ENERGY

Let us understand the concept of energy with some examples.

• When a fast moving cricket ball hits a stationary wicket, the wicket is thrown away.
• An object when raised to a certain height gets the capability to do work.

From all these examples we conclude that an object that possesses energy can exert a force on another object. When this happens energy is transferred from the former to the latter.

The second object may move as it receives energy and therefore do same work. Thus, the first object had a capacity to do work. This implies that any object that possesses energy can do work.When a man does a work, he feels tired. he feels that he has lost something which he must regain to work more. A weak man gets exhausted after doing only a small amount of work. A strong man can continue to work for longer duration.

“Something that a working man loses is called energy”.

DEFINITION:

Capacity of doing work or total work done by a man or by an agent, is called the energy of the man or the agent.

(a) Units of energy :

C.G.S. unit of energy is erg and S.I. unit of energy is joule.

(i)     kilo Watt × hour (kWh) is commercial unit of energy.

1 kWh = 1000 watt × 60 × 60 s.

= 3.6 × 106 watt × s

1 kWh = 3.6 × 106 J.  

(ii)   Electron volt (eV) is also the unit of energy. The energy of an electron, when it is accelerated by a potential difference of 1 volt, is known as one eV

1 eV = 1.6 × 10-19 J.

FORMS OF ENERGY:

There are various forms of energy :

• Mechanical Energy. It is the energy possessed by a body due to its position (potential energy) or motion (kinetic energy). The sum of these two energies is called mechanical energy.
• Solar Energy. The energy radiated by the Sun is called solar energy. Plants collect and store this energy to make food through photosynthesis.
• Chemical Energy. It is the energy possessed by fossil fuels (coal, petroleum and natural gas) and is also called the fuel energy. The food that we eat has chemical energy stored in it.
• Heat Energy. It is the energy released when coal, oil, gas or wood burn and it produces in us the sensation of warmth.
• Light Energy. It is the form of energy which produces in us the sensation of sight. Sun is the natural source of light.
• Sound Energy. It is the energy possessed by vibrating objects and it produces in us the sensation of hearing.
• Electrical Energy. The energy possessed by charges (either at rest or in motion) is called electric energy.
• Nuclear Energy. The energy produced in the processes of fission and fusion is called nuclear energy.

The kinetic energy and potential energy taken together is known as mechanical energy.