| English | Electromotive force |
|---|---|
| Bahasa Melayu | Daya gerak elektrik |
| 中文 | 电动势 |
Definition
The energy supplied by a source to move one coulomb of charge around a complete circuit; measured in volts (V).
In exam terms
Electromotive force (emf), symbol ε, is the energy supplied by a source to drive one coulomb of charge around a complete circuit. It is defined as ε = E/Q and is measured in volts (V), where 1 V = 1 J C⁻¹.
Despite its name, emf is not a force; it is an energy-per-charge quantity. It equals the terminal potential difference only when no current is drawn (open circuit).
Example of correct use: a cell that supplies 6 J of energy to move 2 C of charge has an emf of ε = E/Q = 6 J ÷ 2 C = 3 V.
Do not confuse it with…
Do not confuse emf with terminal potential difference. The emf is the total energy per coulomb given by the source; the terminal p.d. is the energy per coulomb delivered to the external circuit.
Because of internal resistance, V = ε − Ir, so the terminal p.d. is always less than the emf when current flows.
In Paper 2, if asked to define emf you must refer to the energy supplied per unit charge (per coulomb) for a full circuit, not just say “voltage of the battery”.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)