NDA (II) 2017 General Ability Test · Question 127 of 150

Correct — C, the second law of thermodynamics. The sentence printed in the stem is the Clausius statement of the second law, which says that no process can have as its sole result the transfer of heat from a colder body to a hotter one. The two words that carry the whole weight are 'by itself'. Heat certainly can be pushed uphill — that is exactly what a refrigerator and an air conditioner do — but only because an external agency does work on the system. Left to itself, heat always flows from hot to cold, and that one-way character of natural processes is what the second law asserts.

• (a)Zeroth law of thermodynamics — The zeroth law says that if two bodies are each in thermal equilibrium with a third, they are in thermal equilibrium with one another. It is the law that makes temperature a meaningful quantity and lets a thermometer work; it says nothing about the direction of heat flow.
• (b)First law of thermodynamics — The first law is conservation of energy applied to heat — the heat supplied to a system equals the increase in its internal energy plus the work it does. It is scrupulously neutral about direction: it would be perfectly happy with heat flowing from cold to hot, provided the energy books balanced. Ruling that out needs the second law.
• (d)Third law of thermodynamics — The third law concerns behaviour near absolute zero — the entropy of a perfect crystal tends to zero as the temperature tends to 0 K, and absolute zero cannot be reached in a finite number of steps. It is a statement about a limiting temperature, not about the direction of everyday heat flow.

The four laws of thermodynamics divide up cleanly. The zeroth defines temperature through thermal equilibrium. The first is energy conservation. The second fixes the direction in which natural processes run and is the reason no engine can convert heat into work with perfect efficiency. The third describes the unreachable limit at absolute zero. The second law has two classic equivalent forms — the Clausius statement about heat flow, quoted in this question, and the Kelvin-Planck statement that no engine working in a cycle can take heat from a single reservoir and turn all of it into work.

Almost every question on this topic is really a matching exercise, so learn one identifying phrase for each law. Direction of heat flow, entropy, efficiency of engines, irreversibility — all four belong to the second law. Energy in equals energy out belongs to the first. Thermometers and equilibrium belong to the zeroth. Absolute zero belongs to the third. The refrigerator is the standard follow-up: it does not violate the second law, because the heat it moves uphill is paid for by the electrical work of the compressor.

• The Clausius statement of the second law: no process can have as its sole result the transfer of heat from a colder to a hotter body.
• The Kelvin-Planck statement is the equivalent form: no cyclic engine can convert heat from a single reservoir entirely into work.
• The first law is conservation of energy — heat supplied equals change in internal energy plus work done.
• The zeroth law establishes thermal equilibrium and hence the concept of temperature.
• The third law places the entropy of a perfect crystal at zero as the temperature approaches absolute zero.

• Reading the refrigerator as a violation of the second law, when it is the external work that makes the uphill transfer possible.
• Attributing the direction of heat flow to the first law, which is silent on direction.
• Mixing up the zeroth law with the third; the zeroth is about equilibrium, the third about absolute zero.

Thermodynamics usually appears as a one-line statement to be matched to the right law, or as a pair of statements about a refrigerator or a heat engine.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which law of thermodynamics makes the concept of temperature meaningful by defining thermal equilibrium?

  • (a)Zeroth law
  • (b)First law
  • (c)Second law
  • (d)Third law

  Answer(a) Zeroth law — two bodies each in equilibrium with a third are in equilibrium with each other, which is why a thermometer works.

• practice — not a real PYQ

  A domestic refrigerator transfers heat from its cold interior to the warmer kitchen. This does not violate the second law of thermodynamics because

  • (a)the interior is not perfectly insulated
  • (b)external work is supplied to the compressor to drive the transfer
  • (c)the second law applies only to gases
  • (d)the heat transferred is very small

  Answer(b) external work is supplied to the compressor — the law forbids the uphill transfer only as the sole result of a process.