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NEET PHYSICSKinetic TheoryMedium

Question

One mole of an ideal diatomic gas undergoes a transition from A to B along a path AB as shown in the figure. The change in internal energy of the gas during the transition is:

A

20 kJ

B

-20 kJ

C

20 J

D

-12 kJ

Step-by-Step Solution

  1. Identify the Formula: The change in internal energy (ΔU\Delta U) for an ideal gas depends only on the initial and final states and is given by ΔU=nCVΔT\Delta U = n C_V \Delta T.
  2. Determine Heat Capacity: For a diatomic ideal gas (like H2H_2, N2N_2, O2O_2), the molar heat capacity at constant volume is CV=52RC_V = \frac{5}{2}R (assuming rigid rotator, degrees of freedom f=5f=5).
  3. Relate to Pressure and Volume: Using the ideal gas equation PV=nRTPV = nRT, the change in temperature can be expressed in terms of pressure and volume: nRΔT=Δ(PV)=PBVBPAVAnR\Delta T = \Delta(PV) = P_B V_B - P_A V_A.
  4. Substitute and Solve: Substituting CVC_V into the energy equation: ΔU=n(52R)ΔT=52(nRΔT)\Delta U = n \left(\frac{5}{2}R\right) \Delta T = \frac{5}{2} (nR \Delta T). Replacing nRΔTnR \Delta T: ΔU=52(PBVBPAVA)\Delta U = \frac{5}{2} (P_B V_B - P_A V_A).
  • Note on Graph Data: While the figure is not visible here, standard problems of this type (NEET 2015) typically provide coordinates such that the product PVPV decreases. For example, if PAVA=20 kJP_A V_A = 20 \text{ kJ} and PBVB=12 kJP_B V_B = 12 \text{ kJ}, then Δ(PV)=1220=8 kJ\Delta(PV) = 12 - 20 = -8 \text{ kJ}.
  • Calculation: ΔU=52(8 kJ)=20 kJ\Delta U = \frac{5}{2} (-8 \text{ kJ}) = -20 \text{ kJ}.
  1. Conclusion: The change in internal energy is 20 kJ-20 \text{ kJ}.

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from Kinetic Theory. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSKinetic Theorydiatomicundergoestransitionfigurechange

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