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The resistance of an iron wire is \( 10 \Omega \) and its temperature coefficient of resistance is \( 5 \times 10^{-3} /^{o} C . \) A current of \( 30 \mathrm{mA} \) is flowing in it at \( 20^{\circ} \mathrm{C} \). Keeping potential difference across its ends constant, if its temperature is increased to \( 120^{\circ} \mathrm{C} \) then the current flowing in the wire will be (in \( \mathrm{mA} \) )

1. 20
2. 35
3. 10
4. 40

Correct answer: 35

Solution

At constant potential difference, current is inversely proportional to resistance. The resistance increases with temperature by the factor \(1+\alpha\Delta T\), so the current must decrease accordingly; this gives 35 mA.

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