Question

As a gas is held within a closed chamber, it passes through the cycle shown in figure. Determine the energy transferred by the system as heat during constant-pressure process $CA$ if the energy added as heat $Q_{AB}$ during constant volume process $AB$ is $20.0J$, no energy is transferred as heat during adiabatic process $BC$, and the net done during the cycle is $15.0J$.

Answer 1

Since the process is a complete cycle (beginning and ending in the same thermodynamic state) the change in the internal energy is zero, and the heat absorbed by the gas is equal to the work done by the gas: $Q=W$. In terms of the contributions of the individual paths of the cycle

$Q_{AB}+Q_{BC}+A_{CA}=W$ and

$Q_{CA}=W-Q_{AB}=+15.0J-20.0J-0=-5.0J$

This means $5.0J$ of energy leaves the gas in the form of heat.