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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Specific Heat Capacities of Gases

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Specific Heat Capacities of Gases

Half mole of an ideal gas ( γ = 5/3 ) is taken through a cycle...

Half mole of an ideal gas (γ=5/3\gamma = 5/3) is taken through a cycle abcdaabcda in a PP-VV diagram. The states are located at a(5000 cm3,100 kPa)a(5000 \text{ cm}^3, 100 \text{ kPa}), b(10000 cm3,100 kPa)b(10000 \text{ cm}^3, 100 \text{ kPa}), c(10000 cm3,200 kPa)c(10000 \text{ cm}^3, 200 \text{ kPa}), and d(5000 cm3,200 kPa)d(5000 \text{ cm}^3, 200 \text{ kPa}). Take R=253 J K−1 mol−1R = \dfrac{25}{3} \text{ J K}^{-1} \text{ mol}^{-1}. Determine the amount of heat supplied in the processes abab and bcbc.
Half mole of an ideal gas ( γ = 5/3 ) is taken through a cycle abcda in a P - V diagram. The states are located at

Options

  1. A500 J500 \text{ J}, 1000 J1000 \text{ J}
  2. B1250 J1250 \text{ J}, 1500 J1500 \text{ J}
  3. C750 J750 \text{ J}, 2500 J2500 \text{ J}
  4. D1500 J1500 \text{ J}, 1250 J1250 \text{ J}

Correct answer

B. 1250 J1250 \text{ J}, 1500 J1500 \text{ J}

Step-by-step solution

Given for the ideal gas, γ=53\gamma = \dfrac{5}{3}.

The molar heat capacities are:
Cv=Rγ−1=R53−1=32RC_v = \dfrac{R}{\gamma - 1} = \dfrac{R}{\dfrac{5}{3} - 1} = \dfrac{3}{2}R
Cp=γRγ−1=52RC_p = \dfrac{\gamma R}{\gamma - 1} = \dfrac{5}{2}R

From the PP-VV diagram, the states are:
Pa=100 kPa=105 PaP_a = 100 \text{ kPa} = 10^5 \text{ Pa}, Va=5000 cm3=5×10−3 m3V_a = 5000 \text{ cm}^3 = 5 \times 10^{-3} \text{ m}^3
Pb=100 kPa=105 PaP_b = 100 \text{ kPa} = 10^5 \text{ Pa}, Vb=10000 cm3=10×10−3 m3V_b = 10000 \text{ cm}^3 = 10 \times 10^{-3} \text{ m}^3
Pc=200 kPa=2×105 PaP_c = 200 \text{ kPa} = 2 \times 10^5 \text{ Pa}, Vc=10000 cm3=10×10−3 m3V_c = 10000 \text{ cm}^3 = 10 \times 10^{-3} \text{ m}^3

Process abab is isobaric (constant pressure). The heat supplied is:
Qab=nCpΔT=CpR(nRΔT)=CpRPΔVQ_{ab} = nC_p\Delta T = \dfrac{C_p}{R}(nR\Delta T) = \dfrac{C_p}{R}P\Delta V
Qab=52Pa(Vb−Va)Q_{ab} = \dfrac{5}{2} P_a (V_b - V_a)
Qab=52×105×(10×10−3−5×10−3)Q_{ab} = \dfrac{5}{2} \times 10^5 \times (10 \times 10^{-3} - 5 \times 10^{-3})
Qab=52×105×5×10−3=52×500=1250 JQ_{ab} = \dfrac{5}{2} \times 10^5 \times 5 \times 10^{-3} = \dfrac{5}{2} \times 500 = 1250 \text{ J}

Process bcbc is isochoric (constant volume). The heat supplied is:
Qbc=nCvΔT=CvR(nRΔT)=CvRVΔPQ_{bc} = nC_v\Delta T = \dfrac{C_v}{R}(nR\Delta T) = \dfrac{C_v}{R}V\Delta P
Qbc=32Vb(Pc−Pb)Q_{bc} = \dfrac{3}{2} V_b (P_c - P_b)
Qbc=32×10×10−3×(2×105−105)Q_{bc} = \dfrac{3}{2} \times 10 \times 10^{-3} \times (2 \times 10^5 - 10^5)
Qbc=32×10−2×105=32×1000=1500 JQ_{bc} = \dfrac{3}{2} \times 10^{-2} \times 10^5 = \dfrac{3}{2} \times 1000 = 1500 \text{ J}

Therefore, the heat supplied in processes abab and bcbc are 1250 J1250 \text{ J} and 1500 J1500 \text{ J} respectively.

Answer: 1250 J1250 \text{ J}, 1500 J1500 \text{ J}
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