JEE Advanced · Physics · 13. Thermodynamics
The left and right compartments of a thermally isolated container of length \(L\) are separated by a thermally conducting, movable piston of area \(A\). The left and right compartments are filled with \(\frac{3}{2}\) and 1 moles of an ideal gas, respectively. In the left compartment the piston is attached by a spring with spring constant k and natural length \(\frac{2 L}{5}\). In thermodynamic equilibrium, the piston is at a distance \(\frac{L}{2}\) from the left and right edges of the container as shown in the figure. Under the above conditions, if the pressure in the right compartment is \(P=\frac{\mathrm{k} L}{\mathrm{~A}} \alpha\), then the value of \(\alpha\) is \(\qquad\)

- A 0.2
- B 0.4
- C 0.6
- D 0.8
Answer & Solution
Correct Answer
(A) 0.2
Step-by-step Solution
Detailed explanation

Extension in spring
\(\begin{aligned}
& x=0.5 \mathrm{~L}-0.4 \mathrm{~L} \\
& =0.1 \mathrm{~L}
\end{aligned}\)
FBD of piston

\(\begin{aligned} & \mathrm{kx}+\mathrm{P}_2 \mathrm{~A}=\mathrm{P}_1 \mathrm{~A} \\ & \mathrm{P}_2 \mathrm{~A}=\mathrm{P}_1 \mathrm{~A}-\mathrm{kx}\end{aligned}\)
\(P_2=P_1-\frac{k L}{A(10)} \qquad ....(i)\)
\(\begin{aligned} & \mathrm{P}_1 \mathrm{~V}=\mathrm{n}_1 \mathrm{RT} \\ & \mathrm{P}_2 \mathrm{~V}=\mathrm{n}_2 \mathrm{RT} \\ & \frac{\mathrm{P}_1}{\mathrm{P}_2}=\frac{\mathrm{n}_1}{\mathrm{n}_2}=\frac{3}{2}\end{aligned}\)
\(\mathrm{P}_1=\frac{3}{2} \mathrm{P}_2 \qquad ....(ii)\)
\(\begin{aligned} & P_2=\frac{3}{2} P_2-\frac{k L}{10 A} \\ & \frac{P_2}{2}=\frac{k L}{10 A} \\ & P_2=\frac{k L}{5 A}=\frac{k L}{A} \alpha \\ & \alpha=\frac{1}{5}=0.2\end{aligned}\)
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