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100g of water is heated from 30°C to 50°C. Ignoring the slight expansion of the water, the change in its internal energy is (specific heat of water is 4184 J/kg/K):

1. 8.4 kJ
2. 84 kJ
3. 2.1 kJ
4. 4.2 kJ

Correct answer: 8.4 kJ

Solution

Change in internal energy equals heat added (negligible expansion): Q = m c dT = 0.1 kg * 4184 J/kg/K * 20 K = 8368 J, about 8.4 kJ.

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