Revision NotesExam Questions
IGCSEPhysics: Co-ordinated Sciences (Double Award)CIERevision Notes1. Motion, Forces & EnergyEnergy, Work & PowerEfficiency

Efficiency (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note

Leander

Author

Leander

Last updated

Did this video help you?

Efficiency of energy transfer

  • The efficiency of a system is a measure of the amount of useful and wasted energy in an energy transfer

 

  • Efficiency is defined as:

The ratio of the useful power or energy output from a system to its total power or 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

 

  • The overall efficiency of a typical thermal power station is approximately 30%
    • This means that 70% of the energy transferred from the power station to the National Grid is wasted energy

 

  • In the production of electricity:
    • Energy is used to heat water to produce steam
    • The steam turns a turbine
    • The turbine turns a generator
    • The generator produces electricity 
      • At each stage of this process, energy is dissipated to the surroundings

 

Sankey diagram of electricity production

cie-1-8-6-sankey-diagram-of-power-station-efficiency

Sankey diagram showing the energy transfers involved in generating electricity in a gas-fired power station

Calculating efficiency

Extended tier only

  • Efficiency is represented as a percentage, and can be calculated using two equations

  • Efficiency in terms of energy:

efficiency space equals space fraction numerator useful space energy space output over denominator total space energy space input end fraction space open parentheses cross times 100 percent sign close parentheses

  • Efficiency in terms of power:

efficiency space equals space fraction numerator useful space power space output over denominator total space power space output end fraction space open parentheses cross times 100 percent sign close parentheses

Did this video help you?

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.

 

Answer:

Step 1: Write down the efficiency equation in terms of power

e fficiency space equals space fraction numerator useful space power space output over denominator total space power space input end fraction space open parentheses cross times 100 percent sign close parentheses

Step 2: Rearrange to make power input the subject

power space input space equals fraction numerator space power space output over denominator efficiency space divided by space 100 end fraction  OR  power space input space equals space fraction numerator power space output space cross times space 100 over denominator efficiency end fraction

Step 3: Calculate the power output

power space output space equals space fraction numerator increment E over denominator t end fraction

  • increment E is equal to the change in gravitational potential energy as the load is lifted

increment E subscript P space equals space m g increment h

increment E subscript P space equals space 7.2 space cross times space 9.8 space cross times space 5

increment E subscript P space equals space 352.8 space straight J

  • Therefore:

power space output space equals space fraction numerator 352.8 over denominator 3 end fraction

power space output space equals space 117.6 space straight W

Step 4: Substitute the values into the power input equation 

power space input space equals space fraction numerator 117.6 space over denominator 0.35 end fraction  OR  power space input space equals space fraction numerator 117.6 space cross times space 100 over denominator 35 end fraction

power space input space equals space 336 space straight W

Examiner Tip

Efficiency can be given 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

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:PowerNext:Energy from the Sun
    Leander

    Author: Leander

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.