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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

The figure displays the variation of the internal energy U with the...

The figure displays the variation of the internal energy UU with the volume VV of 2.0 mol2.0 \text{ mol} of an ideal gas in a cyclic process abcdaabcda. The temperatures of the gas at bb and cc are 500 K500 \text{ K} and 300 K300 \text{ K} respectively. Determine the net heat absorbed by the gas during the process. (Take R=8.3 J K−1 mol−1R = 8.3 \text{ J K}^{-1} \text{ mol}^{-1} and ln⁡2=0.693\ln 2 = 0.693)
The figure displays the variation of the internal energy U with the volume V of 2.0 mol of an ideal gas in a cyclic

Options

  1. A1150 J1150 \text{ J}
  2. B3320 J3320 \text{ J}
  3. C2300 J2300 \text{ J}
  4. D5760 J5760 \text{ J}

Correct answer

C. 2300 J2300 \text{ J}

Step-by-step solution

For an ideal gas, the internal energy UU depends only on its temperature TT.

From the given U−VU-V graph, we can identify the thermodynamic processes:
Process a→ba \rightarrow b: UU is constant, which implies TT is constant. This is an isothermal expansion at Ta=Tb=500 KT_a = T_b = 500 \text{ K}.
Process b→cb \rightarrow c: VV is constant. This is an isochoric process.
Process c→dc \rightarrow d: UU is constant, which implies TT is constant. This is an isothermal compression at Tc=Td=300 KT_c = T_d = 300 \text{ K}.
Process d→ad \rightarrow a: VV is constant. This is an isochoric process.

For a cyclic process, the net change in internal energy is zero (ΔU=0\Delta U = 0).
According to the first law of thermodynamics, the net heat absorbed is equal to the net work done:
Qnet=Wnet=Wab+Wbc+Wcd+WdaQ_{\text{net}} = W_{\text{net}} = W_{ab} + W_{bc} + W_{cd} + W_{da}

Calculating the work done in each process:
Wab=nRTbln⁡(VbVa)=nR(500)ln⁡(2V0V0)=500nRln⁡2W_{ab} = nRT_b \ln\left(\dfrac{V_b}{V_a}\right) = nR(500) \ln\left(\dfrac{2V_0}{V_0}\right) = 500 nR \ln 2
Wbc=0W_{bc} = 0 (since volume is constant)
Wcd=nRTcln⁡(VdVc)=nR(300)ln⁡(V02V0)=−300nRln⁡2W_{cd} = nRT_c \ln\left(\dfrac{V_d}{V_c}\right) = nR(300) \ln\left(\dfrac{V_0}{2V_0}\right) = -300 nR \ln 2
Wda=0W_{da} = 0 (since volume is constant)

Net work done:
Wnet=500nRln⁡2−300nRln⁡2=200nRln⁡2W_{\text{net}} = 500 nR \ln 2 - 300 nR \ln 2 = 200 nR \ln 2

Substituting the given values (n=2.0 moln = 2.0 \text{ mol}, R=8.3 J K−1 mol−1R = 8.3 \text{ J K}^{-1} \text{ mol}^{-1}, ln⁡2=0.693\ln 2 = 0.693):
Qnet=Wnet=200×2.0×8.3×0.693Q_{\text{net}} = W_{\text{net}} = 200 \times 2.0 \times 8.3 \times 0.693
Qnet=400×8.3×0.693Q_{\text{net}} = 400 \times 8.3 \times 0.693
Qnet=3320×0.693=2300.76 JQ_{\text{net}} = 3320 \times 0.693 = 2300.76 \text{ J}

Rounding to the nearest given option, the net heat absorbed is 2300 J2300 \text{ J}.

Answer: 2300 J2300 \text{ J}
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