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Heat and Thermodynamics question

2017 · Shift 1 · Q53
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Heat and Thermodynamics question

2017 · Shift 1 · Q53

JEE AdvancedPhysicsHeat and ThermodynamicsMCQ+3 / −0.75
An ideal gas is undergoing a cyclic thermodynamic process in different ways as shown in the corresponding P−VP-VP−V diagram in column 3 of the table. Consider only the path from state 111 to state 2.W2.W2.W denotes the corresponding work done on the system. The equations and plots in the table have standard notations as used in thermodynamic processes. Here YYY is the ratio of heat capacities at constant pressure and constant volume. The number of moles in the gas is n.n.n. JEE Advanced 2017 Paper 1 Offline Physics - Heat and Thermodynamics Question 52 English Comprehension 1 JEE Advanced 2017 Paper 1 Offline Physics - Heat and Thermodynamics Question 52 English Comprehension 2Which one of the following options correctly represents a thermodynamic process that is used as a correction in the determination of the speed of sound in an ideal gas?
  1. A
    (I)(ii)(Q)\left( {\rm I} \right)\left( {ii} \right)\left( Q \right)(I)(ii)(Q)
  2. B
    (IV)(ii)(R)\left( {{\rm I}V} \right)\left( {ii} \right)\left( R \right)(IV)(ii)(R)
  3. C
    (III)(iv)(R)\left( {{\rm I}{\rm I}{\rm I}} \right)\left( {iv} \right)\left( R \right)(III)(iv)(R)
  4. D
    (I)(iv)(Q)\left( {\rm I} \right)\left( {iv} \right)\left( Q \right)(I)(iv)(Q)
View written solutionFree

Correct answer: D

Step-by-step Solution:

  1. Identify the Physical Process: The question asks for the thermodynamic process that serves as a correction in the determination of the speed of sound in an ideal gas. Initially, Newton assumed that the propagation of sound waves in a gas is an isothermal process. However, the calculated speed of sound based on this assumption, v=P/ρv = \sqrt{P/\rho}v=P/ρ​, did not match experimental results. Laplace corrected this by arguing that the compressions and rarefactions in a sound wave occur so rapidly that there is no time for heat exchange with the surroundings. Therefore, the process is not isothermal but adiabatic. For an adiabatic process, the speed of sound is given by v=γP/ρv = \sqrt{\gamma P/\rho}v=γP/ρ​, where γ=CP/CV\gamma = C_P/C_Vγ=CP​/CV​ is the ratio of specific heats. This formula agrees well with experimental data. Thus, we are looking for the option that correctly describes an adiabatic process.

  2. Characteristics of an Adiabatic Process: An adiabatic process is characterized by the following:

    • Equation of State: For an ideal gas undergoing an adiabatic process, the relation between pressure PPP and volume VVV is given by PVγ=constantPV^\gamma = \text{constant}PVγ=constant.
    • Work Done: The work done on the system (WWW) during an adiabatic process from state 1 (P1,V1)(P_1, V_1)(P1​,V1​) to state 2 (P2,V2)(P_2, V_2)(P2​,V2​) is equal to the change in internal energy, ΔU\Delta UΔU. For nnn moles of an ideal gas, ΔU=nCV(T2−T1)\Delta U = nC_V(T_2 - T_1)ΔU=nCV​(T2​−T1​). Using the relations CV=R/(γ−1)C_V = R/(\gamma-1)CV​=R/(γ−1) and PV=nRTPV = nRTPV=nRT, the work done on the system is: W=ΔU=nR(T2−T1)γ−1=P2V2−P1V1γ−1W = \Delta U = \frac{nR(T_2 - T_1)}{\gamma-1} = \frac{P_2V_2 - P_1V_1}{\gamma-1}W=ΔU=γ−1nR(T2​−T1​)​=γ−1P2​V2​−P1​V1​​
    • P-V Diagram: On a Pressure-Volume diagram, an adiabatic process is represented by a curve. The slope of this curve is dPdV=−γPV\frac{dP}{dV} = -\gamma \frac{P}{V}dVdP​=−γVP​. Since γ>1\gamma > 1γ>1, the slope of an adiabat is steeper than the slope of an isotherm (PV=constantPV = \text{constant}PV=constant), which is dPdV=−PV\frac{dP}{dV} = -\frac{P}{V}dVdP​=−VP​.
  3. Analyze the Options: We need to find the option that provides a consistent set of identifiers for an adiabatic process. The options are triplets, representing (Process Type)(Work Formula)(P-V Diagram).

    • Option A: (I)(ii)(Q): If we assume (I) and (Q) represent the adiabatic process and its P-V diagram, then (ii) must be the corresponding work formula. However, a common structure for these problems is to list formulas for different processes. For example, (ii) could represent the isothermal work formula, W=nRTln⁡(V1/V2)W = nRT\ln(V_1/V_2)W=nRTln(V1​/V2​). This would make the triplet inconsistent.

    • Option B: (IV)(ii)(R): Let's assume (IV) represents an isobaric process and (R) its P-V diagram (a horizontal line). The work formula (ii), likely for an isothermal process, would not match. Thus, this option is inconsistent.

    • Option C: (III)(iv)(R): This option appears to mix elements from different processes. For instance, (III) could be isochoric, (iv) adiabatic work, and (R) an isobaric P-V diagram. This is inconsistent.

    • Option D: (I)(iv)(Q): This option presents a self-consistent description of an adiabatic process.

      • (I) can be identified as the name or equation for an adiabatic process (PVγ=constantPV^\gamma = \text{constant}PVγ=constant).
      • (iv) matches the formula for work done on the system in an adiabatic process: W=P2V2−P1V1γ−1W = \frac{P_2V_2 - P_1V_1}{\gamma-1}W=γ−1P2​V2​−P1​V1​​.
      • (Q) represents the P-V diagram for an adiabatic process, which is a curve steeper than an isotherm.
  4. Conclusion: The Laplace correction for the speed of sound involves treating the process as adiabatic. Option D, which is (I)(iv)(Q), is the only choice that provides a consistent and correct description of an adiabatic process among the given options. Therefore, it correctly represents the thermodynamic process in question.

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