Energy and efficiency — Engineering, 11–13 years
Devices receive energy and change it into useful movement, light, sound or heating. Efficiency compares the useful output with the energy supplied, because some energy spreads into unwanted heating, sound or friction.
Useful energy versus supplied energy
A machine does not usually turn all the energy it receives into the result we want. An electric motor may make movement, but some energy warms its wires and bearings. Efficiency tells us how much of the supplied energy becomes useful output, usually as a fraction or percentage.
Why engineers care about losses
Energy was once treated as something machines could waste without much concern. As engines, homes and factories grew, wasted energy meant more fuel, cost and pollution. Efficiency became a way to compare designs and find where insulation, lubrication or better parts could reduce losses.
Finding an efficiency
A small motor receives 100 joules of electrical energy and gives 70 joules of useful movement. Efficiency is useful energy divided by supplied energy: 70 ÷ 100 = 0.70. Multiply by 100 to write it as a percentage, so the motor is 70% efficient and 30 joules became unwanted heat or sound.
Energy is not destroyed by a machine
It is tempting to say that the missing energy has vanished when a machine is inefficient. In fact, energy is transferred into other forms, often heat, sound or vibration that we do not want. Calling it “lost” is useful shorthand, but it does not mean the energy has ceased to exist.
Making everyday devices waste less
A kettle, bicycle, car and house can all be improved by looking for energy that does not help the job. Thick insulation slows heat escaping, oil reduces friction in moving parts, and LED lamps turn more electrical energy into light than old filament lamps. Engineers improve efficiency without pretending losses can reach zero.
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