Question

At constant pressure P the volume of a gas increases from V1 to V2 when ‘Q’ amount of heat is removed from the system. What will happen to internal energy?

(a) Remain the same if Q = P(V1 – V2)

(b) Decrease by an amount P(V2 – V1) – Q

(c) Decrease by an amount Q + P(V2 – V1)

(d) Increase by some unknown amount

The question was posed to me during an internship interview.

The doubt is from First Law of Thermodynamics in division Thermodynamics of Physics – Class 11

Answer 1

Correct option is (c) Decrease by an amount Q + P(V2 – V1)

Easiest explanation: The internal energy will definitely decrease as both factors heat removal and work done by gas support the same. ΔU = ΔQ – ΔW = -Q – P(V2 – V1) = -(Q + P(V2 – V1)). Thus, we can say that internal energy decreases by an amount = Q + P(V2 – V1).