Q: In a thermodynamic process pressure of a fixed mass of a gas is changed in such a manner that the gas release 20J of heat and 8J of work is done on the gas. If initial internal energy of the gas was 30J, what will be the final internal energy ?
Sol: We know that, dQ = dU + dW
Since heat is released by the system, dQ = -20 J
and work is done on the gas, dW = -8 J
dU = -20 – (-8) = -20 + 8 = -12 J
Uf – Ui = – 12 J
Uf = Ui – 12 = 30 – 12 = 18 J