Electromotive Force (Cambridge International A-Level Physics)

Spec point — 10.1.3

Electromotive force

• When charge passes through a power supply such as a battery, it gains energy
• The electromotive force (e.m.f) is the amount of energy transferred per coulomb of charge (C) when charge passes through a power supply
• That energy drives the charge around the complete circuit and is transferred to the components
• e.m.f is measured in Volts (V)

$e.m.f=\frac{\mathrm{energy} \mathrm{transferred}}{\mathrm{charge}}$

• e.m.f is a measure of the energy transferred into the circuit by the power source

• E.m.f is also the potential difference across the cell when no current is flowing

• E.m.f can be measured by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit

Measuring e.m.f in a cell

E.m.f is measured using a voltmeter connected in parallel with the cell

EMF & Potential Difference

• Potential difference is also the energy transferred per unit charge
• But potential difference is a measure of the energy transferred out of the circuit via the components

$\mathrm{potential} \mathrm{difference}=\frac{\mathrm{energy} \mathrm{transferred}}{\mathrm{charge}}$

• When charge passes through a resistor, for example, energy is transferred from the charge (electron) to the resistor

  • The resistor therefore has a potential difference across it

Exam Hint Although voltage and potential difference are the same thing, make sure not to confuse them with e.m.f, which is slightly different!