efficiency

In physics, a general term indicating the degree to which a process or device can convert energy from one form to another without loss, or how effectively energy is used, and wasted energy, such as heat and sound, minimized. It is normally expressed as a fraction or a percentage, where 100% indicates conversion with no loss. The efficiency of a machine, for example, is the ratio of the energy output to the energy input; in practice it is always less than 100% because of frictional heat losses.

For example, 75% of the electrical energy of an electric light bulb is converted into heat and only 25% is converted into light. Therefore, an electric light bulb is not an efficient energy converter. Certain electrical machines with no moving parts, such as transformers, can approach 100% efficiency.

Since the mechanical advantage, or force ratio, is the ratio of the load (the output force) to the effort (the input force), and the velocity ratio is the distance moved by the effort divided by the distance moved by the load, for certain machines the efficiency can also be defined as the mechanical advantage divided by the velocity ratio.

In the special case of a heat engine, the efficiency can never exceed 1 − T2/T1 , where T1 is the absolute temperature of the heat source and T2 is the absolute temperature of the exhaust.