AQA A Level Physics复习笔记4.6.3 Efficiency



  • The efficiency of a system is a measure of how well energy is transferred in a system
  • Efficiency is defined as the ratio of the useful power output from a system to its total power input
    • If a system has high efficiency, this means most of the energy transferred is useful
    • If a system has low efficiency, this means most of the energy transferred is wasted


  • What type of energy is useful or wasted depends on the system
    • When electrical energy is converted to light in a lightbulb, the light energy is useful and the heat energy produced is wasted
    • When electrical energy is converted to heat for a heater, the heat energy is useful and the sound energy produced is wasted


  • Efficiency is represented as a percentage, and can be calculated using the equation:


  • Where power is defined as the energy transferred per unit of time


  • The efficiency equation can also be written in terms of energy:


  • The energy can be of any form e.g. gravitational potential energy, kinetic energy

Worked Example

An electric motor has an efficiency of 35 %. It lifts a 7.2 kg load through a height of 5 m in 3 s.Calculate the power of the motor.

Step 1: Write down the efficiency equation


Step 2: Rearrange for the power input  4.6.2-Worked-Example-Power-Input-Equation

Step 3: Calculate the power out

    • The power output is the energy ÷ time
    • The electric motor transferred electric energy into gravitational potential energy to lift the load


Gravitational potential energy = mgh = 7.2 × 9.81 × 5 = 353.16 J

Power = 353.16 ÷ 3 = 117.72 W

Step 4: Substitute values into power input equation 4.6.2-Power-Input-Worked-Example-Answer-1


Exam Tip

Efficiency can be in a ratio or percentage format.If the question asks for efficiency as a ratio, give your answer as a fraction or decimal.If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it: if the ratio = 0.25, percentage = 0.25 × 100 = 25 %