curriculum-key-fact
One watt is equal to one joule per second (J/s)

1 W = 1 J/s.

A light bulb that has a power of 60 W, transfers 60 J of electrical energy into light and heat energy every second.

Question

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. Calculate the power of both motors.

Both motors do the same amount of work because they lift the same weight through the same distance.

Work done W = Fd

F = 2 N

d = 10 m

W = 2 N x 10 m

W = 20 J

For motor ONE:

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

work done = 20 J

time taken = 5 s

power = {20 J}\div{5 s}

power = 4 W

For motor TWO:

pwer = \frac{\text{work done}}{\text{time taken}}

work done = 20 J

time taken = 10 s

power = {20 J}\div{10 s}

power = 2 W

Both motors do the same amount of work, but motor ONE does it twice as quickly as motor TWO.

Motor ONE transfers twice the energy each second that motor TWO does.

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?

power = \frac{\text{energy transferred}}{\text{time taken}}

energy transferred = 48,000 J

time taken = 1 minute = 60 s

power = {48,000 J}\div{60 s}

power = 800 W

The power rating of the hairdryer is 800 W