Power
When work is done on an object, energy is transferred. The Per unit time or âper secondâ. 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 Ă· 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:
\(power = \frac{work~done}{time}\)
\(P = \frac{W}{t}\)
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.
Learn more on power in this podcast
Listen to the full series on ±«Óătv Sounds.
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:
\(W = F \times d = 2 \times 10 = 20~J\)
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?
\(P = \frac{W}{t}\)
\(P = \frac{48,000}{60}\)
\(P = 800~W\)