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A liquid-nitrogen vessel is built from a 1 cm thick thermocol sheet of thermal conductivity 0.025 J/m-°C-s. It holds liquid nitrogen at 80 K, with a total wall area of 0.80 m² in contact with the nitrogen. The surroundings are at 300 K. Find the rate of heat flow into the liquid nitrogen.

1. 400 W
2. 440 W
3. 480 W
4. 500 W

Correct answer: 440 W

Solution

Plugging into Q/t = K A ΔT / L gives 0.025*0.80*220/0.01 = 440 W.

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