19.2.2 Isobaric Process

FIGURE 19.7b shows an isobaric process in which the volume changes from $V_{\text{i}}$ to $V_{\text{f}}$. The rectangular area under the curve is $p\text{Δ}V$, so the work done during this process is

where $\text{Δ}V=V_{\text{f}}-V_{\text{i}}$. $\text{Δ}V$ is positive if the gas expands $(V_{\text{f}}>V_{\text{i}})$, so $W$ is negative. $\text{Δ}V$ is negative if the gas is compressed $(V_{\text{f}}<V_{\text{i}})$, making $W$ positive.