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A calorimeter of negligible heat capacity holds 100 cc of water at 40 deg C, which cools to 35 deg C in 5 minutes. The water is then replaced by an equal volume of kerosene oil at 40 deg C. Find the time for the kerosene to cool from 40 deg C to 35 deg C. (Densities: water 1000, kerosene 800 kg/m³; specific heats: water 4200, kerosene 2100 J/kg-degC.)

1. 1 min
2. 2 min
3. 3 min
4. 4 min

Correct answer: 2 min

Solution

Heat content (m*c) for kerosene is 168 vs 420 for water, and with the same heat-loss rate the time scales in that ratio, giving 5*(168/420) = 2 min.

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