Energy is a key principle in physics, as it allows work to be done. The rate at which energy is transferred is called power and the amount of energy that is usefully transferred is called efficiency.
When work is done on an object, energy is transferred. The ratePer unit time or ‘per second’. 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 ÷ time taken.. So the more powerful a device is, the more energy it will transfer each second.
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?
