the object is not in equilibrium.

Impulse and Impulsive Force

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Impulse and impulsive Force

1. Impulse is defined as the change momentum

2. From F=ma

Ft=mv-mu (change of momentum)

3. Impulse is the product of the force F acting on a body and the time t for which the force acts.

Hence, impulse = Ft = mv – mu

4. The SI unit of impulse is kg m s⁻1 or N s.

5. Impulsive force is the rate of change of momentum.

Impulsive force = Impulse / time

6. The SI unit of impulse is kg m s⁻² or N.

The Effect of Time on an impulsive Force

1. From the formula for impulsive force, we get
Ft = mv – mu

F = (mv - mu) / t

This shows that the time of action is very important factor in the calculation of the impulsive force.

2.When the time of action is prolonged, the impulsive force will decrease.

3. On the other hand, if the time of action is shortened, the impulsive force will increase.

Ways to Reduce Impulsive Forces

The Design of a car

1. A car is mainly designed for the safety of the driver.

2. The front and the rear parts of the car are made of soft metal so that the car is easily crumpled during an accident.
a) During collision, the time taken for the change in speed (from a high speed to zero) is prolonged. Since the impulsive force
= Distance / Time , the force will decrease when the time increase.

b) This will decrease the impulsive force on the passengers and the driver.

3. The seats of the passengers are strengthened to protect the passengers.

4. Safety belts:
a) Passengers have to fasten the safety belts. When the car stops suddenly, the inertia of the passengers will result in the passengers being flung to the front and hitting the windscreen of the car.

b) Hence, safety belts will slow down the motion of the passengers.

5. Airbags are built in some cars. When an accident happens, the airbags will be filled with air. This will prolong the time of action and reduce the impulsive force on the passenger.

Ways to utilize impulsive force

Material arts player break a few pieces of bricks
- A martial arts player ia able to break a pile of bricks with ease.
- This is because the hand of the player moves very fast and stops when it hits the top brick.
- Hence, the time of contact of the hand with the brick is short and this will increase the impulsive force on the bricks.

- The bricks are easily broken because of the big impulsive force.

The pestle and mortar
- The pestle and mortar are made of hard materials.
- During pounding or grinding, the pestle moves very fast. The mortar stops the motion of the pestle in a short time.

- A strong impulsive force is produced and the food can be broken into pieces easily.

The pile and the pile driver
- A pile driver is made of hard steel alloy.
- The pile driver is released very fast hit the hard pile.
- The time taken to hit the pile is short because both surfaces are hard.
- Hence, a big impulsive force is produced on the pile and it will be driven into the ground to support the foundation of the structure of a tall building.

Cathode Ray Oscilloscope

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Cathode Ray Oscilloscope (CRO) uses a cathode ray tube to produce visible graphical representations of electrical signals.

CRO

http://boson.physics.sc.edu/~hoskins/crfig1.gif

Oscilloscope

http://www.best-microcontroller-projects.com/how-to-use-an-oscilloscope.html

The graphs produced consist of a horizontal axis which is normally a function of time, and a vertical axis which is a function of the input voltage.

Many physical quantities can be converted into a corresponding electric voltage. The oscilloscope is a useful tool in many physics experiments.

The componentd in a cathode ray tube consists of a vacuum glass tube with an electron gun, a deflection system for deflecting the electron beam, and a flourescent coated screen.

Electron gun

In a cathode ray tube, a beam of electrons is produced by heating the filament with a small voltage supply. the power supply can be AC or DC. The electron beam emerging from the electron gun passes between two pairs of deflection plates, i.e. X and Y - plates mounted horizontally and vertically.

Deflection system

CRO has a fluorescent screen. When the screen is struck by a beam of electrons, wave forms will be traced out on the screen. The kinetic energy of the electrons is changed to light energy.

There is a bright spot on the screen when the beam strikes. By changing the vertical gain on the Y-plates, the beam is deflected vertically. The beam can be moved up and down and if it moves fast enough, the dots will appear as a line.

When an AC supply is connected to the Y-plates, the electron beam will move vertically. The amount of vertical movement can be amplified by increasing gain control. The vertical movement of the electron depends on the vertical gain control and it can be adjusted, using the VOLTS/DIV control. The control is adjusted so that the resulting display is neither too small nor too large, but it fits the screen.

The horizontal deflection plates or X-plates produced a left to right movement. The movement is produced by a circuit called the time base inside the oscilloscope. The time base produces a saw tooth wave form. During the rising phase ( the rising line) of the voltage, the spot is driven at a uniform rate from left to right across the screen. During the falling phase, (the straight vertical line downwards) the electron beam returns rapidly from right to left.

Sawtooth wave form

http://www.hasdeu.bz.edu.ro/softuri/fizica/mariana/Mecanica/Waves_4/transp11.7.gif

The spot is moving in a very short duration and will not appear on the screen. The time base has a changing voltage across the X-plates so that the spot moves from left to right across the screen again. The speed at which the spot sweeps across the screen horizontally can be controlled by altering the frequency of the time base.

Properties of Cathode Rays

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Properties of cathode rays

The properties of cathode rays can be studied using apparatus such as the maltese cross tube and cathode ray deflection tube.

Maltese cross tube

The maltese cross tube has a glass bulb and a hot cathode and an anode enclosed in it. The anode has a hole in the centre so that electrons can pass through it anc shoot across the vacuum. In the middle of the bulb is a second anode in the shape of a Maltese cross. At the end of the tube, there is a fluorescent screen.

http://www.schoolphysics.co.uk/photos/Atomic%20physics/Maltese_cross_tube.jpg

The streams of electrons which leave the cathode and shoot across the vacuum are called cathode rays. The edges of the shadow of the Maltese cross on the screen are sharp. This is because the electrons are travelling in straight lines. Invisible cathode rays travelling across the tube, cast a shadow of the cross on the screen. When electrons strike the screen, the fluorescent screen will glow and light is emitted.

The beam of electrons can be moved by a magnetic field.

Deflection tube

http://www.practicalphysics.org/imageLibrary/jpeg300/1839.jpg

The properties of a beam of electrons in an electric field can be investigated using a deflection tube as shown above. The cathode is connected to about 6 V AC power supply (AC: Alternating current). The electron gun produces a narrow beam of electrons.

The vertical screen is coated with fluorescent material which will glow when electrons strike it. It can show the path of the beam. There are two horizontal metal plates one above the other. When a voltage is applied accross the metal plates the electron beam will be deflected towards the positive plate.

Cathode Ray Oscilloscope: Thermionic Emission

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Electrons are attracted to the nucleus of an atom. There is a strong attractive force between the electrons and the positive charges of the nucleus. In order to move these electrons farther from the atom, energy is needed. This can be done by heating a metal using electric current. We can look at a vacuum diode and see how electrons move between the cathode and anode.

Thermionic emission

A vacuum diode consists of a glass bulb containing two electrodes. One electrode is called the anode and the other is the cathode. The cathode is made up of tungsten filament. The cathode can be heated by a small current connected to the filament. This filament when heated will release electrons from its surface. These electrons can be attracted to the anode when there is a high ptential difference applied between the anode and the cathode.

http://www.radio-electronics.com/info/data/thermionic-valves/vacuum-tube-theory/diode_vacuum_tube.gif

The filament is connected to a 6 volt external battery (usually). When it is heated, a large number of electrons are free to move. As a result, a cloud of electrons is found outside the metal surface of the filament. Many of these electrons are held back by the attractive force of the atomic nucleus. Some of the electrons gained enough energy and escape from its surface. This effect is called thermionic emission.

Thus, Thermionic emission can be defined as the escape of high energy electrons from the surface of a tungsten filament.

Thermionic emission can be used to produce a continuous flow of electrons in a cathode ray tube. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply, a narrow beam of fast electrons will move to the anode. The beam of electrons moving from the cathode to the anode is called cathode rays.

Thus, Cathode ray is the beam of electrons moving from the cathode to the anode in a cathode ray tube.

Renewable Energy

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The world is getting its oil and gas resources depleted day after day. This calls for other sustainable sources of energy.

'Sustainable energy" means our energy resources are used efficiently to ensure a continuous supply. Using renewable energy instead of fossil fuels will be a better option because they cause less pollution. Renewable energy sources are continually replenished naturally, which means they are sustainable.These include hydroelectric, solar, wind, geothermal, biomass, wave and tidal energy.

Efficient energy management is required to ensure that the energy sources is not wasted and quickly depleted.

The National Grid Network

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Electricity is a convenient form of energy because it is easily transmitted, switched on and switched off. The problem with electrical energy is that it cannot be efficiently stored for future use. Batteries can only be used when small quantities of electrical energy are needed.

They are not suitable as main source of power supply.This power supply is distributed to places where electricity is needed.

The Chart below shows the transmission and distribution of electrical energy.

The cables from the power station are connected to a nationwide supply network called National Grid Network. Using the National Grid Network, power stations in areas where the demand is high. Also power stations can be built away from cities and towns. The advantage of the National Grid Network is the uninterrupted power supply even when there is a breakdown in one power station. The other power stations will increase their production to ensure continuous power supply.

Please note though that the regulation of the Network Grid differs from countries to countries. Power from the National Grid Network is distributed by a series of substations to meet the reqirements of consumers. The engineers at the central control of the National Grid Network continually asses the demand, direct the flow and reroute electrical energy when breakdown occurs.

Sometimes this system break down due to unanticipated demand for electricity, which is called and overload. The demand of electricity is usually the highest during the day.

Transmission of Electricity

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Most power stations are located far away from populated areas. Electricity is transmitted from the power station to consumers through power lines.

Unfortunately, some electrical energy is always lost as heat when it travels through wires.

Since the heat dissipated depends on the magnitude of the current, it is more efficient to transmit electrical energy at very low currents.

To produce this low current, the voltage has to be increased. Step-up transformers are used for this purpose to increase the voltage at the power plant.

Step-down transformers are used to decrease the voltage before being delivered to the consumers.

In addition, the long thick cables used as transmission lines are made of copper or aluminium because they have low resistance and thus less energy will be lost when current flows through them.

Example 1:

A power station supplies a factory with 1.0 MW of electrical power at a potential difference of 2 kV. The resistance of the cable between the power station and the factory is 5 Ohm. Find the power loss in the cable.

Current to factory : from equation P = IV, I = P / V (P divided by V)

= 1 X 10^6 (ten to the power of six OR ten exponent six) / 2 X 10^3 = 500 A.

Power loss in the cable = I^2R ( I squared times R)

= (500)^2 X 5

=1.25 X 10^6 W (Watt)

Generation of Electricity

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The electrical energy we used is produced in power plants. These power plants convert fossil fuel, such as coal and diesel into electricity. The electrical energy produced is transported into where it is needed. Electricity also can be generated through natural sources such as Water (hydroelectric), Wind energy and Solar energy.

Electricity is produced using generators. The working principle of a generator is basically like below:

Magnet + Copper wire + Motion = Electricity

A generator has huge magnet that is turned by a turbine.

As the magnet turns inside a coil of wire, electricity is produced by electromagnetic induction. Many sources of energy are used to turn these turbines, each with its own advantages and disadvantages.

Energy Losses In a Transformer

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When we use the equation VpIp = VsIs we are assuming that the transformer is an ideal transformer. An ideal transformer is one which is 100% efficient. In practice, the efficiency of a transformer is less than 100%.

A transformer is designed so that as little energy as possible is lost. There are many ways that a transformer can lose energy.

(a) Power losses occur because the changing magnetic field will also induce currents in the iron core. These induced currents are known as eddy currents. Eddy currents will generate heat and reduce the transformer's efficiency. In order to reduce the formation of eddy currents, a laminated core is used.

(b) Current flowing through th primary and secondary coils will generate heat. Low resistance copper wires is used to reduce this effect.

(c) The core is magnetised and demagnetised alternately when AC current flows through the primary coil. Energy is lost during this process. This is known as Hysterisis. This effect is reduced by using a soft iron core.

(d) There may be a leakage of magnetic flux in the primary coil. A special core design is used in a transformer to ensure that all the primary flux is linked with the secondary coil.

Primary and Secondary Current in Transformers

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Although we can make the secondary voltage greater than the primary voltage in a transformer, this does not mean energy is created. Energy is still conserved.

In an ideal transformer, there is no loss of energy. Hence,

Power supplied to the primary coil = Power used in the secondary coil

That is,

VpIp = VsIs

Vp = Primary voltage , Ip = Primary current

Vs = Secondary voltage, Is = Secondary current

(Remember, Power, P = IV)

Which brings us to the ratio of current which is

Is / Ip = Vp / Vs

This means that if the voltage is stepped-up, the current in the secondary coil is stepped-down by the same ratio.

Comparing the transformer equation.

Vs / Vp = Ns / Np

We ultimately get

Is / Ip = Np / Ns

(Ns = number of coils in secondary coil)

(Np = number of coils in primary coil)

Analysing Momentum II

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Conservation of Momentum

1. The term conservation is derived from the root word “conserve” which means constant.
2. The principle of conservation of momentum states that in the absence of an external force, the total momentum of a system remains unchanged.
3. An example of external force is friction and this can be contact friction or air friction.
4. An isolated or closed system the sum of external forces is zero, thus, the principle of conservation of momentum is true for a closed system.

Collisions

1. There are two types of collision:
(a) Elastic collision
(b) Inelastic collisions

2. In Elastic collision: Two objects collide and move apart again after a collision. Momentum is conserved. Total energy is conserved. Kinetic energy is conserved.

Formula: m1u1+m2u2 = m1v1+m2v2

Elastic Collision

3. In Inelastic collision: Two objects combine and stop or move together with a same velocity after a collision. Momentum is conserved. Total energy is conserved. Kinetic energy is not conserved (the total kinetic energy after the collision is less than the total kinetic energy before collision, excess energy is released as heat, sound energy etc).

Formula: m1u1+m2u2 = (m1+m2)v

Inelastic Collision

Step Up and Step Down Transformers

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In a transformer, the secondary voltage can be more or less than primary voltage. The voltage is dependent on the number of turns in each coil. In an ideal transformer, the relationship between the voltages and the ratio of the turns of the primary and secondary coils is given as below:

Where:

Vs = Secondary voltage

Vp= Primary voltage

Ns = Number of turns in secondary coil

Np = Number of turns in primary coil

From the above equation, if Ns is greater than Np, the Vs is greater than Vp. In this case we have a step-up transformer. If Ns is less than Np, we have a step-down transformer, where the secondary voltage is less than the primary voltage.

Understanding Inertia

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SITUATION 1

Have you ever stood in a bus which starts suddenly from rest? You are likely to fall backwards. If the moving bus stops suddenly, you are likely to fall forward.

SITUATION 2

Have you noticed that a bigger vehicle (Truck) is more difficult to stop than a light vehicle (Motorcycle)?

What Causes This to Happen?

Explanation:

When the bus moves suddenly from rest, our feet are carried forward but the inertia of our body tends to keep us at rest. This causes our body to fall backwards. When the bus stops suddenly, our feet are brought to rest, but the inertia of our body tends to continue its forward motion. This causes our body to fall forward.

The two situations above show that our body has an inbuilt resistance to any change in its state of rest or motion. This reluctance is called inertia.

“The inertia of an object is the tendency of the object to remain at rest or, if moving, to continue its uniform motion in a straight line.”

The concept of inertia was explained by Sir Isaac Newton in the first law of motion.

MASS AND INERTIA

It is to be put in mind that inertia is dependent upon the mass of the object. The larger the mass, the larger its inertia. Hence, we can see that its harder to push a heavy box than to push a lighter box.

EFFECTS OF INERTIA

Many phenomena in our daily lives involve inertia. We make use of the positive effects of inertia to solve some of our daily problems. On the other hand, there are many negative effects of inertia that can endanger our lives and wee need to find ways to reduce them.

Examples are: It’s more effective to fit the head of a hammer (with a higher mass) tightly onto the wooden handle by hitting the bottom of the handle against a hard surface. The head which has a larger mass remain in its state of motion and thus presses itself more tightly around the handle.

Inertia also can be observed in ice skaters where inertia enables ice skaters to keep gliding over the surface of ice at an almost constant speed in a straight line effortlessly.

Ways to reduce inertia in vehicle:

1. Seat belts help to tighten the passenger during collision. This is to prevent the passenger from being thrown forward due to inertia.

2. Air bag is fitted inside the steering wheel. It provides a cushion to prevent the driver from hitting the steering wheel.