Table of Contents

Unit 1: Graphs of linear motion.......................................................................................................3

Unit 2: Friction force and Newton’s laws of motion...........................................................................8
Unit 3: Applications of atmospheric pressure.................................................................................20
Unit 4: Renewable and non-renewable energy sources...................................................................28
Unit 5: Heat transfer and quantity of heat......................................................................................38
Unit 6: Laws of thermodynamics....................................................................................................52
Unit 7: Electromagnetic induction..................................................................................................61
Unit8: Electrical power transmission..............................................................................................74
Unit 9: Electrostatics.....................................................................................................................81
Unit 10: House electric installation................................................................................................89
Unit 11: Basic alternating current circuits.....................................................................................97
Unit12: Refraction of light............................................................................................................107
Unit 13: Telecommunication Channels.........................................................................................128
Unit 14: Properties of physical processes affecting plant growth...................................................140
Unit 15: Environmental phenomena and related physical concept................................................146
 Unit 1: Graphs of linear motion

Key Unit Competence

By the end of this unit, the learner should be able to plot and analyses the
graphs of linear motion.

1.1 Uniform and non-uniform linear motion

Some definitions

(1)Uniform linear motionis a type of motion in which the body moves with
constant velocity. In other words, it moves with zero acceleration or along a
straight path with constant speed.

Example: Motion of a car moving along a straight section of road at a

constant speed

(2)Non-uniform linear motion is the kind of motion in which a body moves

with a varying velocity.

Example: a bouncing ball.

(3)Speedis the rate of change of distance with time. Speed is a scalar

quantity.

(4) Velocityis the rate of change of displacement. Velocity is a vector

quantity.

(5)Average Velocity is the displacement per unit time. Let x1 and x2 be its
positions at instants t1 and t2, respectively. Then mathematically we can
express average velocity as
Example1:The position of an object moving along the x-axis is defined as
x= 20t2 where t is the time measured in seconds and position is expressed
inmeters. Calculate the average velocity of the object over the time interval
from 3sto 4s.

Example 2:A person runs on a 300m circular track and comes back to the
starting point in 200s. Calculate the average speed and average velocity.
Ans: 1.5 m/s
(4)Acceleration: it is time rate of change of velocity.Acceleration is a vector
quantityand its SI unit is ms-2.
The average acceleration of an object is given by,

The negative acceleration is often called decelerationor retardation.

Therefore, we can say that an increase in the rate of change of velocity is
acceleration, whereas the decrease in the rate of change of velocity is
retardation.

Example 1:The velocity of a car moving towards the East increases from zero
to 12m/s in 3.0 s. Calculate its average acceleration.

Equations of Motion

Useful information for motion of an object

• starts from rest u = 0
• comes to rest v = 0
• is dropped u = 0
• reaches its maximum height v = 0
Example 1
 10 seconds after coming around a corner, a racing car reaches a velocity
in a particular direction of 70 m/s. If it accelerates at a uniform rate of 5
m s–2 in the same direction, at what speed did it come round the corner?

Example 2

A train starts from a station and after 1.0 minute has travelled a
distance of 0.50 km., Calculate its acceleration (assumed to be uniform).

Example 3
A person standing on the top of a building 50 m high throws a stone
with a velocity of 15 m/s vertically downwards. How long will the
stone take to reach the ground?
1.2 Plotting graphs of motion

The relation between graphs of position, velocity and acceleration as

functions of time is summarized in Figure below

Exercises

1. Consider a case of an object moving as shown in table below

2. A motorcycle, travelling east, starts from rest, moves in a straight line

with a constant acceleration and covers a distance of 64 m in 4 s.
Calculate a. its acceleration
b. its final velocity
c. at what time the motorcycle had covered half the total distance
d. What distance the motorcycle had covered in half the total time.

3.A velocity-time graph for a ball rolling along a track is shown below.
The graph has been divided up into 3 sections, A, B and C for easy
reference. (Disregard any effects of friction.)
(a) Use the graph to determine the following: i. the speed 5 s after the start
ii. The distance travelled in Section A iii. the acceleration in Section C
(b) At time t1, the velocity-time graph intersects the time axis. Use an
appropriate equation of motion to calculate the value of time t1 (in s).
(c) Sketch a displacement-time graph for the motion of the ball for these
12 s.

Unit test

1. The slopes of a velocity-time graph is the

A. speed of body B. velocity of body C. acceleration of body D. distance the
body travelled
2. A body moves with uniform
acceleration if A. Its momentum
remains constant.
B. It covers equal distances in equal time.
C. The velocity change by equal amount in equal times.
D. The net force on the body is zero.
3. A car is parked 10 m from home for 10 minutes. Draw a displacement-
time, velocity-time and acceleration-time graphs for the motion. Label all
the axes.
4. A bus travels at a constant velocity of 12 m/s for 6 seconds. Draw the
displacement-time, velocity-time and acceleration-time graph for the
motion. Label all the axes.
5. The position versus time graph for the motion of a car is given below.
Draw the corresponding velocity vs. time and acceleration versus time graphs,
and then describe the motion of the car.
6. Table below shows the instantaneous speed of a vehicle at intervals of 1
second.

(a) Plot a graph of speed against time.

(b) Use your graph to determine:

(i) How fast the vehicle is travelling at 2.6 s and 4.8 s.

(ii) How far the vehicle travelled between the two instants in part (i)

(iii) Slope of the graph.

Unit 2: Friction force and Newton’s laws of motion

Key Unit Competence

By the end of this unit, the learner should be able to perform experiments
involving
Newton’s laws of motion and friction force.

2.1 Newton’s First law of motion

Review on force

A force is anything that can cause a change to objects. Forces can:

• change the shape of an object

• move or stop an object

• change the direction of a moving object.

The S.I unit of force is the newton (symbol N).

Newton’s first law of motion which states that a body remains in its
state of rest or uniform motion in a straight line unless acted upon by
an external force.

Alternatively, an object at rest wants to stay at rest, while an object in motion

wants to stay in motion.

The idea of inertia

Inertia (Latin word meaning laziness), is the tendency of a body to maintain its
state of rest or uniform motion when there are no external forces acting on it.

Factors affecting inertia of a body

(a) Mass of a body

Mass of a body is a measure of its inertia. The more mass the more
inertia.

(b) Acceleration of a body

As the acceleration of a body increases so does its tendency to continue at a

constant velocity.

(c) Force applied on a body

When the force applied on a body is increased, its tendency to remain at rest is
reduced. This would result to movement of the body from its resting
state.Friction acting on a body
It is a force that makes a body to slow down.

Question1: The passengers get thrown to the side when the car they are
driving in goes around a corner.

A. What happens before the car turns?

B.What happens while the car turns ?

C.Why passengers get thrown to the side?

Answer
Question2

1. What physical quantity is a measure of the inertia of a body?

2. Can a force change only the direction of velocity of an object
keeping its magnitude constant?
3. State the different types of changes, which a force can bring in
a body when applied on it.

2.2 Linear momentum and impulse

Definition of Linear Momentum

It is a measure of how difficult it is to stop an object.

The linear momentum of a particle or object of mass m moving with a

velocity v is defined to be the “product of the mass and velocity”: p=mv

Linear momentum is a vector quantity.

Its SI unit is kg .m/s.

Example 1

David weights 60 kg and travels with velocity 1.0 m/s towards Manoj
who weights 40 kg, and is moving with 1.5 m/s towards David.
Calculate their momenta.

Solution

For David

Example 2

A rubber ball of mass 0.2 kg strikes a rigid wall with a speed of 10

m/s and rebounds along the original path with the same speed.
Calculate the change in momentum of the ball.
Ans: – 4

kgm/s

Impulse and momentum

If a person exerts a force F on moving an object in time interval Δ t the

velocity of the object changes. We say that its momentum changes too.
An impulse is the product of the force F and the time interval Δ t in which
it acts. Its magnitude is I =FΔt

The S.I unit of impulse is Newton –Second (Ns).An impulse can also be
equal to the area under the force (F) versus time (t) graph.

Relation between impulse and momentum Δp=FΔt= ΔI

2.3. Newton’s Second Law of Motion

Newton’s Second Law of Motion states that ‟The rate of change of

momentum of a body is directly proportional to the applied force and
takes place in the direction of forces.”

Let a body of mass m having an initial velocity u be acted upon by a

constant force F for a time t.
S.I unit of F is newton (N)

Definition of one newton

One newton is the force which when it acts on a mass of 1 kg, it gives
an acceleration of 1m/s2.

Applying Newton’s Second Law

Example 1

Calculate the acceleration produced by a force of 20 N on an object of

mass 300 kg.

Solution

Example 2

A ball of mass 0.4 kg starts rolling on the ground at 20 m/s and

comes to a stop after 10s. Calculate the force, which stops the ball,
assuming it constant in magnitude throughout.
Exercises

1. Two objects of different masses have the same momentum. Which of

them is moving faster?

2. A boy throws up a ball with a velocity v 0. If the ball returns to the

thrower with the same velocity, will there be any change in (a)
momentum of the ball?

(b) Magnitude of the momentum of the ball?

3. When a ball falls from a height, its momentum increases. What

causes increase in its momentum?

4. In which case will there be larger change in momentum of the object?

(a) A 150 N force acts for 0.1 s on a 2 kg object initially at rest.

(b) A 150 N force acts for 0.2 s on a 2 kg object initially at rest.

A car of mass 1 500 kg is brought to rest from a velocity of 25 m/s

by a constant force of 3 000 N. Determine the change in momentum
produced by the force and the time it takes the car to come to rest.

2.4. Newton’s third law of motion

Based on his study of interactions between bodies, Newton formulated

third law of motion that states as: To every action, there is an equal
and opposite reaction.

Here by „action’ and „reaction‟ we mean force.

A book placed on a table exerts a force F 1 (equal to its weight mg) on
the table, while the table exerts a force F2 on the book.
For this reason, it is better to state Newton’s third law, as when two
objects interact, the force exerted by one object on the other is equal
in magnitude and opposite in direction to the force exerted by the
latter object on the former.

Effects of third law of motion

Example 1

(i) When a bullet is fired from a gun with a certain force (action),
there is an equal and opposite force exerted on the gun in the
backward direction (reaction).

(ii) When a man jumps from a boat to the shore, the boat moves
away from him. The force he exerts on the boat (action) is
responsible for its motion and his motion to the shore is due to the
force of reaction exerted by the boat on him.

(iii) The swimmer pushes the water in the backward direction

with a certain force (action) and the water pushes the swimmer in
the forward direction with an equal and opposite force (reaction).

Example 2

Fig. below shows a block of mass 4.5 kg resting on a smooth

horizontal surface. It is attached to another block of mass 1 kg
using a light inextensible string passing over a frictionless pulley.

Determine

(a) The acceleration of the system.

(b) The tension in the string.

Solution

(a) Since there is no frictional force hence this weight is the resultant force

Resultant force = mass × acceleration

15 N = (1.5 + 4.5) kg × a

Acceleration, a = 2.5 m/s2

(b) The resultant force acting downwards on the hanging mass is (15 – T)
N.

Resultant force = mass × acceleration

15 N – T = 1.5 × 2.5

Tension T = 15 N – 3.75 N = 11.25 N

Activity1

Identify the action - reaction forces in each of the following situations:

(a) A man kicks a football

(b) Earth pulls the moon

(c) A ball hits a wall

Activity2

When a high jumper leaves the ground, where does the force, which
throws the jumper upwards, come from?

2.5. Conservation of Momentum

(a)Law of conservation of momentum

It states the total momentum of the system is always a constant.

Alternatively final momentum = initial

momentum (b) Applications of law of

conservation of momentum

Collision:
It is an interaction or impact between two or more bodies resulting in an
exchange of momentum and energy.

There are two types of collision

Elastic collision in which total kinetic energy (as well as total

momentum) is the same before and after the collision.

Inelastic collision in which total kinetic energy is not the same before
and after the collision (even though momentum is constant).

General case

Consider two particles of masses m1and m2moving with initial velocities u1

and u2 respectively; let v1and v2 be their final velocities respectively after
collision.

Equation defining conservation of

momentum
Example1

A truck of mass 5000kg is travelling along a straight level track at

6.0 m/s. It collides with a second truck of mass 7000kg, which is at
rest. After the collision, the two trucks continue along the track
together. Calculate their final speed. Is the collision elastic or
inelastic? Solution m1u1 + m2u2 = m1v1 + m2v2

(5000 x 6) + (7000 x 0) = (5000 + 7000) v, v = 2.5 m/s

Example2

A 5kg mass moving with a velocity of 10m/s collides with a 10kg mass
moving with a velocity of 4m/s in the same direction along the same line.

After collision, the 5kg mass moves with a velocity of 7.0m/s.

(a) Calculate the velocity of the 10kg mass.

(b)If the two masses join on impact, find their common velocity after
impact.

(c) If the 10kg mass was initially moving to the left and that the two joined
on impact, find their common velocity after impact.

Ans: (a)5.5m/s. (b)6m/s (c)0.67m/s

Example3

A rugby player of mass 85kg, running east at 6m/s, tackles another

player of mass 75kg who is running directly towards him at 8.4m/s.
if the two players cling together after the tackle, calculate the
common velocity.

Ans: 0.75m/s towards west.

Example4

A bullet of mass 30g is fired at 600m/s from a gun of mass 5.0kg at rest.
Calculate the velocity of the gun when the bullet is fired.

Ans: -3.6m/s

The negative sign indicates that the gun moves backwards.

Propulsion by reaction and recoiling gun (or riffle)

1. Recoiling riffle

Consider the riffle which is held horizontally and when the shoot (bullet)
is from the riffle and an explosion occurs.
Where m: mass of bullet

M: Mass of the

riffle V: Speed of

riffle and v: speed of

bullet after shooting

2. Rocket propulsion

The motion of a rocket is an application of Newton’s third law of

motion and law of conservation of linear momentum. A rocket is a
projectile that carries the rocket fuel and the oxidizer, which
supplies the oxygen needed for combustion.

Just before launching, the momentum of the rocket is zero. When

the rocket is fired, it forces a jet of hot gases with a high velocity
through the nozzle. The jet of gases acquires a momentum
downwards. Hence, the rocket acquires momentum of equal
magnitude in the opposite direction.

Exercises

1. A mass of 8 kg travelling to the right at 2.5 m/s collides with a 2

kg mass travelling to the left at 3.0 m/s. Find (a) the momentum of
each mass.

(b) Their total momentum.

2. An object of mass 20 kg collides with a stationary object of mass

10 kg .The two objects join and move at a velocity of 5 m/s. Find the
initial velocity of the moving object.

3. A bullet of mass 10 g is shot from a gun of mass 20 kg with a

nuzzle velocity of 100 m/s. If the barrel is 20 cm long, determine:
(a) The acceleration of the bullet.

(b) The recoil velocity of the gun.

2.6. Friction force

Force of friction is a contact force and always acts along the surfaces in a
direction opposite to that of the motion of the body.

Friction opposes the motion of objects, causes wear and tear and is
responsible for loss of mechanical energy.

Types of friction forces

(a)Static friction force

The force arises between two objects in contact at rest.

It is an equilibrium force.

The magnitude of this force is given by

(b)Kinetic

friction force

The force arises between two objects in contact when one is sliding relative
to another.

It is an opposing force.
Example1

A force of 25 N just limits the motion of a block of mass 50 kg, which is

being dragged on the horizontal ground. Calculate the coefficient of
static friction force.

Solution

Fs = μsR

25 N =

μs × 500

N μs =

25 N 500

N = 0.05

μs = 0.05

Example2

Fig. below shows a block of mass 200 kg being dragged at constant

velocity with a force 40 N at angle 60º to the horizontal.

Determine the coefficient of kinetic friction (μk).

Solution
Fr = μkR

Since the body is not accelerating,

Friction = horizontal component of F.

Fr =Fh = Fcosθ = 40 × cos 60 = 40 × 0.5 = 20 N

R = Weight - vertical component of 40 N

= 2000 N - 40sin60°

= 1965.36 N

Substituting for Fr and R in, Fr = μkR, we get:

20 N = μk ×

1965.36 N

μk = 0.01

The coefficient of kinetic friction is 0.01.

Unit test

1. A bullet of mass 20 g travelling horizontally at a speed of 200

m/s embeds itself in a block of wood of mass 850 g suspended from
a light inextensible string so that it can swing freely. Find the:

(a) Velocity of the bullet and block immediately after collision.

(b) Height through which the block rises.

2. A trolley of mass 2 kg travelling from left to right at 4 m/s

collides elasticity with another trolley of mass 4 kg travelling from
right to left at 1 m/s. If the speed of the 2 kg trolley after collision is
0.8 m/s, what is the speed of the 4 kg trolley?

3. State: (a) three advantages of frictional force (b) three

disadvantages of frictional force

Unit 3: Applications of atmospheric pressure

Key Unit Competence

By the end of the unit, the learner should be able to explain the
existence of pressure in gas and the application of atmospheric
pressure.

3.1 Proof of Existence of Atmospheric Pressure

The existence of the atmospheric pressure can be proved by the following

experiments.

• Crushing can experiment

• Water cover with cardboard does not flow out
Magdeburg Hemisphere

a. Crushing can experiment

Conclusion

When a can filled with hot water is closed and is cooled down rapidly by
pouring cold water on it, it will crush instantly.

This experiment proves that there is a huge atmospheric pressure

exerts on everything on the surface of the earth.
 b. Water cover with cardboard does not flow out

Conclusion

The cardboard does not fall and the water remains in the glass even
though it is not supported by anything.

This is because the force caused by the atmospheric pressure acts

on the surface of the cardboard is greater than the weight of the
water in the glass. This experiment proves that atmospheric
pressure is present on the surface of the earth.

c. Magdeburg Hemisphere

Conclusion

When the air inside the hemisphere is pumped out so that it becomes a
vacuum, the hemisphere cannot be separated even by a very great force.
This is because when the air is pumped out, the pressure inside the
hemisphere becomes very low.
The atmospheric pressure exerts a strong force on the outer surface of the
hemisphere, holding the hemisphere tightly together

Factors influencing atmospheric pressure

(a) Altitude

The fewer number of gas molecule at higher altitude means fewer

molecular collisions and a decrease in atmospheric pressure.

(b) Temperature

When atmospheric air is heated (such as by radiation from the sun)

the space between the neighboring air molecules increases and
reduces air density. Lowering the air density decrease the amount
of pressure exerted by the air i.e. atmospheric pressure.

(c) Water vapor concentration (humidity)

The atmospheric pressure decreases with increase of humidity (water

vapor in the atmosphere).

(d) Wind Pattern

Winds causes convergence (moving together) and divergence (moving

apart) of air at the earth’s surface.

When the wind converges, air molecule increases exerting more

pressure on the surface whereas it exerts less pressure when the
wind diverges since air molecules decrease in number.

Activities

1. Explain why it is difficult to cook food while on top of a mountain.

2. Discuss how the temperature and wind patterns affects atmospheric

pressure.

3. A student in senior three started nose bleeding while they were in a trip
at the top of Mt. Karisimbi.

(a) Explain the possible reason for her nose bleeding

(b) Discuss how you can help her to stop nose bleeding.
3.3 Instruments for measuring atmospheric pressure

(a) Mercury Barometer

A mercury barometer consists of a thick-walled glass tube, which is

closed at one end. The tube is completely filled with mercury and
inverted repeatedly to remove air bubbles. The tube is then
completely filled again with mercury and inverted into a trough
containing mercury.

If the barometer is at sea level, the mercury column drops until it

reaches a height of 76 cm above the lower level of the mercury
meniscus.

When the barometer is placed in a region with lower atmospheric

pressure e.g. high on the mountain, the height of the mercury
column in the tube drops to a level showing the atmospheric
pressure at that place.

(b) Fortin Barometer

Fortin barometer is also used to measure where high level of accuracy is

required.
Before taking the reading, the level of mercury surface in the
reservoir is adjusted by turning the adjusting screw until the
surface of the mercury just touches the tip of the ivory pointer. The
mirror-like mercury surface produces an image of the tip, which
helps to make the adjustment very accurate. The height of mercury
is then read from the main scale and the Vernier scale. Any change
in air pressure makes the surface to move up and down hence this
adjustment is necessary before the barometer is read.

(c) Aneroid Barometer

The aneroid barometer is another example of a portable barometer.

It consist of a sealed metal chamber in the form of a flat cylinder
with flexible walls. The chamber is partially evacuated and the
spring helps in preventing it from collapsing.

The chamber expands and contracts in response to changes in

atmospheric pressure. The movement of the chamber walls is
transmitted by a mechanical lever system, which moves a pointer
over a calibrated scale.

An aneroid barometer is also used as a weatherglass to forecast the

weather. Rainy clouds form in areas of low-pressure air. This is
shown by the fall in the barometer reading, which often means that
bad weather is coming.

Applications of atmospheric pressure

 a. Drinking straw

When a person sucks through the straw, the pressure in the straw
become low. The atmospheric pressure outside which is
higher will force the water into the straw and consequently into the
mouth.

b. Syringe

When the piston is pulled up, the atmospheric pressure inside the
cylinder will decrease.

The atmospheric pressure outside pushes the liquid up into the syringe.

c. Lift Pump
 d. Siphon

Working Mechanism of Siphon

e. Sucker hook
When the sucker is pressed into place, the air inside is forced out.
As a result, the pressure inside the sucker becomes very low. The
sucker is then held in position by the high atmospheric pressure on
the outside surface.

f. Rubber Sucker

g. Vacuum Cleaner
When the vacuum cleaner is switched on, the fan sucks out the air
from the space inside creating a partial vacuum. The atmospheric
pressure outside, which is greater, then forces air and dust
particles into the filter bag. This traps the dust particles but allows
the air to flow out through an exit at the back.

Unit test
For questions, 1 - 3, select the most appropriate answer
1. The following are factors influencing atmospheric pressure. Which one
is not?
A. Temperature C. Cloud cover
B. Water vapor concentration D. Altitude
2. Which one of the following instruments is used for measuring
atmospheric pressure?
A. Temperature C. Thermometer
B. Barometer D. Lactometer
3. Which one of the following statements is correct?
A. When altitude increases atmospheric pressure also increases.
B. When altitude increases, atmospheric pressure remains constant.
C. When altitude increases atmospheric pressure also decreases.
D. There is no relationship between increase in altitude and atmospheric
pressure.
 4. Use the words given to fill in the
spaces. Atmosphere, barometer,
atmospheric, density
Earth surface is surrounded by a thick layer of air called ___ .The
_____ of air varies earth's surface to the outer place. The pressure
exerted by air is called _____ pressure and its measured using an
instrument called _____.
5. Outline and briefly discuss four applications of atmospheric pressure
in our daily lives.
6. Explain how altitude affects atmospheric pressure.

Unit 4: Renewable and non-renewable energy sources

Key Unit Competence

By the end of the unit, the learner should be able to explain the
existence of pressure in gas and the application of atmospheric
pressure.

4.1. Sources of energy

What is a good source of energy?

A good source of energy is one
• which would do a large amount of work per unit volume or
mass,
• be easily accessible,
• be easy to store and transport, and
• Perhaps most importantly, be economical.
Natural resources of energy can be classified as renewable
resources and non- renewable resources of energy.

4.2. Difference between Renewable and Non-renewable sources

No Renewable energy sources Non-renewable energy sources

It can be used again and again It cannot be used again and again
1.
 throughout its life. but one day it will be exhausted.
These are the energy resources, which They are the energy resources,
2. cannot be exhausted. which can be exhausted one day.
It has low carbon emission and hence It has high carbon emission and
3. environment friendly. hence not environment friendly.
It is present in limited quantity and
4. It is present in unlimited quantity. vanishes one day
5. Cost is low. Cost is high.
Renewable energy resources are pollution The non-renewable energy
6. free. resources are not pollution free.
Life of resources is finite and
7. Life of resources is infinite. vanishes one day.
It has low maintenance cost as
compared with the renewable
8. It has high maintenance cost. energy resources.
Large land area is required for the Less land area is required for its
9. installation of its power plant. power plant installation.
Solar energy, wind energy, tidal energy Coal, petroleum, natural gases are
etc. are the examples of renewable the examples of non-renewable
10. resources. resources

4.3. Examples of renewable sources of energy

1.Hydroelectric power plant

It is a transformation of the energy stored in a depth of water into

electricity. The potential energy, or energy due to height, can be
extracted by flowing the water through turbines as it moves from a
higher level to a lower one. See figure bellow
Below is a simple block diagram showing energy changes in hydroelectric
power station.

Advantages

• The installation is not expensive: Big efficiency about 90%

• Source of water is available
• No pollution of atmosphere
• Require no fuel

Disadvantages

• Very expensive to set up because of high costs in setting suitable

places, purchase of necessary materials and construction of the
station and supply power lines.
• Some energy is lost in sound and internal energy.
• Weak power produced; its function is accorded to the seasons
2.Solar energy

The sun: the sun is the biggest source of energy in our environment.
Energy given off by the sun is called solar energy.
Heat energy from the sun is used for making drying things, heating
water (Solar heaters) and for making rain in our environment. The
light from the sun is used for seeing and by plants in the process of
photosynthesis. Light from the sun can be used to make electricity.
Cell containing these crystals is called a solar cell.

Solar cell (Photovoltaic cells and solar heating panels)

A solar cell (photovoltaic cell) is a device, which converts solar energy

directly into electricity.

A photovoltaic cell consists of two thin sheets of a semiconductor

sandwiched together, usually silicon (see figure below)

As the photons from the sun hit the solar cell, its electrons are
freed, passing through the bottom of the cell to the electrical wire,
thus producing electricity. The greater the intensity of light, the
more current is produced.

Advantages
• The power source of the sun is absolutely free.
• solar cells have no moving parts,
• The installation is not expensive
• no pollution of atmosphere

Disadvantages

• Used only in sun season; weak efficiency

• Storage system is expensive
 3. Geothermal energy

It is heat energy from the earth. Heat energy from the hot interior of the
earth can rise the temperature of underground rocks in the earth’s
crust to very high temperatures.

To use this energy, water is pumped down to these rocks and is

converted to steam by this heat. The steam can then be directed to
generators to produce electricity. In Rwanda , you can find hot
water springs in western province (Mashyuza in Bugarama) with a
power generation potential of about 170-320MW.

Advantages

• The pollution can be controlled by putting water or steam back into

the earthHigh production in energy

Disadvantages

• The installation is very expensive; It requires a very greater space

• The sources are not easily accessible; there is loss of internal energy
3. Wind energy

Wind power is the conversion of wind energy into a useful form of

energy. Most modern electrical wind power is generated by
converting the rotation of turbine blades into electrical currents by
means of an electrical generator.
Moving air (wind) has kinetic energy, which can be used to move windmill
vanes, resulting in the driving of generators, which produce electricity.

Windmill, in wind electric power station is machine that converts wind

into useful electrical energy.

The moving air exerts a very large force on the blades or vane of the
turbines and makes them rotate.

Energy change

Advantages of wind energy

• Very cheap way of generation of electricity

• Do not require fuels and can be run at low cost with minimum
maintenance costs.
• Safe because they do not produce substances which pollute air or
water.
Disadvantages

• Very expensive to set up because of a lot of work must be done to

select a proper site with correct wind velocity and abundant wind
supply.
• The wind does not always blow, and wind direction may vary.

5. Tidal or water wave energy

The tides are the daily rise and fall of the earth’s ocean waters.
Tides are caused by gravitational forces between the moon, the
earth, and the sun, and by the spinning of the earth.

WORKING:
-Tidal energy is harnessed by constructing a dam across a narrow
opening to the sea. A turbine fixed at the opening of the dam
converts tidal energy to electricity. Advantages
Tidal energy is cheap; pollution is free and renewable source of
energy.

6. Biomass

It is the energy released from plants (wood, corn, etc.) through combustion
or other chemical process.

A Biogas plant is an airtight tank in which organic wastes mixed with

water can be digested/ fermented through anaerobic digestion caused by
bacteria action.

Anaerobic digestion is a process in which organic materials like

cow dung, agriculture residue or human feaces are digested in the
absence of oxygen to produce biogas(methane and ethanol) in the
gas tight chamber called digester. There are three principal
products of anaerobic digestion; biogas, digestate, and water.

Advantages of using Biogas:

• Biogas does not contribute to the atmospheric pollution, less air

pollution
• Available source; it replaces the deforestation (wood energy)
• Production of fertilizers it is not expensive it is renewable.
• The biogas is a type of fuels, which can be burnt to produce heat
energy for cooking and other uses.

Disadvantages:
 • When it is not well installed, it can cause many problems (breathing
diseases) because of gas rejected.

4.4. Examples of Non-renewable sources of energy

Resources, such as fossil fuels, cannot be replaced by natural processes

at the same rate it is consumed. Example: Fossils energy, nuclear
energy and natural gases.
1.Fossil fuels

They are remains of plant or animal that existed in a past geological

age and that has been excavated from the soil.

Types of fossil fuels

Coal is a solid organic rock made up mostly of carbon. Coal was formed
from the waste of plants that lived in forests and swamps millions of years
ago.

Petroleum

It is also called crude oil. It was produced after millions of years by

the bacterial decomposition of animals and plants, which were
buried underground to great depths inside the earth due to the
earthquakes, cyclones and storms.

Disadvantage of Burning fossil fuels

• Air pollution is caused by burning of coal or petroleum

products.
• They cause a greenhouse effect of gases like carbon dioxide.

The pollution caused by burning fossil fuels can be somewhat reduced

• by increasing the efficiency of the combustion process
• Using various techniques to reduce the escape of harmful
gases and ashes into the surroundings.

2.A nuclear energy

Nuclei of elements like Uranium are a source of nuclear energy.

Nuclear energy is released by reactions in the nuclei of the atoms.
These reactions are fission and fusion.

A. Nuclear fission: In this process the nucleus of heavy atom (such

as uranium, plutonium) when bombarded with low energy
 neutrons split into lighter nuclei with the liberation of large
amount of energy.
This energy can be controlled & converted into electricity in the
nuclear power plant

B. Nuclear fusion: In this process, the two lightweight nuclei

combine to form a stable heavier nucleus with the liberation of
large amount of energy.
The conditions for the occurrence of a nuclear fusion reaction are:
(a) Temperature of fusing nuclei should be raised to 107 K.
(b) Fusing nuclei be accelerated to high speeds using particle
accelerators.

Advantages of nuclear energy system over fossil fuel energy system

• On equal mass basis, nuclear energy system produces more energy

than fossil fuels.
• Nuclear energy systems consume very little fuel. Once loaded, a
nuclear reactor operates for years together. Fossil fuel systems need
a few hundred tons of coal every day.
• No problem of environment
• Great power is produced
• Small amounts fuel required
• Power station requires little space
• Economical for producing bulk electrical energy.

Disadvantages of nuclear energy system over fossil fuel energy system

• Pollution caused by radiation leak from a nuclear reactor is much
more serious than the pollution caused by burning fossil fuels.
• There is a very serious problem regarding storage and disposal of
nuclear waste.
• There is no serious problem for the disposal of ash produced in
fossil fuel energy systems
• Escape of harmful radiation; pollution of water
• The installation is very expensive
• Require more careful sitting and very expensive equipment and
structures. Extremely expensive to ensure operation and disposal of
the dangerous waste products

4.5. Energy Transformations

Energy transformation is the change of energy from one form to
another. Examples of forms of energy include electrical, thermal,
nuclear, mechanical, electromagnetic, sound, and chemical energy.

All energy transformations obey the law of conservation of energy

that states that energy cannot be created nor destroyed but simply
changes from one form to another i.e. total energy in a closed system
is conserved. A device that converts energy from one form to another
is known as a transducer.

Examples of energy transformations

1. Potential energy to kinetic energy and vice
versa (a) Swinging Pendulum

When the pendulum bob is set to oscillate, the potential energy is

transformed to kinetic energy and vice versa.

(b) Conversion of electrical energy into mechanical energy and vice versa.
A device that converts mechanical energy into electrical energy is a
generator. It uses the concept of electromagnet induction where
electric current is induced in a conductor moving inside a magnetic
field or a conductor cutting through magnetic field linking two
points.

Activity 1:To make a simple motor

Materials:
• Safety pins, nails (screws)
• Battery holder
• Wood block
• Disk magnet
• Wire
• Scotch tape
• Sharp knife/razor blade
Steps
Figure below shows a simple electric motor that we are going to make
following the steps below.

Electric motor

1. Wind a wire to form a coil (solenoid) on a pen, making 6 or 9 turns and

leave some inches of wire free at each end.
2. Carefully, pull the coil off the pin (former) and make its shape
permanently by wrapping it around the loop.
3. Hold the coil at the edge of a table so that the coil is straight up and
down (not flat on the table) and one of the free wire ends lying on the
table.

4. With a sharp knife, remove the top half of the insulation from the free
wire end. Be careful to leave the bottom half of the wire with the
enamel insulation intact. Do the same thing to the other free wire end.
5. Bend two safety pins from the middle.
6. Use nails (screws) to mount the bent safety pins on the wood block so
that the loops face each other and are about 1 inch apart.
7. Attach the wires from the battery holder to the supports (bent safety
pins).
8. Swing the safety pins apart a little and insert the coil into both rings.
9. Insert the battery into the holder. Place the magnet on top of the wood
just underneath the coil. Make sure the coil can spin freely and it just
misses the magnet.
10. Spin the coil (armature) gently. What do you observe?
11. Discuss the energy transformation in the simple motor

Activity 2:
To demonstrate transformation of mechanical energy to electrical energy

Materials:
•Galvanometer
•Connecting wires
•Coil (solenoid)
•Bar magnet
•Insulated copper wire
Steps
1. Make a coil (solenoid) using an insulated copper wire.
2. Connect the ends of the solenoid using connecting wires to a sensitive
galvanometer.
3. Quickly introduce (push) the bar magnet into the solenoid and stop
(Fig. (a)).
4. Withdraw the magnet quickly from the coil and stop (Fig. (b)).

5. Move both the bar magnet and the coil at the same speed and in the
same direction. Observe and explain what happens to the galvanometer.

Unit Test
For questions 1 - 10, select the question that you think it is right.
1. Energy sources that once used can replenish themselves and can be
used again and again are termed as A. Non-renewable B. Renewable
C. Finite D. Kinetic
2. Which of the energy sources listed is not a renewable source of energy
A. Oil B. Solar C. Wind D. Tidal E. Geothermal
3. What is the other name for non-renewable
A. Non-renewable B. Finite C. Infinite
4. Energy sources that once used cannot be replenished are called;
A. Non-renewable B. Renewable C. Infinite D. Potential E. Kinetic
5. What natural source is harnessed to generate hydroelectric power
(HEP)?
A. Wind B. Water C. Light D. Heat
6. What is the other name of the renewable energy source generated from
using volcanic heat found under the earth’s surface?
A. Wind B. Hydro-electric power (HEP) C. Tidal D. Solar E. Geothermal

7. What is the name of the renewable energy supply generated by

capturing sunlight in panels that covert the sunlight into electricity?
A. Wind B. Hydro-electric power C. Tidal D. Solar E.
Geothermal

8. Which statement below is not an advantage of tidal energy?

A. Tidal barrages have the potential to generate a lot of energy
B. Tidal barrages can double as bridges
C. Tidal barrages can help to prevent flooding
D. Tidal energy is renewable and once in use can be used for generations
9. What type of energy source comes from radioactive minerals such
as uranium and releases energy when the atoms of the
radioactive minerals are split by nuclear fission?
A. Biomass B. Natural gas C. Geothermal D. Hydro-electric power E.
Nuclear
10. What type of energy source is formed from fossilized plants and is
found sandwiched between other types of rock in the earth? A. Oil B.
Coal C. Geothermal D. Biomass E. Nuclear
11. Describe the advantages and disadvantages of using fossil fuels to
generate electricity.
12. Describe how fossil fuels are formed.
13. Name three compounds that are formed from the chemical compounds
in petroleum.
14. If fossil fuels are still forming, why are they considered to be a
nonrenewable resource?
15. Solar energy has provided almost all the sources of energy on the
earth. Explain.
16. Explain two advantages and two disadvantages of using solar energy.
17. Explain why geothermal energy is unlikely to become a major energy
source?
18. Describe three ways solar energy can be used.
19. Explain how the generation of electricity by hydroelectric, tidal, and
wind sources are similar to each other.