Power & Efficiency

Test yourself


The "energy transferred per second" is equivalent to saying "the rate of energy transfer". Both are definitions for power.

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Rate of energy transfer

  • The rate at which energy is transferred from one form to another is called power.
  • This is equivalent to saying the energy transferred per second.
  • The energy transferred is also known as the work done, W.
    • So, power can also be described as the work done per second (or the rate of doing work).
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  • Power is the rate of doing work.
    • Power = work done ÷ time taken
    • P=ΔWΔtP = \frac{\Delta W}{\Delta t}
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  • Power is measured in Watts (W).
    • 1 Watt is defined as 1 Joule transferred per second.
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Force × velocity

  • Power is also equal to the force applied times the velocity of the object the force is applied to.
    • P=FvP = F v
  • This equation can be more useful if you're given the speed and not the distance travelled.


When energy is transferred, there are some losses. For example, some energy may be dissipated as heat or sound.

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Useful energy

  • Efficiency is the fraction of energy that is used for something useful.
    • The higher the efficiency, the better.
  • Efficiency is calculated by:
    • Efficiency = useful output powerinput power\frac{useful\space output\space power}{input \space power}
  • A kettle supplied with 50 W of power and converts 40 W to heat water has an efficiency of 4050=0.8\frac{40}{50} = 0.8.
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  • Efficiency can also be expressed as a percentage:
    • Efficiency (%) = useful output powerinput power×100\frac{useful\space output\space power}{input \space power} \times 100
  • A kettle supplied with 50 W of power and converts 40 W to heat water has an efficiency of 80%.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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