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In a circuit, a voltmeter of resistance 10000 ohm and an ammeter of resistance 20 ohm are used to measure a resistance R. The ammeter (in series with R) reads 0.10 A and the voltmeter (across the series R-and-ammeter combination) reads 12 V. Find R.

1. 100 ohm
2. 122 ohm
3. 140 ohm
4. 116 ohm

Correct answer: 100 ohm

Solution

The ammeter drops 0.10*20 = 2 V, so R drops 10 V; with 0.10 A through it, R = 10/0.10 = 100 ohm.

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