efficiency (physics) – the ratio of useful energy output to total energy input

(pronounced uh-fish-un-see)

To remember the meaning of efficiency (physics), use the following mnemonic:

Efficiency is a measure of how well a device or system converts input energy into useful energy, expressed as a percentage or decimal. 

No device or system is 100% efficient because some energy is always wasted, typically as heat, sound, or light that isn't needed for the system/device's intended purpose. 

Efficiency can be calculated using the following formula:

`\text(Efficiency) = (\text(useful energy output)/(\text(total energy input)))   \text(x)   100`

For example, if an electric motor receives 100 joules of electrical energy but only converts 75 joules into useful kinetic energy, with 25 joules wasted as heat and sound, its efficiency would be 75%

`75/100  \text(x)  100 = 75%`

Understanding efficiency is important because it helps us identify how much energy is being wasted in different biological and mechanical processes and devices, which has implications for reducing energy costs.