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A heater delivering 60 W keeps an object at a steady 50 deg C while the surroundings are at 20 deg C. The heater is then turned off. Using Newton's law of cooling, find the rate at which the object loses heat once its temperature has fallen to 30 deg C.

1. 20 W
2. 30 W
3. 40 W
4. 60 W

Correct answer: 20 W

Solution

From steady state k = 60/30 = 2 W per deg C; at 30 deg C the difference is 10 deg C, so loss = 2 * 10 = 20 W.

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