Terminal PD
The potential difference measured across the terminals of a power source (battery or cell) in a circuit where current is flowing. Given by V = ε − Ir, where ε is the source's EMF, I is the current, and r is the internal resistance. Terminal PD is less than EMF when current flows but equals EMF under open-circuit conditions (I = 0).
Formula
V = ε − Ir
Real World
When you connect a voltmeter directly across a phone charger's output under load, the reading falls below its rated 5 V because current flowing through the charger's internal resistance reduces the terminal PD.
Exam Focus
Explain why terminal PD is less than EMF by explicitly referencing the 'lost volts' (Ir) — simply quoting the formula without explanation will not score explanation marks.
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