Multiple Choice Questions (MCQs) for JEE Physics: Thermodynamics
Access targeted multiple-choice questions for Thermodynamics designed to align with the latest JEE academic syllabus for JEE Physics. These objective practice sets help students evaluate their conceptual understanding and improve exam readiness.
Practice Thermodynamics MCQs for JEE Physics
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Multiple Choice Questions
Assertion (A): If a gas container in motion is suddenly stopped, the temperature of the gas rises.
Reason (R): In the process, the kinetic energy of ordered mechanical motion is converted into the kinetic energy of random motion of gas molecules.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. The ordered motion of the whole gas stops.
2. Its kinetic energy is shared among the molecules as random motion.
3. That raises the temperature, so R explains A.
Teacher's Note:
1. Heat is random molecular energy.
Assertion (A): The value of ΔQ is always zero in an adiabatic process.
Reason (R): An adiabatic process is always a cyclic process.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (c) A is true, but R is false.
Explanation:
1. In an adiabatic process there is no heat exchange, so ΔQ = 0: A is true.
2. An adiabatic process need not be cyclic; an adiabatic expansion is not cyclic: R is false.
Teacher's Note:
1. Cyclic means the system returns to its initial state.
Assertion (A): Thermodynamic processes in nature are irreversible.
Reason (R): Dissipative effects such as friction cannot be eliminated.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Real processes involve friction, viscosity and heat flow across finite temperature differences.
2. Such dissipation cannot be removed, so the processes cannot be reversed exactly.
Teacher's Note:
1. Reversible processes are idealisations.
Assertion (A): Reversible systems are difficult to find in the real world.
Reason (R): Most processes are dissipative in nature.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Dissipation (friction, turbulence, finite temperature differences) is present in almost all real processes.
2. This is exactly why reversible systems are hard to find.
Teacher's Note:
1. A reversible process needs a quasi-static change with no dissipation.
Assertion (A): Air quickly leaking out of a balloon becomes cooler.
Reason (R): The leaking air undergoes adiabatic expansion.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. The air escapes rapidly, so there is no time for heat exchange: the expansion is adiabatic.
2. In adiabatic expansion the gas does work at the cost of internal energy, so it cools.
Teacher's Note:
1. The same effect cools gas from a spray can.
Assertion (A): The Carnot cycle is useful in understanding the performance of heat engines.
Reason (R): The Carnot cycle provides a way of determining the maximum possible efficiency achievable with reservoirs of given temperatures.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. A Carnot engine is the most efficient engine between two given temperatures, with efficiency 1 − T₂/T₁.
2. This upper limit is what makes it useful for judging real engines.
Teacher's Note:
1. No real engine can exceed the Carnot efficiency.
Assertion (A): It is not possible for a system, unaided by an external agency, to transfer heat from a body at a lower temperature to another at a higher temperature.
Reason (R): It is not possible to violate the second law of thermodynamics.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. The statement in A is the Clausius form of the second law.
2. R says the second law cannot be violated, which is why the transfer cannot occur unaided.
Teacher's Note:
1. A refrigerator needs work input.
Assertion (A): It is impossible for a ship to use the internal energy of sea water to operate its engine.
Reason (R): A refrigerator is a heat engine working in the reverse direction.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Extracting heat from a single reservoir (sea water) and converting it fully into work violates the Kelvin–Planck statement, so A is true.
2. A refrigerator is indeed an engine run in reverse, so R is true.
3. But R does not explain A.
Teacher's Note:
1. Both statements are true but unrelated in logic.
Assertion (A): When a bottle of cold carbonated drink is opened, a slight fog forms around the opening.
Reason (R): Adiabatic expansion of the gas causes lowering of temperature, which starts condensation of water vapour.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. The escaping gas expands rapidly (adiabatically) and cools.
2. The cooling condenses water vapour into tiny droplets, forming the fog.
Teacher's Note:
1. Fast expansion of a gas cools it.
Assertion (A): In an isothermal process the heat energy supplied to the system is converted completely into work.
Reason (R): According to the first law of thermodynamics, Q = W + ΔU.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. For an ideal gas in an isothermal process ΔT = 0, so ΔU = 0.
2. Then Q = W + ΔU gives Q = W, so all the heat becomes work.
Teacher's Note:
1. Internal energy of an ideal gas depends on temperature only.
Assertion (A): During rapid pumping of air in tyres, the air inside the tyre is hotter than atmospheric air.
Reason (R): An adiabatic process occurs at a very high rate.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Rapid compression leaves no time for heat to flow out, so it is nearly adiabatic.
2. The work done on the gas raises its internal energy and temperature.
Teacher's Note:
1. Fast processes are nearly adiabatic.
Assertion (A): The ratio C_P/C_V for a monoatomic gas is more than that for a diatomic gas.
Reason (R): The molecules of a monoatomic gas have more degrees of freedom than those of a diatomic gas.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (c) A is true, but R is false.
Explanation:
1. γ = 1 + 2/f: monoatomic 5/3 = 1.67, diatomic 7/5 = 1.40, so A is true.
2. A monoatomic molecule has 3 degrees of freedom, fewer than the 5 of a diatomic molecule, so R is false.
Teacher's Note:
1. Fewer degrees of freedom give a larger γ.
Assertion (A): The ratio C_P/C_V is more for helium gas than for hydrogen gas.
Reason (R): The atomic mass of helium is more than that of hydrogen.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Helium is monoatomic (γ = 1.67) and hydrogen is diatomic (γ = 1.40), so A is true.
2. Helium's atomic mass is indeed greater, so R is true.
3. But γ depends on degrees of freedom, not on mass, so R does not explain A.
Teacher's Note:
1. γ depends on the structure of the molecule.
Assertion (A): For an ideal gas C_P is greater than C_V.
Reason (R): Work is done in the expansion of the gas at constant pressure.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. At constant pressure the gas expands and does work, in addition to the heat that raises the internal energy.
2. Hence more heat is needed per degree: C_P − C_V = R.
Teacher's Note:
1. Mayer's relation: C_P − C_V = R.
Assertion (A): The specific heat of a gas at constant pressure is more than that at constant volume.
Reason (R): The energy required to raise the temperature by one unit at constant pressure is greater because some heat is used in doing work.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. The extra heat at constant pressure is the work done by the gas in expanding.
2. That explains why C_P exceeds C_V.
Teacher's Note:
1. Same reasoning as Mayer's relation.
Assertion (A): Heat is not a state thermodynamic function.
Reason (R): The heat given to a system depends on the process.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. A state function depends only on the state of the system.
2. Heat depends on the path followed, so it is not a state function.
Teacher's Note:
1. Work and heat are path functions.
Assertion (A): In an adiabatic process the temperature of the system decreases when work is done by the system.
Reason (R): A part of the internal energy is utilised in doing the work.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. With Q = 0, work done by the gas equals the decrease in internal energy.
2. A smaller internal energy means a lower temperature.
Teacher's Note:
1. Adiabatic expansion cools the gas.
Assertion (A): Adiabatic free expansion of an ideal gas is irreversible.
Reason (R): PV = constant for a reversible adiabatic expansion.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (c) A is true, but R is false.
Explanation:
1. Free expansion into a vacuum cannot be reversed without external work, so A is true.
2. For a reversible adiabatic expansion the law is PV^γ = constant, not PV = constant, so R is false.
Teacher's Note:
1. PV = constant is the isothermal law.
Assertion (A): The temperature is constant in a quasi-static isothermal process.
Reason (R): No heat is exchanged with the surroundings in a quasi-static isothermal process.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
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Answer: (c) A is true, but R is false.
Explanation:
1. An isothermal process is by definition at constant temperature, so A is true.
2. Heat is exchanged with a reservoir to keep the temperature constant; it is the adiabatic process that has no heat exchange, so R is false.
Teacher's Note:
1. Isothermal means constant T, not Q = 0.
Assertion (A): For an isolated system the internal energy remains constant.
Reason (R): Heat is exchanged through the walls of an isolated system.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. An isolated system exchanges neither energy nor matter, so its internal energy is constant: A is true.
2. No heat crosses the walls of an isolated system, so R is false.
Teacher's Note:
1. Isolated means no exchange with the surroundings.
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