Efficiency (OCR A Level Physics)

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Efficiency

  • The efficiency of a system is a measure of how successfully 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

  • Determining which 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 thermal energy in 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:

Efficiency equation 2, downloadable AS & A Level Physics revision notes

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

Power Equation

  • 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 

Efficiency equation

Step 2: Rearrange for the power input  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 Power Input Worked Example Answer

Examiner Tip

Efficiency can be in a ratio (between 0 and 1) or percentage format (between 0% and 100%). 

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 %.

Remember that efficiency has no units. It is a ratio with both the numerator and denominator with the same units.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.