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Heat and Thermodynamics question

2020 · 9 Jan · Shift 2 · Q45
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Heat and Thermodynamics question

2020 · 9 Jan · Shift 2 · Q45

JEE MainPhysicsHeat and ThermodynamicsNumerical+4 / −1
Starting at temperature 300 K, one mole of an ideal diatomic gas (γ\gammaγ= 1.4) is first compressed adiabatically from volume V1 to V2 =V116{{{V_1}} \over {16}}16V1​​. It is then allowed to expand isobarically to volume 2V2. If all the processes are the quasi-static then the final temperature of the gas (in oK) is (to the nearest integer) ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 1818TO1819

  1. Given data
  • Initial temperature: T1=300 KT_1 = 300\,\text{K}T1​=300K
  • Number of moles: n=1n=1n=1
  • Diatomic ideal gas with γ=1.4=75\gamma = 1.4 = \dfrac{7}{5}γ=1.4=57​
  • First process: adiabatic compression from V1V_1V1​ to V2=V116V_2 = \frac{V_1}{16}V2​=16V1​​
  • Second process: isobaric expansion from V2V_2V2​ to V3=2V2V_3 = 2V_2V3​=2V2​

We need the final temperature T3T_3T3​.


  1. Temperature after adiabatic compression

For a quasi-static adiabatic process of an ideal gas, TVγ−1=constantTV^{\gamma-1} = \text{constant}TVγ−1=constant

So, T1V1γ−1=T2V2γ−1T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}T1​V1γ−1​=T2​V2γ−1​

Hence, T2=T1(V1V2)γ−1T_2 = T_1\left(\frac{V_1}{V_2}\right)^{\gamma-1}T2​=T1​(V2​V1​​)γ−1

Now, V1V2=16,γ−1=1.4−1=0.4=25\frac{V_1}{V_2} = 16, \qquad \gamma -1 = 1.4-1 = 0.4 = \frac{2}{5}V2​V1​​=16,γ−1=1.4−1=0.4=52​

Therefore, T2=300×160.4T_2 = 300\times 16^{0.4}T2​=300×160.4

Since 16=24  ⟹  160.4=(24)2/5=28/516 = 2^4 \implies 16^{0.4} = (2^4)^{2/5} = 2^{8/5}16=24⟹160.4=(24)2/5=28/5

Numerically, 160.4≈3.031416^{0.4} \approx 3.0314160.4≈3.0314

Thus, T2≈300×3.0314=909.4 KT_2 \approx 300\times 3.0314 = 909.4\,\text{K}T2​≈300×3.0314=909.4K


  1. Temperature after isobaric expansion

For an isobaric process, TV=constant\frac{T}{V} = \text{constant}VT​=constant

So, T3T2=V3V2\frac{T_3}{T_2} = \frac{V_3}{V_2}T2​T3​​=V2​V3​​

Given V3=2V2V_3 = 2V_2V3​=2V2​, therefore T3=2T2T_3 = 2T_2T3​=2T2​

Hence, T3≈2×909.4=1818.8 KT_3 \approx 2\times 909.4 = 1818.8\,\text{K}T3​≈2×909.4=1818.8K


  1. Final answer

To the nearest integer, T3≈1819 KT_3 \approx 1819\,\text{K}T3​≈1819K


  1. Comparison with stored answer

Stored correct answer: 1818 to 18191818 \text{ to } 18191818 to 1819

Our derived answer is 181918191819, which lies in the given accepted range. So it agrees with the stored answer.

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