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

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

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 is

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

This gives us

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

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

Power equation in terms of resistance

The squared value for both current and voltage in these equations means that, for any given resistance, if the current or voltage doubles, the power will be four times as great
Conversely, for a given power, if the resistance doubles then current will be 4 times less but voltage will be 4 times greater

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 Tip

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.

Electrical Power (Edexcel A Level Physics)

Revision Note