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Efficiency & Power (Edexcel GCSE Physics)

Revision Note

Leander Oates

Last updated

Efficiency & Power

  • 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 energy output from a system to its total energy output

  • 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

 

  • Efficiency can be represented as a decimal or as a percentage

  • Efficiency can be calculated using energy or power   The equations for efficiency are: 

    efficiency space equals space fraction numerator useful space energy space transferred space by space the space device over denominator total space energy space supplied space to space the space device end fraction   efficiency space equals space fraction numerator useful space power space output over denominator total space power space input end fraction

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: List the known quantities

  • Efficiency = 35%

  • Mass, m = 7.2 kg

  • Height, h = 5 m

  • Time, t = 3 s

  • Gravitational field strength, g = 10 N/kg 

Step 2: Write down the efficiency equation 

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

Step 3: Rearrange to make power input the subject

useful space power space input space equals space fraction numerator power space output space cross times space 100 over denominator efficiency space end fraction  

Step 4: Calculate the power output

P space equals fraction numerator space E over denominator t end fraction

  • Energy was transferred from the kinetic store of the motor to the gravitational potential store of the load

increment G P E space equals space m space cross times space g space increment h

increment G P E space equals space 7.2 space cross times space 10 space cross times space 5

increment G P E space equals space 360 space straight J 

  • Therefore, the power output is

P space equals 360 over 3

P space equals space 120 space straight W 

Step 5: Substitute values into power input equation

useful space power space input space equals space fraction numerator power space output space cross times space 100 over denominator efficiency space end fraction

useful space power space input space equals space fraction numerator 120 space cross times space 100 over denominator 35 space end fraction

useful space power space input space equals space 343 space straight J

Examiner Tips and Tricks

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

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 (just the % sign)

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    Leander Oates

    Author: Leander Oates

    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.