NEET Thermodynamics PYQs & Practice Questions

10 free NEET Physics questions on Thermodynamics, starting with previous year questions (2016–2026). Tap “Show answer” to check each one. No login needed.

Topics: Laws · Carnot Engine · First Law of Thermodynamics · Zeroth Law of Thermodynamics · Thermodynamic Processes · Heat Engine and Efficiency · Second Law of Thermodynamics · Specific Heat Capacities · Refrigerator and Coefficient of Performance · Quasi-static Processes and P-V diagrams · Heat and Latent Heat · Work Done in Thermodynamic Cycle (Isochoric Process)

1. Q1NEET 2026Cyclic Process on P-V Diagram

   One mole of an ideal monatomic gas undergoes a cyclic process a→b→c→d→a as shown in the P-V diagram: a(V=2 m³, P=300 N/m²) → b(V=5 m³, P=300 N/m²) → c(V=5 m³, P=100 N/m²) → d(V=2 m³, P=100 N/m²) → a. The total heat supplied to the gas is:

   • A. 500 J
   • B. 600 J
   • C. 800 J
   • D. 400 J
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   Answer: A — 500 J

   The cycle a→b→c→d→a traces a rectangle in the P-V plane in the clockwise sense, so the net work done by the gas over one complete cycle equals the enclosed area: W = ΔP × ΔV = (300-100) × (5-2) = 200 × 3 = 600 J. Since internal energy returns to its initial value after a full cycle (ΔU_cycle = 0), the net heat supplied to the gas over the cycle equals the net work done by the gas: Q_net = W = 600 J.

2. Q2NEET 2026Work Done in Adiabatic Process

   In an adiabatic expansion, the temperature of one mole of an ideal monatomic gas (γ=5/3) decreases from 60K to 50K. The work done by the gas in the process is: (Take the universal gas constant as R=8.3 J mol⁻¹ K⁻¹)

   • A. 83 J
   • B. 124.5 J
   • C. 166 J
   • D. 41.5 J
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   Answer: A — 83 J

   For an adiabatic process, work done by the gas W = -ΔU = -nC_vΔT. For a monatomic gas, C_v = (3/2)R. ΔT = 50-60 = -10 K. W = -(1)(3/2)(8.3)(-10) = 124.5 J.

3. Q3NEET 2026Rate of increase of internal energy

   An electric heater supplies heat to a system at a rate of 100 W. If the system performs work at a rate of 75 J/s, then the rate at which internal energy increases will be:

   • A. 75 W
   • B. 25 W
   • C. 100 W
   • D. 125 W
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   Answer: A — 75 W

   dU/dt=dQ/dt-dW/dt=100-75=25 W.

4. Q4NEET 2025First Law of Thermodynamics

   Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radius rA and rB, respectively. On supplying an equal amount of heat to both the systems reversibly under constant pressure, the pistons of gas A and B are displaced by 16 cm and 9 cm, respectively. If the change in their internal energy is the same, then the ratio rA/rB is equal to -

   • A. 4/3
   • B. 3/4
   • C. 2/√3
   • D. √3/2
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   Answer: A — 4/3

   Q=ΔU+ΔW, with Q and ΔU same for both, ΔW is also same ⟹ PΔVA = PΔVB ⟹ ΔVA=ΔVB ⟹ πrA²xA = πrB²xB ⟹ rA²/rB² = xB/xA = 9/16 ⟹ rA/rB = 3/4.

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5. Q5NEET 2024Work Done in Thermodynamic Cycle (Isochoric Process)

   A thermodynamic system is taken through the cycle abcda. The work done by the gas along the path bc is: [P-V diagram: a(100cm³,100kPa) → b(400cm³,100kPa) → c(400cm³,300kPa) → d(100cm³,300kPa) → a]

   • A. -90 J
   • B. -60 J
   • C. zero
   • D. 30 J
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   Answer: B — -60 J

   Along path bc, volume remains constant at 400 cm³ (isochoric process), so work done W = PΔV = 0.

6. Q6NEET 2023Carnot Engine Efficiency

   A Carnot engine has an efficiency of 50% when its source is at a temperature 327°C. The temperature of the sink is:

   • A. 15 °C
   • B. 100 °C
   • C. 200 °C
   • D. 27 °C
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   Answer: C — 200 °C

   Efficiency n = 1 - T2/T1. 0.5 = 1 - T2/600 (since 327°C = 600K). T2/600 = 0.5, so T2 = 300K = 27°C.

7. Q7NEET 2022Adiabatic vs Other Processes - PV Diagram

   An ideal gas undergoes four different processes from the same initial state, shown in a P-V diagram as curves 1, 2, 3 and 4, with curve 1 being a vertical line (isochoric), and curves 2, 3, 4 being decreasing curves of increasing steepness from the initial point, with curve 4 being nearly horizontal (isobaric). Those processes are adiabatic, isothermal, isobaric and isochoric. The curve which represents the adiabatic process among 1, 2, 3 and 4 is:

   • A. 1
   • B. 2
   • C. 3
   • D. 4
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   Answer: A — 1

   For the same initial state and same volume change, the adiabatic curve is steeper than the isothermal curve because gamma > 1 for an adiabatic process. Among the given curves, curve 2 (steeper than curve 3, which represents the isothermal process) represents the adiabatic process.

8. Q8NEET 2022Adiabatic Process

   An ideal gas undergoes four different processes from the same initial state as shown in the figure below. Those processes are adiabatic, isothermal, isobaric and isochoric. The curve which represents the adiabatic process among 1, 2, 3 and 4 is (P-V diagram with curve 1 vertical, curves 2,3 decreasing from same point, curve 4 horizontal at top).

   • A. 1
   • B. 2
   • C. 3
   • D. 4
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   Answer: A — 1

   For curves starting from the same point on a P-V diagram, the adiabatic curve is steeper than the isothermal curve. Curve 2 is the adiabatic process.

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9. Q9NEET 2020Laws

   For an ideal gas undergoing isothermal expansion, which remains constant?

   • A. Pressure
   • B. Internal Energy
   • C. Entropy
   • D. Volume
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   Answer: A — Pressure

   In isothermal process T=constant. For ideal gas, U depends only on T (U=nCvT), so ΔU=0. First law: Q=W. All heat absorbed goes to doing work. PV=nRT=constant.

10. Q10NEET 2020Carnot Engine

    Carnot engine efficiency 40%, source temperature 500K. Sink temperature is:

    • A. 200 K
    • B. 300 K
    • C. 400 K
    • D. 250 K
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    Answer: A — 200 K

    η=1-T₂/T₁. 0.4=1-T₂/500. T₂/500=0.6. T₂=300K. Carnot efficiency depends only on absolute temperatures.