# Energy

Energy is the capacity of matter to perform work as a result of an applied force on it. It can be considered to be a measurable property of a physical system. This property may be modified due to the application of a force which may result in a change in the motion, the relative position, or the dimensions of an object. Though the basic forms of energy are kinetic energy (the energy associated with motion) and potential energy (the energy possessed by a body by virtue of its relative position), there are several other important forms in nature. They are mechanical, thermal, electrical, chemical, radiant and nuclear energy. All forms in which energy exists are inter-convertible by suitable physical or chemical processes.

## Various forms of energy

• Mechanical energy, as that possessed by the rotor of an electric motor when switch on, is that which is involved in doing mechanical work.
• Thermal energy is the energy which is supplied in the form of heat with a corresponding rise in temperature.
• Electrical energy is that which is possessed by a body due to its charge, and which can flow due to a change in electrical potential.
• Chemical energy is that form of energy which is present by virtue of chemical bonds between atoms of a substance. It can be expended as heat, light or sound energy. A good example is that of fire-crackers bursting.
• Radiant energy is that of electromagnetic waves, such as light, infra-red rays and ultra-violet rays.
• Sound energy is the energy of waves due to alternate compression and expansion of a medium such as air or water.
• Nuclear energy is that released by fission or fusion reactions of atomic nuclei.

## Units of energy

In the SI system, the unit of energy is joule (J). When expressing a joule in terms of the fundamental units, the following relation holds good:

• In nuclear physics, energy is expressed in terms of meV (million electron volts).
• In the cgs (centimetre gram second) system, an ‘erg’ is used
• Electricity is measured in terms of kwh (kilowatt-hours),
• Heat energy is often denoted in terms of calories. The unit calories may also be used to refer to energy in nutrition.
• In terms of food energy, it is more often ‘kilocalories’ that is used.

In the process of transformation from one form to another, it is ultimately either kinetic or potential energy which is added or subtracted, but the sum total of the two always remains a constant. This is the law of conservation of energy.

Traditional theories did not consider the inter-convertibility of matter and energy until Einstein discovered the Theory of Relativity. According to this – for phenomena involving velocities that are smaller than that of light – matter and energy can be considered transferable, without any destruction of either.

## History and uses of energy

In olden times, energy used to be seen in the observable effects of forces in nature. Man knew how to harness energy in order to make his life easier. However, it was during the Middle Ages after the four-element theory was discarded that real progress began. Physicists began to recognise more forms of energy and their inter-convertibility. The Age of the Steam Engine and the flurry of exciting new inventions and discoveries that took place in the 19th century are testimony to the fact that man became increasingly aware of the ways energy can be put to use. New processes involving wind energy, geothermal energy, tidal wave energy and solar energy have come to the fore in recent times.

It is up to man as to how he wants to use it. For instance, nuclear energy can be an efficient, cost-effective means of developing electrical power. But it can also be used to spread destruction in a nuclear war.