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Question #41989

Difficulty: 8/10 | 🟢 Free

The Question

An ideal monatomic gas of molar mass MM undergoes a cyclic process. During the high-pressure isothermal expansion phase at temperature T0T_0, the volume increases from V0V_0 to 2V02V_0 at a constant volumetric rate dVdt=C\frac{dV}{dt} = C. Simultaneously, a microscopic valve of area AA is opened, allowing gas to effuse into a vacuum. The valve is closed during the rest of the cycle. If the effusion rate is given by dndt=−PA2πMRT\frac{dn}{dt} = -\frac{P A}{\sqrt{2\pi M R T}}, what is the ratio of the work done by the gas during this expansion phase to the work that would have been done if the valve remained closed? (Let β=ACRT02πM\beta = \frac{A}{C} \sqrt{\frac{R T_0}{2\pi M}})

Options

  • A.

    1−2−ββln⁡2\frac{1 - 2^{-\beta}}{\beta \ln 2}

  • B.

    1−2−ββ\frac{1 - 2^{-\beta}}{\beta}

  • C.

    1−e−ββln⁡2\frac{1 - e^{-\beta}}{\beta \ln 2}

  • D.

    1−2−β2βln⁡2\frac{1 - 2^{-\beta}}{2 \beta \ln 2}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Variable_Constraint

Develop pure symbolic logic skills by mastering algebraic constraints without relying on numerical data.

Concepts Behind This Question

Cyclic Process Geometry|Effusion and Diffusion|First Law Application

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to handle open thermodynamic systems where the number of moles nn is not a state constant but a path-dependent variable.

A common trap is treating nn as a constant or using an average value when calculating the PdVP dV work integral.

By coupling Graham's law of effusion with the macroscopic ideal gas law, the problem forces the setup of a first-order differential equation to find n(V)n(V).

Failing to substitute this continuously varying n(V)n(V) into the work integral leads to incorrect logarithmic or linear approximations, exposing a fundamental misunderstanding of variable-mass thermodynamics.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterThermodynamics - Class 11 Part 2
ConceptCyclic Process Geometry
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree