Conservation and dissipation of energy

In any system change, some energy is always dissipated — usually to the thermal energy store of the surroundings — making the surroundings warmer. This wasted energy becomes increasingly spread out and harder to use for further useful transfers.

• •Lubrication (e.g. oil in a motor) reduces friction between moving surfaces, so less energy is dissipated as heat and sound.
• •Thermal insulation (e.g. double glazing, loft insulation) reduces the rate at which a useful thermal energy store is transferred to the surroundings.
• •The thermal conductivity of a material affects how quickly energy is transferred by conduction through it — the higher the thermal conductivity, the faster energy is transferred (and the faster a building loses heat through it). Thick walls made from a material with low thermal conductivity reduce the rate of energy transfer out of a building; thin metal sheets would lose heat very quickly.

Efficiency has no units, since it is a ratio of two energy (or power) values in the same units — multiply by 100 to express it as a percentage. Efficiency can never be greater than 1 (or 100%), since a device cannot usefully output more energy than is supplied to it.

• •Efficiency can be increased by reducing unwanted energy transfers, e.g. lubrication to reduce friction, or thermal insulation to reduce unwanted heat loss.
• •Efficiency can also be increased by recovering wasted energy, e.g. capturing waste thermal energy from a power station and using it to heat nearby buildings.