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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Laws of Thermodynamics

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Laws of Thermodynamics

A cylindrical tube of volume V having adiabatic walls contains an...

A cylindrical tube of volume VV having adiabatic walls contains an ideal gas. The internal energy of this ideal gas is given by 1.5nRT1.5 nRT. A fixed diathermic wall divides the tube into two equal parts. Initially, the pressure and temperature are p1,T1p_1, T_1 on the left side and p2,T2p_2, T_2 on the right side. The system is allowed sufficient time for the temperatures on both sides to become equal. Find the final pressures on the left and right sides respectively. (Let λ=p1T2+p2T1\lambda = p_1 T_2 + p_2 T_1)
A cylindrical tube of volume V having adiabatic walls contains an ideal gas. The internal energy of this ideal gas is

Options

  1. Ap1T2(p1−p2)λ\dfrac{p_1 T_2 (p_1 - p_2)}{\lambda} and p2T1(p1−p2)λ\dfrac{p_2 T_1 (p_1 - p_2)}{\lambda}
  2. Bp1T1(p1+p2)λ\dfrac{p_1 T_1 (p_1 + p_2)}{\lambda} and p2T2(p1+p2)λ\dfrac{p_2 T_2 (p_1 + p_2)}{\lambda}
  3. Cp1T2(p1+p2)λ\dfrac{p_1 T_2 (p_1 + p_2)}{\lambda} and p2T1(p1+p2)λ\dfrac{p_2 T_1 (p_1 + p_2)}{\lambda}
  4. Dp2T2(p1+p2)λ\dfrac{p_2 T_2 (p_1 + p_2)}{\lambda} and p1T1(p1+p2)λ\dfrac{p_1 T_1 (p_1 + p_2)}{\lambda}

Correct answer

C. p1T2(p1+p2)λ\dfrac{p_1 T_2 (p_1 + p_2)}{\lambda} and p2T1(p1+p2)λ\dfrac{p_2 T_1 (p_1 + p_2)}{\lambda}

Step-by-step solution

Let the volume of each part be V0=V2V_0 = \dfrac{V}{2}.

The number of moles of gas in the left part is:
n1=p1V0RT1n_1 = \dfrac{p_1 V_0}{R T_1}

The number of moles of gas in the right part is:
n2=p2V0RT2n_2 = \dfrac{p_2 V_0}{R T_2}

Since the outer walls are adiabatic and the total volume is constant, the total internal energy of the system remains conserved. Let the final common temperature be TfT_f.

Uinitial=UfinalU_{initial} = U_{final}
n1CvT1+n2CvT2=(n1+n2)CvTfn_1 C_v T_1 + n_2 C_v T_2 = (n_1 + n_2) C_v T_f

Solving for TfT_f:
Tf=n1T1+n2T2n1+n2T_f = \dfrac{n_1 T_1 + n_2 T_2}{n_1 + n_2}

Substituting the expressions for n1n_1 and n2n_2:
n1T1+n2T2=p1V0R+p2V0R=V0R(p1+p2)n_1 T_1 + n_2 T_2 = \dfrac{p_1 V_0}{R} + \dfrac{p_2 V_0}{R} = \dfrac{V_0}{R}(p_1 + p_2)

n_1 + n_2 = \dfrac{V_0}{R} \left( \dfrac{p_1}{T_1} + \dfrac{p_2}{T_2} \right) = \dfrac{V_0}{R} \left(\dfrac{p_1 T_2 + p_2 T_1}{T_1 T_2} \right)

Therefore, the final temperature is:
Tf=T_f =\dfrac{\dfrac{V_0}{R}(p_1 + p_2)}{\dfrac{V_0}{R} \left( \dfrac{p_1 T_2 + p_2 T_1}{T_1 T_2} \right)} = \dfrac{(p_1 + p_2) T_1 T_2}{p_1 T_2 + p_2 T_1}

Given λ=p1T2+p2T1\lambda = p_1 T_2 + p_2 T_1, we can write:
Tf=T_f =\dfrac{(p_1 + p_2) T_1 T_2}{\lambda}

Now, applying the ideal gas law to find the final pressure on the left side (p1fp_{1f}) and right side (p2fp_{2f}), keeping in mind that the volume of each part remains V0V_0:

For the left side:
\dfrac{p_1}{T_1} = \dfrac{p_{1f}}{T_f} \Rightarrowp1f=p_{1f} =\dfrac{p_1}{T_1}Tf=T_f =\dfrac{p_1}{T_1} \left[ \dfrac{(p_1 + p_2) T_1 T_2}{\lambda} \right] = \dfrac{p_1 T_2 (p_1 + p_2)}{\lambda}

For the right side:
\dfrac{p_2}{T_2} = \dfrac{p_{2f}}{T_f} \Rightarrowp2f=p_{2f} =\dfrac{p_2}{T_2}Tf=T_f =\dfrac{p_2}{T_2} \left[ \dfrac{(p_1 + p_2) T_1 T_2}{\lambda} \right] = \dfrac{p_2 T_1 (p_1 + p_2)}{\lambda}

Answer: \dfrac{p_1 T_2 (p_1 + p_2)}{\lambda} and \dfrac{p_2 T_1 (p_1 + p_2)}{\lambda}
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