1. Different Forms of Energy
In the present chapter we have studied mechanical energy. Apart from this, there are other
forms of energy which are as under :
Internal energy: A substance possesses energy due to internal motions of its constituent
particles and also due to interactions among these particles. For example, the constituent
particles vibrate about their mean positions and hence they possess vibrational kinetic energy.
They also possess potential energy due to mutual attraction and repulsion among themselves.
The sum of all such energies of constituent particles of a body is known as internal energy of
the body .
Heat or thermal Energy: The kinetic energy associated with the random motion of the
constituent particles of a body, is known as heat or thermal energy of the body. (What then is
the difference between internal energy and heat energy ?)
Chemical Energy: Energy of a stable chemical compound is always less than the sum of
energies of its constituent elements in free state. This difference is known as chemical energy
or chemical binding energy. The chemical processes may be endothermic or exothermic,
depending on whether the energy of the products is more or less than the energy of reactants.
Electrical Energy: Electric current can be utilized in getting some work or in getting
different forms of energy. Thus, the energy associated with electric current is known as
electrical energy.
Equivalence of mass and energy :Albert Einstein, from his studies on the theory of
relativity, showed that mass and energy are two different forms of same fundamental
quantity. Mass and energy are mutually convertible according to the following relation .
E = mc2
Where c is velocity of light in vacuum. Its value is 3 x 108
rn/s
Nuclear energy :Microscopic particles like neutrons and protons interact with each other at
distances of the order of 10-15m and constitute a nucleus. The mass of a given nucleus is less
then the sum of the masses of its constituent particles in free state. The energy equivalent to
this mass difference is known as nuclear energy or nuclear binding energy.
When heavy nuclei like uranium are bombarded by neutrons, they absorb the incident
neutrons and become unstable. As a result the compound nucleus (uranium + neutron) is
broken up into two almost equal parts and in this process some neutrons are also emitted. A
huge amount of nuclear energy is released in this process. This process is known as nuclear
fission. This is the process which takes place in atom bomb. In nuclear reactor also fission
process is allowed to take place under control.

2. Lighter nuclei like hydrogen, deuteron, combine with each other at high temperatures and
form a helium nucleus. In this process, too, nuclear energy is released. This process is known
as nuclear fusion. The energy produced in the Sun and stars is due to this process.
Similarity between different forms of energy
After studying different forms of energy, it is found that there is some similarity between
them. At the microscopic level all these energies are in the form of kinetic energy and I or
potential energy. Examining the problem at the fundamental level it is found that what we
call chemical energy is ultimately the potential energy. Then what are internal and heat
energies ? Are they not potential and kinetic energies ?
The equivalence of different forms of energy indicate that there might be a limited number of
fundamental forces in nature.
Conservation of energy :
While stating the law of conservation of mechanical energy, we had clarified that for the law
to hold, the system should be mechanically isolated and the force should be conservative. In
this condition we have,
∆𝑘 + ∆𝑈 = 0
When non-conservative forces like friction are present, a part of the work is used up in doing
work against frictional force and rest of the work done is used in changing kinetic and
potential energies. The energy spent against friction is converted into heat energy (Q).
Hence on the
L.H.S. of eqn. we must add Q.
∆𝑘 + ∆𝑈 + 𝑄 = 0
This equation can still be generalized by inclusion of other forms of energy.
.∆𝑘 + ∆𝑈 + 𝑄 + ∆ (𝑜𝑡ℎ𝑒𝑟 𝑓𝑜𝑟𝑚𝑠 𝑜𝑓 𝐸𝑛𝑒𝑟𝑔𝑦) = 0
This generalized equation represents the law of conservation of energy which is stated as
under :
One form of energy can be transformed in the other form of energy. Whatever some amount
of one form of energy disappears, the equivalent amount of other form of energy is produced.
In other words, the total energy of an isolated system remains constant. The universe is an
isolated system. So, "the total energy of the universe remains constant." This statement is
known as the law of conservation of energy.