Thermodynamics — Set 2
Physics · ऊष्मागतिकी · Questions 11–20 of 40
When an ideal gas expands freely into a vacuum, what is the work done by the gas?
Correct Answer: C. It is zero
In free expansion into vacuum, external pressure is effectively zero, so the gas does no work. Work is W = ∫ Pext dV, which becomes zero here. The process is irreversible even though work is zero.
For a reversible adiabatic process of an ideal gas, which relation is correct?
Correct Answer: B. PV^γ = constant
In a reversible adiabatic process, pressure and volume follow PV^γ = constant for an ideal gas. This comes from combining the first law with the condition Q = 0. The exponent γ depends on the gas heat capacities.
What is the efficiency of a Carnot engine operating between hot reservoir Th and cold reservoir Tc (in kelvin)?
Correct Answer: B. η = 1 - Tc/Th
A Carnot engine is a reversible engine, and its efficiency depends only on reservoir temperatures. The formula is η = 1 - Tc/Th when temperatures are in kelvin. No real engine can exceed this efficiency between the same reservoirs.
The coefficient of performance (COP) of a refrigerator is defined as which ratio?
Correct Answer: D. Qc/W
A refrigerator’s COP is the heat removed from the cold space divided by work input. This is written as COP = Qc/W. A higher COP means more cooling for the same work.
Which statement matches the Kelvin-Planck form of the second law of thermodynamics?
Correct Answer: B. No cyclic engine can convert all heat from a single reservoir into work
Kelvin-Planck statement says complete conversion of heat into work in a cycle from one reservoir is impossible. Some heat must be rejected to a colder reservoir. This rules out a perfect heat engine with 100% efficiency.
Which statement matches the Clausius form of the second law of thermodynamics?
Correct Answer: B. Heat cannot spontaneously flow from a colder body to a hotter body without external work
Clausius statement says heat cannot by itself flow from cold to hot. To move heat from cold to hot, external work is needed, as in a refrigerator. This is why refrigerators require electricity or some work input.
For a reversible process, how is the infinitesimal change in entropy defined?
Correct Answer: C. dS = δQrev/T
Entropy change for a reversible process is defined by dS = δQrev/T. Here δQrev is the reversible heat transfer and T is absolute temperature. This definition makes entropy a state function.
For a reversible process, what is the total entropy change of the universe?
Correct Answer: C. It is zero
In a reversible process, entropy produced is zero, so the universe has no net entropy change. The system and surroundings exchange entropy without creating it. Irreversible processes make the total entropy change positive.
For an isolated system, which statement about entropy is generally correct?
Correct Answer: C. Entropy cannot decrease
An isolated system exchanges neither heat nor work with surroundings, so total entropy cannot decrease. It stays constant for reversible changes and increases for irreversible changes. This is a key implication of the second law.
Which description best defines a reversible process?
Correct Answer: D. A process that can be reversed leaving no net change in system and surroundings
A reversible process can be reversed so that both system and surroundings return exactly to their initial states. It proceeds through equilibrium states with no dissipative effects. Real processes are usually irreversible due to friction, viscosity, or finite temperature differences.