Conservation of Energy

The principle of conservation of energy states that energy cannot be created or destroyed. It can only be transferred from one store to another or transformed from one form to another. The total energy of a closed system remains constant.

Energy transfers

Every process involves energy moving between stores. A ball falling from a shelf transfers energy from the gravitational potential energy store to the kinetic energy store. The total energy (kinetic plus potential plus any thermal energy generated by air resistance) stays the same throughout the fall.

Examples

Process	Energy transfer
Kettle boiling water	Electrical store to thermal store
Car braking	Kinetic store to thermal store (in brakes and tyres)
Pendulum swinging	Gravitational PE to kinetic and back
Solar cell powering a lamp	Nuclear store (Sun) to light and thermal
Stretched spring released	Elastic PE store to kinetic store

Efficiency

In practice, useful energy is always less than total energy input because some energy is dissipated (usually as thermal energy to the surroundings). Efficiency measures how much of the input energy ends up in the useful output:

efficiency = useful energy output / total energy input

This can also be written using power: efficiency = useful power output / total power input.

Sankey diagrams

A Sankey diagram shows energy transfers as arrows. The width of each arrow is proportional to the amount of energy it represents. The useful output arrow points in the main direction; wasted energy arrows branch off to the sides. Conservation of energy means the total width of all output arrows equals the width of the input arrow.