# 1.21 Power

## Power

• The power of a mechanical process is the rate at which energy is transferred
• Since work done is equal to the energy transferred, power can also be defined as the rate of doing work or the work done per unit time
• Power is measured in Watts (W)
• Since power is energy used per unit time, 1 W = 1 J s–1
• Power can be calculated using the equation: #### Worked example

A car engine exerts the following force for 1.0 km in 200 s. Determine what is the average power developed by the engine. #### Exam Tip

Power is also used in electricity, with labels on lightbulbs which indicate their power, such as 60 W or 100 W, which indicate the amount of energy transferred by an electrical current rather than by a force doing work. Just remembering 'energy per unit time' will help, as it doesn't matter what kind of energy is being transferred.

When working with power equations the numbers are often very large, so expect to see kW, MW and even GW in questions.

(In case you forgot... kW means × 103. MW means × 106, and GW means × 109. If you weren't sure, go and revise S.I. Units!) ### Get unlimited access

to absolutely everything:

• Unlimited Revision Notes
• Topic Questions
• Past Papers 