Electromotive Force (Cambridge (CIE) AS Physics)

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 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

• 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