Work, energy and power Flashcards – AQA A-level Physics (7408)

What is power?

The rate of doing work or transferring energy.

What is the equation for average power over a time interval?

$P = \frac{\Delta W}{\Delta t}$ where $\Delta W$ is work done or energy transferred in time $\Delta t$.

What equation relates power to force and speed, and which force component is used?

$P = Fv$, where $F$ is the force component in the direction of motion.

What is the unit of power, expressed in joules per second?

The watt: $1\,\mathrm{W} = 1\,\mathrm{J\,s^{-1}}$.

What is the equation for efficiency in terms of power?

$\text{efficiency} = \frac{\text{useful output power}}{\text{input power}}$

Why is efficiency dimensionless?

Input power and useful output power have the same units, which cancel in the ratio.

How do you express an efficiency ratio as a percentage?

Multiply the ratio by 100.

What happens to input power that is not transferred as useful output?

It is transferred in less useful ways.