4 · IGCSE Physics
Energy transfers and efficiency
Choose a device and watch its Sankey diagram redraw: arrow widths are proportional to energy, so you can see how much of the input ends up as useful or wasted energy as you change the efficiency, power and time.
E(in) = P × t
efficiency = useful energy out ÷ total energy in
(× 100 for a percentage)
E(in) = E(useful) + E(wasted)
Try this
- Choose the lamp and compare η = 5 % (filament) with η = 40 % (LED): how many times wider is the useful light arrow?
- Double the time t for any device. Check that every energy doubles but the efficiency does not change.
- Set the car engine to 25 %. Predict how many joules are wasted for every 1 J of useful kinetic energy, then check the readouts.
Exam tip: on any Sankey diagram check that the input equals the sum of all the outputs, and remember efficiency has no unit and can never be more than 1 (100 %).