Which of the following situations is allowed by the Second Law of Thermodynamics

Question

Which of the following situations is allowed by the Second Law of Thermodynamics?

a. Designing a heat engine that exploits the the temperature differences at the top and bottom of the oceans.

b. Converting 100 joules of heat energy into 100 joules fo work.

C. Converting 100 joules of work into 100 joules of heat energy.

d. Removing heat from cold outside air to heat the inside of a house in winter.

e. Eliminating 100% of the waste heat from a power plant with super-insulating materials.

Explain.

Explanation / Answer

The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines.

a. Allowed. Since heat flows from higher temp to lower temp, we can tap this heat in an ocean but with less effiiciency.

b. Not allowed.  It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.

c. Allowed. Work can be converted to heat without any loss

d. Allowed. But it is not 100% efficient. Some work has to be done to do so.

e. Not allowed. Even the best insulating materials will lose some heat. Some work has to be done

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