Electrical Energy & Power (AQA A Level Physics): Revision Note

Energy & power

• In mechanics, power P is defined as the rate of doing work

  • The potential difference is the work done per unit charge

  • Current is the rate of flow of charge

• Therefore, the electrical power is defined as the rate of change of work done:

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

• Where:

  • $P$ = power (W)

  • $E$ = energy transferred (J)

  • $t$ = time (s)

  • $W$ = work done (J)

• The work done is the energy transferred so the power is the energy transferred per second in an electrical component

• The power dissipated (produced) by an electrical device can also be written as

$P=IV$

• Where:

  • $P$ = power (W)

  • $I$ = current (A)

  • $V$ = potential difference (V)

• Using Ohm's Law $V=IR$ to rearrange for either $V$ or $I$ and substituting into the power equation, means power can be written in terms of resistance $R$

$P=I^{2}R$

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

• Where:

  • $R$ = resistance (Ω)

• This means for a given resistor if the current or voltage doubles the power will be four times as great

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

• Rearranging the energy and power equation, the energy can be written as:

$E=VIt$

• Where:

  • $E$ = energy transferred (J)

  • $V$ = voltage (V)

  • $I$ = current (A)

  • $t$ = time (s)