## Energy of a body

Energy of a body – 1. Energy of a body is defined as its capacity for doing work. Because the energy of a body represents the total amount of work done, hence
a) It is a scalar quantity just like work.
b) Its dimensions are the same as that of work i.e., [ML2T-2].
c) It is measured in the same unit as the work i.e. Jule and erg.

2. It should note here that Energy and power are different from each other. Energy of a body implies the total amount of work that the body can do and it has nothing to do with the time taken to perform the work. On the other hand, the power of the body depends on the time in which the work is done.

3. Energy can exist in different forms such as mechanical energy, heat energy, sound energy, electrical energy, light energy etc.

## Different forms of energy

a) Mechanical energy: The sum of the potential and kinetic energies of the matter in bulk is called mechanical energy. In most of situations we face, the cause of mechanical energy is either gravitation or electromagnetic in nature. The mechanical energy is concerned with the macroscopic motion of the object. The sound energy also a form of mechanical energy.

b) Internal energy: The sum of the potential and kinetic energies of the molecules or atoms, in a block of matter, is measure of internal energy of the block. Even when the block is stationary, its molecules, atoms etc. are in a state of continuous random motion. It is found that internal energy in directly related with temperature of the bodies. Greater the internal energy, higher is the temperature of the body.

c) Thermal energy: The thermal energy and mechanical energy of a system differ from each other in the sense that former is linked with disorderly motion while the latter is associated with the orderly motion. Further, the thermal energy is also very closely related with the internal energy. The only difference between the two is that thermal energy is linked with the disorderly motion of the molecules while internal energy is related with both the motion of the molecules as well as their configuration or arrangement.

d) Chemical energy: Whenever a chemical compound is formed i.e., atoms are combined into a molecule, some energy is required which is called as chemical energy. It may also be defined as the energy necessary for binding the atoms into a molecule. A very good example of release of chemical energy is the production of heat energy during burning of the coal.

e) Electrical and electromagnetic energy: We know that both the electrical charges and electric currents exert force on each other. Whatever work is done in moving or arranging a system of electric charges against their mutual force is called electric potential energy. Exactly, similarly the arrangement or setting of electric current involves the electromagnetic energy.

f) Mass- energy: Einstein’s mass energy equivalence principle has established that mass is a measure of energy contained in it. It is possible to transform the mass into energy under suitable conditions. i.e., when an electron and a positron combine together, they are themselves annihilated and thus product pure electromagnetic energy.

g) Nuclear energy: The neutrons and protons exert an attractive force on each other when present in the nucleus. Energy associated with them is called nuclear energy. In order to keep the nucleons together within a range of 10-15 m, a part of their mass is converted into energy.