When work is done on an object, energy is transferred. Power is the amount of work done in one second. Efficiency is a measure of how much useful energy is converted.

Part of

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 =

work done = 20 J

time taken = 5 s

power =

power = 4 W

For motor

**TWO**:pwer =

work done = 20 J

time taken = 10 s

power =

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 =

energy transferred = 48,000 J

time taken = 1 minute = 60 s

power =

power = 800 W

The power rating of the hairdryer is 800 W