EMF
The total energy per unit charge converted from chemical (or other) energy to electrical energy by a source such as a battery or cell, measured in volts (V). EMF (ε) is the 'open circuit' potential difference when no current flows. In a circuit with internal resistance, the terminal PD (V) is less than EMF: V = ε − Ir, where r is internal resistance and I is current.
Formula
V = ε − Ir
Real World
A standard AA alkaline cell (e.g. Duracell) has an EMF of 1.5 V, but under the load of a high-drain device like a digital camera the terminal voltage drops noticeably as internal resistance dissipates energy.
Exam Focus
Never confuse EMF with terminal PD — state that EMF is measured only at open circuit (I = 0) to secure definition marks.
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