Energy

Anything capable of doing work is said to posses energy. Energy exists in various forms and may be considered as ‘stored work’ awaiting to be used. For example:

• Chemical energy is absorbed or released when a reaction occurs (e.g. the combustion of fossil fuels).
• Atomic energy is liberated by splitting or combining atomic particles. Splitting atomic particles is called fission and combining atomic particles is called fusion. These reactions produce large amounts of heat energy.
• Heat energy can be subdivided into sensible heat energy and latent heat energy. Sensible heat energy is the energy taken in or given out when a solid, liquid or gas rises in temperature or falls in temperature. Latent heat energy is the energy required to change the state of a solid, liquid or gas at a constant temperature.
• Mechanical energy can be subdivided into potential energy, kinetic energy and strain energy:

Potential energy (also known as conserved energy) is the energy possessed by a body due to position. For example when work has been done to raise the hammer of a pile driver the work is stored in the hammer as potential energy.

Kinetic energy is the energy possessed by a body by virtue of its velocity. For example if the hammer of the pile driver is allowed to fall, the potential energy is reduced in direct proportion to the loss of height. The kinetic energy increases in direct proportion to the gain in velocity. The potential energy is being converted into kinetic energy due to the motion of the hammer. At the instant of the impact of the hammer on the pile all the potential energy is converted into kinetic energy which is used to do work, namely to drive the pile into the ground. Kinetic energy =1⁄2mv².

Strain energy (also known as conserved energy) is the energy stored in a piece of strained elastic material. For example when a metal bar is stretched by a tensile force, work is done on the bar. Provided that the strain is kept within the elastic range for the material, the work done in stretching the bar is stored up as strain energy. This energy can be regained on removal of the straining force. The same argument applies when a bar is twisted as in a torsion spring.

Conservation of energy

The law of conservation of energy states that: Energy can be neither created nor destroyed but is convertible from one form to another.

For example, in a power station, the chemical energy of the fuel is released by combustion and is converted into thermal energy which, in turn, produces steam in a boiler. The steam drives a turbine which converts the energy in the steam into mechanical energy. This mechanical energy is converted into electrical energy by the alternator driven by the turbine. During these conversion processes there will be loss of energy due to the inefficiency of the equipment used. This ‘lost’ energy cannot be destroyed (law of conservation of energy) but is converted into other forms of energy that do not contribute to useful work. For example work done in overcoming friction by heating the bearing surfaces and the lubricating oil.