- 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
- Where power is defined as the energy transferred per unit of time
- The energy can be of any form e.g. gravitational potential energy, kinetic energy
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
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
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 %