CIE AS Physics (9702) exams from 2022

Revision Notes

5.1.3 Efficiency

Efficiency of a System

  • The efficiency of a system is the ratio of the useful energy output from the system to the total energy 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
  • Multiplying this ratio by 100 gives the efficiency as a percentage

Efficiency equation, downloadable AS & A Level Physics revision notes

Efficiency equation in terms of energy

 

  • Efficiency can also be written in terms of power (the energy per second):

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

Efficiency equation in terms of power

 

Worked example

Worked example - efficiency of a system, downloadable AS & A Level Physics revision notes

Exam Tip

Efficiency can be in a ratio or percentage format. If the question asks for an 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, e.g. Ratio = 0.25, Percentage = 0.25 × 100 = 25 %

Solving Problems Involving Efficiency

  • Recall the two equations for calculating efficiency are:

Efficiency equation, downloadable AS & A Level Physics revision notes

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

  • Which to use will depend on whether you’re given a system calculating energies or power as shown in the examples below

 

Worked example

Worked example - efficiency hydraulic ram, downloadable AS & A Level Physics revision notes

  • The pump is what converts the water’s kinetic energy into gravitational potential energy. Since its efficiency is 20%, you would multiply the kinetic energy by 0.2 since only 20% of the kinetic energy will be converted (not 20% of the gravitational potential energy)

 

Exam Tip

Equations for kinetic and potential energies are important for these types of questions. Also familiarise yourself with the different equations for power depending on what quantities are given.

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