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An iron wire has resistance 10 ohm and a temperature coefficient of resistivity of 5 x 10⁻³ per deg C. At 20 deg C it carries a current of 30 mA when a fixed potential difference is applied across its ends. Keeping that potential difference unchanged, the wire is heated to 120 deg C. What current (in mA) now flows through it?

1. 20
2. 15
3. 10
4. 40

Correct answer: 20

Solution

Raising the temperature increases resistance; at fixed voltage the current falls in inverse proportion.

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