Energy and power
When work is done on an object, energy is transferred. The ratePer 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 powerThe 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 power is:
\(power = \frac{work~done}{time}\)
\(power = \frac{W}{t}\)
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.
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 hair dryer transfers 48,000 J of energy in one minute. What is the power rating of the hairdryer?
\(P = \frac{W}{t}\)
\(P = \frac{48,000}{60}\)
\(P = 800~W\)
Learn more on power in this podcast
Listen to the full series on ±«Óătv Sounds.