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Electrical Power (Edexcel International A Level (IAL) Physics) : Revision Note

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Electrical Power

  • Work is also defined as a transfer of energy

  • When components transfer electrical energy to other stores, work must be done because energy is transferred

  • Therefore, potential difference is the work done per unit charge

V italic space italic equals italic space W over Q

  • Current is the rate of flow of charge

I italic space italic equals italic space Q over t

  • These equations can be combined to give the work done by a component in an electric circuit

W space equals space V I t space

  • Power is the rate of doing work

P italic space italic equals italic space W over t

  • This gives us

P italic space italic equals italic space fraction numerator V I t over denominator t end fraction italic space italic equals italic space V I

  • Therefore, the power dissipated (produced) by an electrical device is given by:

Power equation, downloadable AS & A Level Physics revision notes

  • Using V = IR to rearrange for either V or I and substituting into the power equation means we also write power in terms of resistance R

Power in terms of resistance, downloadable AS & A Level Physics revision notes

  • Since current and voltage are both squared in these equations, this means that

    • for a given resistance, if the current or voltage doubles, the power will increase by a factor of four

    • for a given power, if the resistance doubles, the current will decrease by a factor of square root of 2, and the voltage will increase by a factor of square root of 2

Worked Example

Two lamps are connected in series to a 150 V power supply.

WE - power question image, downloadable AS & A Level Physics revision notes

Which statement is correct?

A. Both lamps light normally

B. The 15 V lamp blows

C. Only the 41 W lamp lights

D. Both lamps light at less than their normal brightness

Answer: A

Worked example - power (2), downloadable AS & A Level Physics revision notes

Examiner Tips and Tricks

You can use the mnemonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations.

Which equation to use will depend on whether the value of current or voltage has been given in the question.

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    Joanna

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    Joanna obtained her undergraduate degree in Natural Sciences from Cambridge University and completed her MSc in Education at Loughborough University. After a decade of teaching and leading the physics department in a high-performing academic school, Joanna now mentors new teachers and is currently studying part-time for her PhD at Leicester University. Her passions are helping students and learning about cool physics, so creating brilliant resources to help with exam preparation is her dream job!