Power
When work is done on an object, energy is transferred. The Per unit time or 鈥榩er second鈥. For example, if 2,000 J are transferred over a period of 10 s, then the rate of transfer is 200 J/s or 200 W. This value is the power rating. at which this energy is transferred is called The energy transferred each second, measured in watts (W). Power = work done 梅 time taken.. 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:
\(power = \frac {work \ done}{time \ taken}\)
\(P = \frac {E}{t}\)
This is when:
- power (P) is measured in watts (W)
- work done (E) 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.
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Example
Two electric motors are used to lift a 2 N weight through a vertical height of 10 m.
Motor one does this in 5 seconds.
Motor two does this in 10 seconds.
For both motors, the energy transferred - the work done - is 20 J.
\(E = F \times d = 2 \times 10 = 20 \ J\)
For motor one:
\(P = \frac {E}{t} = \frac {20}{5} = 4 \ W\)
For motor two:
\(P = \frac {E}{t} = \frac {20}{10} = 2 \ W\)
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?
\(P = \frac {E}{t}\)
\(P = \frac {48,000}{60}\)
\(P = 800 \ W\)