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Heat and Thermodynamics question

2020 · Shift 1 · Q45
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Heat and Thermodynamics question

2020 · Shift 1 · Q45

JEE AdvancedPhysicsHeat and ThermodynamicsNumerical+4 / −1
Consider one mole of helium gas enclosed in a container at initial pressure P1 and volume V1. It expands isothermally to volume 4V1. After this, the gas expands adiabatically and its volume becomes 32V1. The work done by the gas during isothermal and adiabatic expansion processes are Wiso and Wadia, respectively. If the ratio WisoWadia{{{W_{iso}}} \over {{W_{adia}}}}Wadia​Wiso​​ = f ln 2, then f is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 1.77

  1. Given: one mole of helium gas.

For helium, being a monoatomic ideal gas, γ=CPCV=53\gamma = \frac{C_P}{C_V} = \frac{5}{3}γ=CV​CP​​=35​

We need the ratio WisoWadia=fln⁡2\frac{W_{\text{iso}}}{W_{\text{adia}}} = f\ln 2Wadia​Wiso​​=fln2


  1. Isothermal expansion from V1V_1V1​ to 4V14V_14V1​.

For one mole of ideal gas, work done in isothermal expansion is Wiso=nRT1ln⁡(VfVi)W_{\text{iso}} = nRT_1 \ln\left(\frac{V_f}{V_i}\right)Wiso​=nRT1​ln(Vi​Vf​​) Here n=1n=1n=1, and initially P1V1=RT1P_1V_1=RT_1P1​V1​=RT1​, so Wiso=P1V1ln⁡(4V1V1)W_{\text{iso}} = P_1V_1\ln\left(\frac{4V_1}{V_1}\right)Wiso​=P1​V1​ln(V1​4V1​​) Wiso=P1V1ln⁡4=2P1V1ln⁡2W_{\text{iso}} = P_1V_1\ln 4 = 2P_1V_1\ln 2Wiso​=P1​V1​ln4=2P1​V1​ln2


  1. State after isothermal expansion.

Since the process is isothermal, P2V2=P1V1P_2V_2 = P_1V_1P2​V2​=P1​V1​ with V2=4V1V_2 = 4V_1V2​=4V1​ So, P2=P1V14V1=P14P_2 = \frac{P_1V_1}{4V_1} = \frac{P_1}{4}P2​=4V1​P1​V1​​=4P1​​ Also, temperature remains T1T_1T1​.


  1. Adiabatic expansion from 4V14V_14V1​ to 32V132V_132V1​.

For adiabatic process, TVγ−1=constantTV^{\gamma-1} = \text{constant}TVγ−1=constant Thus, T2(4V1)γ−1=T3(32V1)γ−1T_2(4V_1)^{\gamma-1} = T_3(32V_1)^{\gamma-1}T2​(4V1​)γ−1=T3​(32V1​)γ−1 So, T3=T2(432)γ−1=T1(18)2/3T_3 = T_2\left(\frac{4}{32}\right)^{\gamma-1} = T_1\left(\frac{1}{8}\right)^{2/3}T3​=T2​(324​)γ−1=T1​(81​)2/3 Now, (18)2/3=14\left(\frac{1}{8}\right)^{2/3} = \frac{1}{4}(81​)2/3=41​ Hence, T3=T14T_3 = \frac{T_1}{4}T3​=4T1​​


  1. Work done in adiabatic expansion.

For adiabatic expansion of ideal gas, Wadia=nCV(T2−T3)W_{\text{adia}} = nC_V(T_2-T_3)Wadia​=nCV​(T2​−T3​) For one mole of monoatomic gas, CV=3R2C_V = \frac{3R}{2}CV​=23R​ Therefore, Wadia=3R2(T1−T14)W_{\text{adia}} = \frac{3R}{2}\left(T_1-\frac{T_1}{4}\right)Wadia​=23R​(T1​−4T1​​) Wadia=3R2⋅3T14=98RT1W_{\text{adia}} = \frac{3R}{2}\cdot \frac{3T_1}{4} = \frac{9}{8}RT_1Wadia​=23R​⋅43T1​​=89​RT1​ Using RT1=P1V1RT_1=P_1V_1RT1​=P1​V1​, Wadia=98P1V1W_{\text{adia}} = \frac{9}{8}P_1V_1Wadia​=89​P1​V1​


  1. Take the ratio.

WisoWadia=2P1V1ln⁡2(9/8)P1V1\frac{W_{\text{iso}}}{W_{\text{adia}}} = \frac{2P_1V_1\ln 2}{(9/8)P_1V_1}Wadia​Wiso​​=(9/8)P1​V1​2P1​V1​ln2​ WisoWadia=169ln⁡2\frac{W_{\text{iso}}}{W_{\text{adia}}} = \frac{16}{9}\ln 2Wadia​Wiso​​=916​ln2

Comparing with WisoWadia=fln⁡2\frac{W_{\text{iso}}}{W_{\text{adia}}} = f\ln 2Wadia​Wiso​​=fln2 we get f=169≈1.78f = \frac{16}{9} \approx 1.78f=916​≈1.78


  1. Comparison with stored answer.

Stored correct answer = 1.771.771.77.

Our derived value is f=169=1.777…f = \frac{16}{9} = 1.777\ldotsf=916​=1.777… which rounds to 1.781.781.78, and to two decimal places may also be reported as 1.771.771.77 depending on truncation.

So the stored answer is essentially consistent.

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