Question

When a system is taken through the process abc shown in figure, 80 J of heat is absorbed by the system and 30 J of work is done by it. If the system does 10 J of work during the process adc, how much heat flows into it during the process?

Answer 1

Initial point is a and the final point is c.

As internal energy is a state function so it depends only on the initial and final points and not on the path followed by the system. This implies that change in internal energy for path abc and path adc is the same.
Using the first law of thermodynamics, we get
∆Q = ∆U + ∆W
Here, ∆Q is the amount of heat absorbed and ∆U is the change in internal energy of the system. Also, ∆W is the work done by the system.

For path abc:
∆Q = 80 J, ∆W = 30 J
∆U = (80 − 30) J = 50 J

For path abc:
∆U =50 J (same as for path abc)
∆W = 10 J

∴ ∆Q = 10 J + 50 J = 60 J (∆U = 50 J)