Electrical Power (Edexcel AS Physics): Revision Note

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

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

• Current is the rate of flow of charge

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

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

$W=VIt$

• Power P is defined as:

The rate of doing work

• In equation form, this can be written as:

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

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

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

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

$P=IV$

• 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