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12 years ago
On a cold winter day, the outside temperature is -20°C and the inside temperature is maintained at 20°C. There is a net heat flow to the outside through the walls, roof, etc., of 25 kW. What is the rate of increase of the entropy of the universe as a result of this?
A) 13 W/K
B) 25 W/K
C) 85 W/K
D) 99 W/K
E) 0 W/K
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9 years ago
The rate of entropy change by the house is given by:

dS_house/dt = (1/T_house)*dq_house/dt

where dq_house = -25kW

Similarly, the rate of entropy change of the surroundings is given by:

dS_surr/dt = (1/T_surr)*dq_surr/dt

where dq_surr/dt = +25kW

The rate of entropy change of the universe is the sum of these:

dS_tot/dt = dS_surr/dt + dq_surr/dt = 25,000W*(1/T_surr - 1/T_house)

dS_tot/dt = 25,000W*(1/253.15K - 1/293.15K) = 13.48W/K

Answer (C) is the correct answer (to 2 significant figures)
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