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Two cells of emf approximately 5 V and 10 V are to be compared accurately using a potentiometer whose wire is 400 cm long. Which of the following arrangements is correct?

1. The driver battery should have an emf of 8 V.
2. The driver battery may have an emf of 15 V, with a series resistance adjusted so that the potential drop across the whole wire slightly exceeds 10 V.
3. The first 50 cm of the wire alone should have a potential drop of 10 V.
4. A potentiometer is meant for comparing resistances, not voltages.

Correct answer: The driver battery may have an emf of 15 V, with a series resistance adjusted so that the potential drop across the whole wire slightly exceeds 10 V.

Solution

The driver voltage must be larger than the largest cell emf and the series resistance is tuned so the wire's total drop is just over 10 V, allowing both cells to balance on the wire.

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