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JEE Advanced 2017 Paper 2 · Physics · Thermodynamics and Kinetic Theory of Gases · medium

One mole of a diatomic ideal gas (rotational but no vibrational modes active) undergoes a process in which its pressure is directly proportional to…

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One mole of a diatomic ideal gas (rotational but no vibrational modes active) undergoes a process in which its pressure is directly proportional to its volume, P=αVP=\alpha V. During the process its volume doubles. The fraction of the heat absorbed that is converted into work done by the gas is
  1. A. 13\dfrac13
  2. B. 14\dfrac14
  3. C. 16\dfrac16
  4. D. 29\dfrac29
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Answer: (C) 16\dfrac16

With P=αVP=\alpha V and PV=RTPV=RT we get T∝V2T\propto V^2, so doubling the volume quadruples the temperature: ΔT=3T0\Delta T=3T_0, where T0=P0V0/RT_0=P_0V_0/R. The work done is W=∫V02V0αV dV=32αV02=32P0V0=32RT0W=\int_{V_0}^{2V_0}\alpha V\,dV=\tfrac32\alpha V_0^2=\tfrac32 P_0V_0=\tfrac32RT_0. The internal energy change for a diatomic gas is ΔU=52RΔT=152RT0\Delta U=\tfrac52R\Delta T=\tfrac{15}{2}RT_0. By the first law, Q=ΔU+W=152RT0+32RT0=9RT0Q=\Delta U+W=\tfrac{15}{2}RT_0+\tfrac32RT_0=9RT_0. Therefore W/Q=3/29=16W/Q=\tfrac{3/2}{9}=\tfrac16. Equivalently the molar heat capacity along this path is CV+R/2=3RC_V+R/2=3R, giving Q=3R⋅3T0=9RT0Q=3R\cdot3T_0=9RT_0. As a check, write dQ=CVdT+P dVdQ=C_VdT+P\,dV. Since T∝V2T\propto V^2, dT/T=2 dV/VdT/T=2\,dV/V, so P dV=RT dV/V=12R dTP\,dV=RT\,dV/V=\tfrac12R\,dT, and the molar heat capacity of the path is C=52R+12R=3RC=\tfrac52R+\tfrac12R=3R, which again gives Q=3R⋅3T0=9RT0Q=3R\cdot3T_0=9RT_0. Thus exactly one sixth of the heat supplied appears as work, and the other five sixths raise the internal energy of the gas.
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