ASPhysicsEdexcelRevision Notes3. Electric CircuitsCurrent, Potential Difference, Resistance & PowerElectrical Power

Electrical Power (Edexcel AS Physics): Revision Note

Exam code: 8PH0

Written by: Joanna

Updated on 4 May 2026

Electrical Power

Work is 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

$V = \frac{W}{Q}$

Current is the rate of flow of charge

$I = \frac{Q}{t}$

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

$W = V I t$

Power P is defined as:

The rate of doing work

In equation form, this can be written as:

$P = \frac{W}{t}$

Substituting the equation for work done by an electrical component gives:

$P = \frac{V I t}{t} = V I$

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

$P = I V$

Where
- $P$ = power dissipated by the component (W)
- $I$ = current in the component (A)
- $V$ = potential difference across the component (V)
Substituting V = IR into the power equation allows us to write power in terms of resistance R

$P = I^{2} R$

$P = \frac{V^{2}}{R}$

The squared current and voltage in these equations mean that:
- for a given resistance, if the current or voltage doubles, the power will quadruple
- for a given power, if the resistance quadruples, then the current will be halved, but the voltage will be doubled

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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Electrical Power (Edexcel AS Physics): Revision Note

Electrical Power

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Author: Joanna