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A thermodynamic process is shown in the figure. The pressures and volumes corresponding to some points in the figure are Pₐ = 3 × 10⁴ Pa, Vₐ = 2 × 10⁻³ m³, Pᵦ = 8 × 10⁴ Pa, Vᵦ = 5 × 10⁻³ m³. In process AB, 600 J of heat is added to the system and in process BC, 200 J of heat is added to the system. The change in internal energy of the system in process AC would be

  1. 560 J
  2. 800 J
  3. 600 J
  4. 640 J

Correct answer: 560 J

Solution

The change in internal energy (ΔU) for the entire process AC can be calculated using the first law of thermodynamics: ΔU = Q - W. The heat added (Q) in the entire process AC is the sum of heat added in AB and BC, i.e., Q = 600 J + 200 J = 800 J. The work done (W) is the area under the PV curve, which is the work done in AB (W_AB = PΔV = 3 × 10⁴ × (5 × 10⁻³ - 2 × 10⁻³) = 90 J) plus the work done in BC (W_BC = 0, as volume is constant). Thus, W = 90 J. Therefore, ΔU = 800 J - 90 J = 560 J.

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