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JEE Advanced 2024 · Thermodynamics

One mole of a monatomic ideal gas undergoes the cyclic process J → K → L → M → J, as shown in the P−TP-T diagram. Match the quantities mentioned in List-I with their values in List-II and choose the correct option. [R is the gas constant.] List-I: (P) Work done in the complete cyclic process; (Q) Change in the internal energy of the gas in the process JK; (R) Heat given to the gas in the process KL; (S) Change in the internal energy of the gas in the process MJ. List-II: (1) RT0−4_{0} − 4RT0ln⁡2;(2)0;(3)3_{0} \ln_{2}; (2) 0; (3) 3RT0;(4)−2_{0}; (4) −2RT0ln⁡2;(5)−3_{0} \ln_{2}; (5) −3RT0ln⁡2._{0} \ln_{2}. Options: (1) P→1; Q→3; R→5; S→4. (2) P→4; Q→3; R→5; S→2. (3) P→4; Q→1; R→2; S→2. (4) P→2; Q→5; R→3; S→4.

Diagram for this Thermodynamics question (JEE Advanced 2024).

Answer: B. P→4P \to 4; Q→3Q \to 3; R→5R \to 5; S→2S \to 2

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Written solution

Step 1: Analyze the process J→KJ \to K.

This is an isobaric process at constant pressure P=P0P = P_0, where the temperature increases from T0T_0 to 3T03T_0.

For 1 mole1\text{ mole} of a monatomic ideal gas, the change in internal energy is:

ΔUJK=nCVΔT\Delta U_{JK} = n C_V \Delta T ΔUJK=1⋅32R(3T0−T0)\Delta U_{JK} = 1 \cdot \frac{3}{2}R (3T_0 - T_0) ΔUJK=3RT0\Delta U_{JK} = 3RT_0

This matches quantity (Q) with value (3).

The work done in this process is:

WJK=P0(VK−VJ)=R(3T0−T0)W_{JK} = P_0(V_K - V_J) = R(3T_0 - T_0) WJK=2RT0W_{JK} = 2RT_0

Step 2: Analyze the process K→LK \to L.

This is an isothermal process at temperature T=3T0T = 3T_0, where the pressure increases from P0P_0 to 2P02P_0.

Since the temperature is constant, the change in internal energy is:

ΔUKL=0\Delta U_{KL} = 0

By the First Law of Thermodynamics, the heat given to the gas is:

QKL=WKL=nR(3T0)ln⁡(VLVK)Q_{KL} = W_{KL} = n R (3T_0) \ln\left(\frac{V_L}{V_K}\right) QKL=3RT0ln⁡(PKPL)Q_{KL} = 3RT_0 \ln\left(\frac{P_K}{P_L}\right) QKL=3RT0ln⁡(P02P0)Q_{KL} = 3RT_0 \ln\left(\frac{P_0}{2P_0}\right) QKL=−3RT0ln⁡2Q_{KL} = -3RT_0 \ln 2

This matches quantity (R) with value (5).

Step 3: Analyze the process L→ML \to M.

This is an isobaric process at constant pressure P=2P0P = 2P_0, where the temperature decreases from 3T03T_0 to T0T_0.

The work done in this process is:

WLM=2P0(VM−VL)=R(T0−3T0)W_{LM} = 2P_0(V_M - V_L) = R(T_0 - 3T_0) WLM=−2RT0W_{LM} = -2RT_0

Step 4: Analyze the process M→JM \to J.

This is an isothermal process at temperature T=T0T = T_0, where the pressure decreases from 2P02P_0 to P0P_0.

Since the temperature is constant, the change in internal energy is:

ΔUMJ=0\Delta U_{MJ} = 0

This matches quantity (S) with value (2).

The work done in this process is:

WMJ=nRT0ln⁡(PMPJ)W_{MJ} = n R T_0 \ln\left(\frac{P_M}{P_J}\right) WMJ=RT0ln⁡(2P0P0)W_{MJ} = RT_0 \ln\left(\frac{2P_0}{P_0}\right) WMJ=RT0ln⁡2W_{MJ} = RT_0 \ln 2

Step 5: Calculate the total work done in the cyclic process.

Sum the work done in each of the four steps:

Wcycle=WJK+WKL+WLM+WMJW_{\text{cycle}} = W_{JK} + W_{KL} + W_{LM} + W_{MJ} Wcycle=2RT0−3RT0ln⁡2−2RT0+RT0ln⁡2W_{\text{cycle}} = 2RT_0 - 3RT_0 \ln 2 - 2RT_0 + RT_0 \ln 2 Wcycle=−2RT0ln⁡2W_{\text{cycle}} = -2RT_0 \ln 2

This matches quantity (P) with value (4).

Answer: The correct matching is (P) →\to (4), (Q) →\to (3), (R) →\to (5), (S) →\to (2).

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