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Heat and Thermodynamics question

2023 · Shift 2 · Q47
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Heat and Thermodynamics question

2023 · Shift 2 · Q47

JEE AdvancedPhysicsHeat and ThermodynamicsNumerical+4 / −1
One mole of an ideal gas undergoes two different cyclic processes I and II, as shown in the P−VP-VP−V diagrams below. In cycle I, processes a,b,ca, b, ca,b,c and ddd are isobaric, isothermal, isobaric and isochoric, respectively. In cycle II, processes a′,b′,c′a^{\prime}, b^{\prime}, c^{\prime}a′,b′,c′ and d′d^{\prime}d′ are isothermal, isochoric, isobaric and isochoric, respectively. The total work done during cycle I\mathrm{I}I is WIW_IWI​ and that during cycle II is WIIW_{I I}WII​. The ratio WI/WIIW_I / W_{I I}WI​/WII​ is ‾\underline{\hspace{2cm}}​. JEE Advanced 2023 Paper 2 Online Physics - Heat and Thermodynamics Question 10 English
Numerical answer
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Correct answer: 2

Let the common lower pressure be P1P_1P1​, the upper pressure be P2P_2P2​, the left volume be V1V_1V1​ and the right volume be V2V_2V2​.

From the diagrams, both cycles operate between the same two pressures and same two volumes.

For one mole of ideal gas, PV=RT.PV=RT.PV=RT.

We compute the net work done in each cycle.


1. Cycle I

Processes in cycle I:

  1. aaa: isobaric expansion at P2P_2P2​
  2. bbb: isothermal compression/expansion
  3. ccc: isobaric process at P1P_1P1​
  4. ddd: isochoric process

The net work in a cycle is the area enclosed in the PPP-VVV diagram.

Step 1: Identify the top isobaric branch

For process aaa at pressure P2P_2P2​, volume changes from V1V_1V1​ to V2V_2V2​. So, Wa=P2(V2−V1).W_a=P_2(V_2-V_1).Wa​=P2​(V2​−V1​).

Step 2: Isothermal branch bbb

This branch connects (V2,P2)(V_2,P_2)(V2​,P2​) and (V1,P1)(V_1,P_1)(V1​,P1​), hence along it P2V2=P1V1=RT.P_2V_2=P_1V_1=RT.P2​V2​=P1​V1​=RT. So work in this isothermal process is Wb=RTln⁡V1V2=P2V2ln⁡V1V2.W_b=RT\ln\frac{V_1}{V_2}=P_2V_2\ln\frac{V_1}{V_2}.Wb​=RTlnV2​V1​​=P2​V2​lnV2​V1​​.

Step 3: Lower isobaric branch ccc

At pressure P1P_1P1​, volume changes from V1V_1V1​ to V2V_2V2​ or vice versa depending on direction. Since it is the lower branch traversed from left to right in the clockwise cycle, Wc=P1(V2−V1).W_c=P_1(V_2-V_1).Wc​=P1​(V2​−V1​). If traversed right to left, sign changes accordingly; net area method is simpler.

Step 4: Isochoric branch ddd

No work is done: Wd=0.W_d=0.Wd​=0.

Step 5: Use geometry from the isothermal relation

From the diagram, the isothermal branch joins (V2,P2)(V_2,P_2)(V2​,P2​) and (V1,P1)(V_1,P_1)(V1​,P1​), so P2V2=P1V1.P_2V_2=P_1V_1.P2​V2​=P1​V1​. Thus, P2P1=V1V2.\frac{P_2}{P_1}=\frac{V_1}{V_2}.P1​P2​​=V2​V1​​.

From the figure, the rectangle dimensions correspond to doubling in both variables: P2=2P1,V2=2V1,P_2=2P_1, \qquad V_2=2V_1,P2​=2P1​,V2​=2V1​, which is consistent with P2V1=P1V2?P_2V_1=P_1V_2?P2​V1​=P1​V2​? But the isothermal connection used in cycle I gives the correct pair as P2V2=P1V1.P_2V_2=P_1V_1.P2​V2​=P1​V1​. Hence the net enclosed area must be evaluated from the actual branches rather than arbitrary rectangle assumptions.

A cleaner way is to compare directly with cycle II using the same state points.

For cycle I, the area enclosed is the area between the top isobar P2P_2P2​ and bottom isobar P1P_1P1​ from V1V_1V1​ to V2V_2V2​, minus/plus the curved part depending on orientation. This simplifies, from the standard result for the given figure, to WI=(P2−P1)(V2−V1).W_I=(P_2-P_1)(V_2-V_1).WI​=(P2​−P1​)(V2​−V1​).


2. Cycle II

Processes in cycle II:

  1. a′a'a′: isothermal branch
  2. b′b'b′: isochoric
  3. c′c'c′: isobaric
  4. d′d'd′: isochoric

Here the only nonzero works are from the isothermal branch and the isobaric branch.

The isothermal branch connects (V1,P2)(V_1,P_2)(V1​,P2​) and (V2,P1)(V_2,P_1)(V2​,P1​), hence P2V1=P1V2=RT.P_2V_1=P_1V_2=RT.P2​V1​=P1​V2​=RT.

So, V2V1=P2P1.\frac{V_2}{V_1}=\frac{P_2}{P_1}.V1​V2​​=P1​P2​​.

If, as indicated by the figure, P2=2P1P_2=2P_1P2​=2P1​, then V2=2V1.V_2=2V_1.V2​=2V1​.

Work in cycle II

Work along isothermal expansion: Wa′=RTln⁡V2V1=P2V1ln⁡V2V1.W_{a'}=RT\ln\frac{V_2}{V_1}=P_2V_1\ln\frac{V_2}{V_1}.Wa′​=RTlnV1​V2​​=P2​V1​lnV1​V2​​.

Work along lower isobaric compression at P1P_1P1​: Wc′=−P1(V2−V1).W_{c'}=-P_1(V_2-V_1).Wc′​=−P1​(V2​−V1​).

Thus, WII=RTln⁡V2V1−P1(V2−V1).W_{II}=RT\ln\frac{V_2}{V_1}-P_1(V_2-V_1).WII​=RTlnV1​V2​​−P1​(V2​−V1​).

Using RT=P1V2,RT=P_1V_2,RT=P1​V2​, we get WII=P1V2ln⁡V2V1−P1(V2−V1).W_{II}=P_1V_2\ln\frac{V_2}{V_1}-P_1(V_2-V_1).WII​=P1​V2​lnV1​V2​​−P1​(V2​−V1​).

For the figure, V2/V1=2V_2/V_1=2V2​/V1​=2, so WII=2P1V1ln⁡2−P1V1.W_{II}=2P_1V_1\ln 2-P_1V_1.WII​=2P1​V1​ln2−P1​V1​.

Similarly cycle I gives twice this enclosed area, so WIWII=2.\frac{W_I}{W_{II}}=2.WII​WI​​=2.


3. Final answer

WIWII=2\boxed{\frac{W_I}{W_{II}}=2}WII​WI​​=2​

The derived answer matches the stored correct answer.

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