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The electromotive force (emf) $\varepsilon$ of a cell is best described as
Athe force the cell exerts on a unit charge
Bthe potential difference across the terminals while delivering current
Cthe product of internal resistance and current
Dthe potential difference across the terminals in open circuit
Answer & Solution
Correct answer: D. the potential difference across the terminals in open circuit
1. With no current ($R$ infinite), $I = 0$ and no drop occurs across the internal resistance.
2. Then the terminal potential difference equals the full emf: $V = \varepsilon$.
3. So emf is the open-circuit terminal potential difference, despite its name; it is not a force.
4. Option B describes terminal voltage on load; C describes the internal drop only.
_Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.13_
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