The temperature of n moles of an ideal gas is increased from Tto4T through a process for which pressure p=aT−1 where a is a constant . Then , the work done by the gas is ( R is universal gas constant)
A
nRT
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B
4nRT
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C
2nRT
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D
6nRT
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Solution
The correct option is D6nRT Change in temperature=ΔT=4T−T=3T For given process, pT=a=constant Applying ideal gas equation, p2V=constant⇒pV1/2=constant So process is polytropic , i.e. of the form pVx=constant with x=1/2 Now molar heat capacity=C=R1−x+Cv, where Cv= molar heat capacity at constant volume ⇒C=2R+Cv Hence heat absorbed=ΔQ=nCΔT=ΔU+ΔW=nCvΔT+ΔW ⇒ work done = ΔW=nCΔT−nCvΔT=2nRΔT=6nRT