Energy and power

When work is done on an object, energy is transferred. The rate at which this energy is transferred is called power. So the more powerful a device is, the more energy it will transfer each second.

Calculating power

The equation used to calculate the power is:

\ power \ = \ \frac{work \ done}{time} \

\ P \ = \ \frac{W}{t} \

This is when:

  • power (P) is measured in watts (W)
  • work done (W) is measured in joules (J)
  • time (t) is measured in seconds (s)

One watt is equal to one joule per second (J/s). This means that for every extra joule that is transferred per second, the power increases by one watt.

An electric motor rests on a table lifting a weight. The motor is connected to a Joulemeter which in turn is connected to a low voltage supply.

Example

Two electric motors are used to lift a 2 N weight at a constant speed through a vertical height of 10 m.

Motor one does this in 5 seconds.

Motor two does this in 10 seconds.

For both motors:

\ W \ = \ Fd \ = \ 2 \times 10 \ = \ 20 \ J \

For motor one:

\ P \ = \frac{W}{t} \ = \ \frac{20}{5} \ = \ 4 \ W \

For motor two:

\ P \ = \frac{W}{t} \ = \ \frac{20}{10} \ = \ 2 \ W \

Since twice as much energy is transferred by motor one each second, it is possible to say that motor one is twice as powerful as motor two.

Question

A hairdryer transfers 48,000 J of energy in one minute. What is the power rating of the hairdryer?

\ P \ = \frac{W}{t} \

\ P \ = \ \frac{48,000}{60} \

\ P \ = \ 800 \ W \

curriculum-key-fact
Power is the amount of energy that is transferred per second.
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