The Efficiency Engine

Power in, useful out

Every appliance does the same thing: it takes in energy and passes it on. Two numbers tell you almost everything about it. Its power tells you how fast energy flows in. Its efficiency tells you what fraction of that energy comes out doing the job you want. The rest is not destroyed, because energy never is; it spreads out into the surroundings, usually by heating them, where it is no use to anyone. Cutting that waste means blocking the way it escapes. And because the energy you pay for is power multiplied by time, these two numbers also decide what the appliance costs to run.

Words for using energy

Five terms for every efficiency and power question.

Order the efficiency method

Put the steps for finding the efficiency of an appliance in order.

• Find the total energy supplied to the appliance
• Find how much of it ends up as useful energy
• Write the useful energy as a fraction of the total
• Turn the fraction into a percentage
• Check the answer is below 100 per cent

Useful or wasted?

Decide whether each transfer is useful or wasted for the job the appliance does. You have three lives.

A motor, worked through

An electric motor is supplied with 800 J of energy and transfers 600 J usefully, lifting a load. As a fraction of the total, the useful part is 600 out of 800, which is 0.75. As a percentage that is 75 per cent. The other 200 J is not lost from existence. It has spread out, mostly by heating the motor and the air around it, where it does nothing useful. That is why the answer can never reach 100 per cent.

Rate the kettle

An electric kettle is supplied with 50 000 J of energy, and 40 000 J of it ends up in the thermal store of the water. What is the efficiency of the kettle, as a percentage?

Two ways energy escapes

Most wasted energy leaves by one of two routes, and each has its own kind of fix.

Match each fix to what it saves

What actually helps?

An engineer wants an electric motor to be more efficient. Which change would do that?

• Lubricating its bearings so less energy goes into warming them
• Supplying it with more energy every second
• Running it for longer each day
• Painting its case a brighter colour

Units at every step

In a calculation, the examiner follows your working line by line. Write the relationship in words first, then put in the numbers, then give the answer with its unit. Keep the units matched before you combine anything: if one quantity is in thousands and the other is not, convert first. A number with no unit, or the wrong one, loses a mark even when the arithmetic is right.

Complete the power facts

Power is the rate of energy transfer, measured in _____. One watt means one _____ of energy transferred every second. So a 2 kW heater transfers 2000 joules every _____. The energy a home uses is usually billed in _____, the energy a 1 kW appliance transfers in an hour.

Cost the heater, line by line

A 2 kW electric heater is switched on for 3 hours. Electricity costs 30p per kilowatt-hour, and the heater is 90 per cent efficient.

True of power and efficiency

Select the TWO statements that are true.

• No real appliance is 100 per cent efficient
• A 2 kW heater transfers energy twice as fast as a 1 kW heater
• A more powerful appliance is always more efficient
• Wasted energy is destroyed

Find the two slips

A student wrote five statements. Tap the TWO that are wrong.

• The motor is 75 per cent efficient, so a quarter of its input is wasted.
• The new lamp is 120 per cent efficient.
• A 60 W bulb transfers 60 joules every second.
• The wasted energy simply disappears.
• Insulating the loft slows the loss of energy by heating.

Choose wisely

Use power and efficiency to make each decision.

• Two kettles cost the same. One is 90 per cent efficient and the other 80 per cent. Which wastes less energy boiling the same water?
• A 1 kW heater runs for 4 hours and a 2 kW heater runs for 1 hour. Which uses more energy?
• A hot water tank in a cold loft loses a lot of energy. Which change would cut the loss most?
• An advert claims a new motor is 110 per cent efficient. What do you conclude?

Explain the efficiency engine

Explain how the power and the efficiency of an appliance together decide how much useful energy it delivers and how much it costs to run. Use the 2 kW, 90 per cent efficient heater as your example, and suggest how wasted energy could be reduced.

• What power tells you, and what efficiency tells you
• The energy used in kilowatt-hours, and its cost
• How much of that energy is useful
• Where the wasted energy goes, and one way to reduce it