Electromotive force (EMF)
EMF & Internal Resistance — AQA A-Level Physics
Key Definition
Electromotive force (EMF) — The electrical energy transferred per unit charge by the source. Unit: volt (V).
$$\varepsilon = \frac{E}{Q}$$
- Where:
- $\varepsilon$ = electromotive force (V)
- $E$ = energy transferred by the source (J)
- $Q$ = charge (C)
- EMFElectromotive force. The energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). by a source in driving chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). around a complete circuit. Measured in volts (V). is NOT a force. It is an energyThe capacity to do work. Measured in joules (J).-per-chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). quantity, measured in volts.
- EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). describes what the source gives to each coulombThe SI unit of charge. One coulomb is the charge transferred by a current of 1 A in 1 second. of charge.
- Potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). describes what each coulombThe SI unit of charge. One coulomb is the charge transferred by a current of 1 A in 1 second. gives to a component.
- EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is the total energy supplied; p.d. is the energy transferred in the external circuit.
Examiner Tips and Tricks
- AQA wants you to say 'energy per unit charge produced by the source' — not 'voltageThe energy transferred per unit charge between two points. Measured in volts (V). Informal term for potential difference. of the battery'.
- The word 'produced' (or 'converted to electrical') is what earns the mark.
Common Mistake
MEDIUM
Students often: Avoid saying EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is the 'force that pushes charge around'.
Instead: EMF is energy per unit charge, not a force. The name is misleading — ignore the word 'force'.
Instead: EMF is energy per unit charge, not a force. The name is misleading — ignore the word 'force'.