BS Physics (Foundation) Series

1. 1
Module # 07
Energy
The ability of a body to do work is called its energy.
OR
Energy is the agent which causes some change in the state of
physical systems.
Forms of Energy
There are many forms of energy such as:
(1) Mechanical Energy
(2) Heat Energy
(3) Electrical Energy
(4) Magnetic Energy
(5) Chemical Energy
(6) Nuclear Energy, etc.
Units
As energy is the ability to do work, so, it has the unit of work, i.e.
Joule.

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Mechanical Energy
A machine works due to the motion of its different parts. The
kinetic energy of these parts is called mechanical energy. When a
fly wheel rotates, its rotational kinetic energy acts as mechanical
energy. Similarly, an electric fan in motion has mechanical
energy.
Types of Mechanical Energy
There are two types of mechanical energy:
(1) Kinetic Energy
(2) Potential Energy
Kinetic Energy
Kinetic energy is the energy which a body has by reason of its
motion,
K.E. = ½ mv2
where m is the mass and v is the velocity. It is measured in joules.
Expression for Kinetic Energy
Let a body of mass 'm’ be moving with a velocity 'V’. Initially, the
body is at rest on a frictionless horizontal surface. When a force F
acts upon it, it moves through distance S.

3. 3
During this process the amount of work done is
W = FS ______ [1]
But according to Newton’s Second Law of Motion, the force F
produces acceleration ‘a’.
F = ma _____ [2]
Here,
Initial velocity of body = Vi = 0
Final velocity of body = Vf = V
Distance covered = S
Now using third equation of motion,
Vf
2
- Vi
2
=2aS, by putting the values of Vf and Vi, we get,
(V)2 - (O)2 = 2aS, or V2 = 2aS
OR
S = V2
/2a _____ [3]
Using equations [2] & [3] in eq. [1], we get,
W = ½ mV2
Now the work done should be equal to the kinetic energy of the
body so,
K.E. = ½ mV2

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It shows that kinetic energy of a body in motion is directly
proportional to its mass and also to the square of its Velocity.
K.E. is a scalar quantity. Its units are the same as those of work
i.e., Joule.
Potential Energy
Potential energy of a body is the energy possessed by it by virtue
of its configuration or simply the energy due to the position of a
body is called potential energy, e.g. if we lift brick from the ground
to the top of a roof, then, work is done against the force of gravity.
This work appears as the potential energy of the brick.
Gravitational Potential Energy
If a body is lifted to a position higher than its initial position in the
gravitational field of the earth, then, work is done on it. This work
is stored in the body as its potential energy. Such a potential
energy is called gravitational potential energy. This energy can be
recovered if the body is allowed to fall.
Expression for Gravitational P.E.
Suppose a body of mass m is situated at some point in the
gravitational field of the earth. Now, if it is lifted with very small
speed vertically through a distance h, then, the force required to

5. 5
raise the body is just equal and opposite to that of the weight (w =
mg) of the body.
Thus, work done on the body is given by
Work = force x displacement = mgh
But, this work is due to the change in position of the body, i.e. this
work is equal to the potential energy of the body at height h with
reference to the initial point.
Thus, it is called gravitational potential energy.
Work = P.E.
Hence, gravitational potential energy or simply P.E. is
mathematically given by;
P.E. = mgh
Absolute Gravitational Potential Energy
In order to define absolute gravitational potential energy of a
body, we are required to select a reference point with zero
potential energy.
Now, if the potential energy of the body in the gravitational field of
the earth is measured with reference to this point at which the
potential energy of the body is zero, then, the potential energy is
called absolute potential energy.

6. 6
Thus, absolute potential energy is defined as the work done in
moving a body from earth's surface to a point far off from the
earth (i.e. at infinite distance) where the value of 'g' is negligible.
The work needed to move the body of mass 'm' from the earth to
an infinite distance or the absolute potential energy of the body at
the earth's surface is equal to GmMe/Re where Me is the mass of
the earth, G is the universal gravitational constant & Re is the
radius of the earth.
Elastic Potential Energy
When work is done in compressing and stretching a spring
against the elastic force, then, potential energy is stored in it.
Such a potential energy is called elastic potential energy.
Example
If we wind our watch, then, its spring is tightened. During this
process, some work is done. This energy is stored in the spring
as its potential energy, which is then used to run the watch.
Electric Energy
It is seen that many machines are run by electricity. It means that
electricity is a form of energy. In heaters, it appears as heat
energy. Electrical energy is used to run fans and is converted into
mechanical energy.

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Chemical Energy
Chemical energy is also a form of potential energy. This energy
can be converted into mechanical or electrical energy, e.g. this
energy is released by burning coal, gas, petroleum, wood, etc.
Atomic or Nuclear Energy
Physicists have provided the modern man with a huge source of
energy called atomic or nuclear energy which is being used for
constructive as well as destructive purposes. For example;
nuclear reactors, nuclear power plants, atomic submarines,
atomic missiles, atomic and hydrogen bombs etc.
The most concentrated form of energy that is available to man is
stored in the nuclei. This energy can be released (or obtained) in
the process of fission (the breaking apart of heavy nuclei) and
fusion (the fusing together of light nuclei).
Geothermal Energy
The source of geothermal energy is the heat produced in the
Earth's interior by radioactive decay. Also, in the similar manner,
heat energy obtained from the hot molten metals inside the earth
crust serves as the source of geothermal energy. This type of
energy, in the form of hot water and steam, is present deep in the
earth.

8. 8
Solar Energy
The source of energy most readily available to us is sunlight. The
energy derived from the sun is called solar energy. It is obtained
from the sun rays. It is responsible for vegetable and animal life
on the earth. Mirrors and lenses have been used to concentrate
the energy of the Sun's rays into small spots.
Satellites generate all the electricity they need through solar cells
which directly convert sunlight into electricity. The efficient use of
solar energy is to produce electricity by using solar cells.
Wind Energy
The non-uniform distribution of heat due to solar energy in
different regions causes the movement of hot and cold air over
the earth's surface. Winds blow from areas of high pressure to
areas of low pressure. At sea, the winds are even stronger than
those on the land surface. An equivalent of 100 billion watts per
year of power in the shape of wind energy is available on the
earth. In the windy regions, wind mills are installed to produce
mechanical energy. This mechanical energy may be used in tube
wells or flour mills. In Pakistan places like Chitral, Gilgit and other
high mountain valleys and coastal areas are suitable for the
exploitation of wind power.

9. 9
Water Power (Hydal Energy)
Until about 100 years ago, the primary sources of energy were
wood, water, wind and the Sun's direct rays. They are still in use,
but only water power in the form of electricity generated by huge
hydro-electric power plants is a significant factor in the world
energy supply.
Tidal Energy
The tidal energy is due to the rotation of the Earth relative to the
Moon. The rise and fall of water in the sea due to attraction of the
moon is a source of energy i.e. the water rises along coasts due
to gravitational interaction (pull) between moon and earth. During
24 hours, the sea water level rises up twice. Massive amount of
energy is stored in water rushing up and receding from the
shores. This energy is the tidal energy and can be usefully
exploited and can be converted into electricity.