First Law of Thermodynamics in Closed System by PK NAG (Chapter 04)

A: 110 kJ is the hinge here. With PK Nag’s sign convention, heat added to the system is positive and work done by the system is positive in magnitude but subtracted in ΔU = Q − W. 150 − 40 lands at +110 kJ. The other choices flip one sign or add both terms, a mistake that shows up when someone drags an open-system energy balance into a closed system.

A: Constant pressure is the discriminator. On a P–V plot, a horizontal line means pressure doesn’t change, not volume. The area under that line is P(V2 − V1). Mixing this up with a T–V sketch is a common documentation read error.

A: 7 kJ is the scale setter. At 1 bar and 300 K, air density is roughly 1.2 kg/m³, so mass is about 1.2 kg. In a rigid vessel W = 0, but ΔU = m cv ΔT still applies: 1.2 × 0.718 × 10 ≈ 8.6 kJ, close to 7–9 kJ. Zero confuses work with internal energy change.

A: Zero is the boundary. In free expansion of a closed, insulated system, there’s no heat transfer and no boundary work. First law collapses to ΔU = 0. Entropy rising doesn’t inject energy; that’s a second-law effect.