An engineer is evaluating the performance of an electrical machine. Which expression gives the percentage efficiency of the machine?
total power outputtotal power input×100%\frac{\text{total power output}}{\text{total power input}} \times 100\%total power inputtotal power output×100%
useful power outputtotal power input×100%\frac{\text{useful power output}}{\text{total power input}} \times 100\%total power inputuseful power output×100%
wasted power outputtotal power input×100%\frac{\text{wasted power output}}{\text{total power input}} \times 100\%total power inputwasted power output×100%
useful power outputwasted power output×100%\frac{\text{useful power output}}{\text{wasted power output}} \times 100\%wasted power outputuseful power output×100%
533 exam-style questions on CIE IGCSE Physics 1.7 Energy, work and power, covering 1.7.1a Energy stores and energy transfers, 1.7.1b Conservation of energy, 1.7.1c Kinetic and gravitational potential energy, 1.7.2 Work done, 1.7.3 Energy resources, 1.7.3a Energy resources and electricity generation, 1.7.3b Nuclear fusion as an energy source, 1.7.3c Efficiency of energy transfer, and 1.7.4 Power. Each one has a worked solution and a mark scheme showing where the marks go.