Efficiency (Cambridge O Level Physics)

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Dan MG

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Dan MG

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Efficiency of Energy Transfer

  • The efficiency of a system is a measure of the amount of 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 a Power Station

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

Sankey diagram showing the efficiency of a gas-fired power station

Calculating Efficiency

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

efficiency = useful energy outputtotal energy input × 100%{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

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

efficiency = useful power outputtotal power input × 100%{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

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

P = Et{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

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 

Efficiency space equals space fraction numerator useful space power space output over denominator total space power space input end fraction cross times 100 percent sign

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

    • ΔE 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

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Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.