Engineering efficiency — Engineering, 14–17 years
Machines transform energy, but some input spreads into heat, sound or friction. Efficiency compares the useful output with the energy supplied.
Useful output is not all the input
Efficiency asks how much of the supplied energy reaches the job we wanted. A motor may receive electrical energy but deliver motion; the rest can warm its wires, make noise or overcome friction. Efficiency is usually written as useful energy divided by input energy, often as a percentage.
The problem of wasted energy
Engineers need to compare devices fairly and limit energy that does no useful work. A kettle, motor or power station can seem powerful while wasting much of its input as heat. Efficiency gives one clear measure, helping engineers find losses and decide whether an improvement is worth its cost.
Calculating a motor’s efficiency
A motor takes in 500 J of electrical energy and gives 350 J of useful lifting energy. Divide useful output by input: 350 ÷ 500 = 0.70. Multiply by 100, giving 70% efficiency. The remaining 150 J was not useful for lifting, though it may have become heat or sound.
The trap: confusing power with efficiency
A device that uses more energy each second can be more powerful, but not necessarily more efficient. It is reasonable to confuse them because both sound like “doing more”. Power measures the rate of energy transfer; efficiency measures the fraction that becomes useful, so the two need separate comparisons.
Choosing and improving devices
Efficiency matters in electric motors, lamps, vehicles and insulated buildings. A more efficient motor can do the same job with less electricity, while better insulation keeps useful heat indoors. It matters when the lost energy costs money or fuel, but efficiency alone does not describe every important feature.
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