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Thermodynamics MCQs for NEET — Physics Questions with Answers

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What is the primary reason for irreversibility in most natural processes?

A. They proceed very slowly. B. They involve only equilibrium states. C. They involve dissipative effects like friction and viscosity, and often take the system to non-equilibrium states. D. They are always isobaric.

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Explanation

The NCERT states, 'Irreversibility arises mainly from two causes: one, many processes (like a free expansion, or an explosive chemical reaction) take the system to non-equilibrium states; two, most processes involve friction, viscosity and other dissipative effects.' These factors make processes irreversible.

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For an isothermal reversible expansion of an ideal gas, the work done (w) is given by which of the following expressions?

A. $w = -p_{ex}\Delta V$ B. $w = nRT \ln(V_f/V_i)$ C. $w = -2.303 nRT \log(V_f/V_i)$ D. $w = q_V$

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Explanation

According to the NCERT (Chemistry) text, for isothermal reversible change, $w_{rev} = -nRT \ln(V_f/V_i) = -2.303 nRT \log(V_f/V_i)$. Option o1 is for irreversible work, and o2 has the wrong sign. Option o4 is for heat at constant volume.

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In a free expansion of an ideal gas, what is the work done (w) and the change in internal energy ($\Delta U$)?

A. $w = 0$, $\Delta U = q$ B. $w > 0$, $\Delta U = 0$ C. $w = 0$, $\Delta U = 0$ D. $w < 0$, $\Delta U > 0$

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Explanation

The NCERT (Chemistry) text states: 'Free expansion: Expansion of a gas in vacuum ($p_{ex} = 0$) is called free expansion. No work is done during free expansion of an ideal gas whether the process is reversible or irreversible ($w=0$ since $p_{ex}=0$). Also, Joule determined experimentally that $q=0$; therefore, $\Delta U = 0$.'

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Which statement about the efficiency of heat engines in relation to reversibility is correct?

A. All heat engines have 100% efficiency, regardless of reversibility. B. Practical engines (involving irreversibility) often achieve higher efficiency than ideal reversible engines. C. heat engine based on idealised reversible processes achieves the highest possible efficiency. D. Reversibility has no impact on the efficiency of a heat engine.

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Explanation

The NCERT (Physics) text states: 'A heat engine based on idealised reversible processes achieves the highest efficiency possible. All other engines involving irreversibility in any way (as would be the case for practical engines) have lower than this limiting efficiency.'

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According to the Kelvin-Planck statement of the Second Law of Thermodynamics, which process is impossible?

A. Absorption of heat from a reservoir and its complete conversion into work. B. Transfer of heat from a hotter object to a colder object. C. work-producing engine with an efficiency less than 100%. D. process where internal energy increases due to heat absorption.

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Explanation

The NCERT (Physics) text directly states the Kelvin-Planck statement: 'No process is possible whose sole result is the absorption of heat from a reservoir and complete conversion of the heat into work.'

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A process is considered quasi-static if:

A. It occurs very rapidly, without allowing time for equilibrium. B. The system and surroundings are in equilibrium at every stage. C. It involves significant temperature differences between the system and surroundings. D. Work is done only by expansion, never by compression.

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Explanation

The NCERT (Physics) text defines a reversible process as 'quasi-static (system in equilibrium with the surroundings at every stage)'. A process being quasi-static is a key characteristic of reversibility, implying constant near-equilibrium.

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Which of the following processes would likely be irreversible?

A. Melting of ice at 0Â°C and 1 atm pressure. B. Diffusion of cooking gas from a cylinder into a room. C. Slow expansion of an ideal gas against an external pressure infinitesimally smaller than the internal pressure. D. Isothermal compression of a gas in a frictionless piston-cylinder arrangement.

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Explanation

The NCERT (Physics) text lists 'Cooking gas leaking from a gas cylinder in the kitchen diffuses to the entire room. The diffusion process will not spontaneously reverse and bring the gas back to the cylinder' as an example of an irreversible process. Options o1, o3, and o4 describe conditions that approach reversibility.

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What is the consequence of irreversibility in a practical heat engine according to thermodynamics?

A. It allows for perfectly spontaneous heat transfer, increasing efficiency. B. It results in an efficiency equal to that of an ideal reversible engine. C. It leads to a lower efficiency compared to the limiting efficiency of a reversible engine. D. It eliminates the need for a cold reservoir.

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Explanation

The NCERT (Physics) text states: 'All other engines involving irreversibility in any way (as would be the case for practical engines) have lower than this limiting efficiency [of an ideal reversible engine].' Irreversibility is associated with dissipative effects which lower efficiency.

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When considering changes in infinite steps during compression, what does the shaded area in a pV-plot (like Fig. 5.5 (c) mentioned in Chemistry NCERT) represent?

A. Heat absorbed by the gas. B. Work done on the gas. C. Change in internal energy of the gas. D. The final pressure of the gas.

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Explanation

The NCERT (Chemistry) text, accompanying Fig. 5.5 (c), states: 'Work done on the gas is represented by the shaded area.' This refers to the integral of $p dV$, which represents work done.

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A Carnot engine, an ideal reversible engine, operates between two temperatures. Which types of processes constitute its cycle?

A. Two isobaric and two isochoric processes. B. Two isothermal and two adiabatic processes. C. Four isothermal processes. D. Four adiabatic processes.

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Explanation

The NCERT (Physics) text states: 'The Carnot cycle consists of two isothermal processes connected by two adiabatic processes.'

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