Power and efficiency

Power is the rate at which energy is transferred, or the rate at which work is done. It is measured in watts (W), and 1 W = 1 J/s: one joule transferred every second.

You must recall both equations:

power = energy transferred ÷ time, or P = E/t

power = work done ÷ time, or P = W/t

Two motors each lift the same load, transferring 600 J to its gravitational potential store. Motor A takes 20 s and motor B takes 12 s.

Motor A:

1. P = E/t
2. P = 600 ÷ 20
3. P = 30 W

Motor B:

1. P = E/t
2. P = 600 ÷ 12
3. P = 50 W

Both motors do the same job, but motor B is more powerful because it transfers the energy faster.

Efficiency tells you how much of the energy put in is transferred usefully. You must recall both forms:

efficiency = useful output energy transfer ÷ total input energy transfer

efficiency = useful power output ÷ total power input

Efficiency has no unit. Give it as a decimal, or multiply by 100 for a percentage.

Example 1: a lamp is supplied with 60 J and gives out 12 J as light.

1. efficiency = 12 ÷ 60
2. efficiency = 0.2, or 20%

Example 2: a motor takes in 500 W and gives out 400 W of useful power.

1. efficiency = 400 ÷ 500
2. efficiency = 0.8, or 80%

Energy cannot be created, so a device can never give out more energy than it takes in. In real devices, some energy is always wasted, usually by heating the device and its surroundings. So the useful output is always less than the input, and the efficiency is always less than 1, or under 100%.

If you work out an efficiency greater than 1, you have put the numbers the wrong way up.

To make a device more efficient, cut down the energy it wastes:

• Lubricate moving parts to reduce friction.
• Insulate hot objects so less energy heats the surroundings.
• Streamline vehicles to reduce air resistance.
• Use wires with low resistance so less energy heats the cables.