The electromotive force of a cell is
Correct answer: B. the energy supplied by the cell per unit charge driven round the whole circuit
- A. the same as the potential difference across its terminals when supplying current
- B. the energy supplied by the cell per unit charge driven round the whole circuit
- C. the force exerted on the electrons in newtons
- D. always smaller than the terminal potential difference
Explanation
Despite its name, emf is an energy per unit charge measured in volts, not a force at all. It exceeds the terminal potential difference whenever current flows, because some energy is dissipated inside the cell against its internal resistance. Only on open circuit, with no current, do emf and terminal voltage become equal.
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About Internal Resistance of Sources
Internal resistance is the opposition to current inside a cell or other source, causing terminal voltage to fall below its emf when current flows. Questions use E = V + Ir, current in complete circuits, lost volts, source combinations, and the difference between internal resistance and the external load resistance.
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