Q: A gas undergoes a change of state during which 100J of heat is supplied to it and it does 20 J of work. The system is brought back to its original state through a process during which 20J of heat is released by the gas. What is the work done by the gas in the second process ?
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Sol: dQ1 = dU1 + dW1
100 = dU1 + 20
⇒ dU1 = 80 J
dQ2 = dU2 + dW2 (∴ dU1 = -dU2)
-20 = -80 + dW2
⇒ dW2 = 60 J