4.24 - Power
Power is the rate of doing work
- Power is the rate of energy transfer, or the rate of doing work (the energy transferred per second).
- Power is measured in watts (W), where 1 W = 1 J/s (one joule of energy transferred each second).
What makes a machine powerful
- A powerful machine transfers a lot of energy in a short time.
- For example, a 1,200 W heater transfers twice as much energy each second as a 600 W heater, so it warms a room more quickly.
The power equation
Where:
- = Power, in watts (W)
- = Energy transferred, in joules (J)
- = Time taken, in seconds (s)
With the work done, , in place of the energy transferred, the same equation is .
Worked example - Power of a motor
A motor transfers 5,400 J of useful energy in 1.5 minutes. Calculate its power output.
Step 1 - Convert the values to the correct units
- Energy is already in joules: J
- Convert time to seconds: s
Step 2 - Substitute into the power equation
The motor has a power output of 60 W - that's 60 J every second.
TermDefinition
PowerThe rate at which energy is transferred or work is done.
WattThe unit of power, equal to a transfer of one joule of energy per second.
Key points to remember
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