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Two cells of equal emf 10 V but unequal internal resistances 20 ohm and 5 ohm are joined in series and then connected to a parallel combination of a 30 ohm resistor and a resistor R. If the terminal voltage across the cell with internal resistance 20 ohm is zero, find R (in ohm).

1. 30
2. 15
3. 20
4. 10

Correct answer: 30

Solution

Zero terminal voltage across the 20 ohm cell means its emf is entirely dropped across its own internal resistance, fixing the current at 0.5 A, which then determines R = 30 ohm.

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