A gas is trapped in a cylinder that has a frictionless piston fitted to it. At the start, the gas occupies a volume $V_1$ and is in equilibrium with the external pressure $p$. The gas is then heated gradually so that it expands while the pressure remains constant, forcing the piston to move back until the gas volume has risen to $V_2$. How much work is done by the gas in this expansion?
- A$p (V_2 - V_1)$
- B$\dfrac{1}{2} p (V_2 - V_1)$
- C$p (V_2 + V_1)$
- D$\dfrac{1}{2} p (V_2 + V_1)$