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A calorimeter coil of resistance R1 = 60 ohm is connected in parallel with a resistor R2 = 30 ohm, and the ammeter in the main line reads 3 A. The coil R1 heats 240 g of water for 7 minutes. Neglecting the battery and ammeter resistance, all heat losses, and the heat capacities of the calorimeter and resistor (specific heat of water = 4200 J/kg/degC), find the rise in temperature of the water (in degC).

1. 25
2. 50
3. 12.5
4. 75

Correct answer: 25

Solution

The parallel combination has resistance 20 ohm, giving 60 V across it; the current through R1 is 1 A, so it dissipates 60 W. Over 420 s that delivers 25200 J, raising 0.24 kg of water by 25 degC.

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