1 mole of an ideal gas performs the following operation from A to B on a $P$–$V$ diagram: point A is at $P = 4\,\text{bar}$, $V = 2\,\text{m}^3$ and point B is at $P = 2\,\text{bar}$, $V = 4\,\text{m}^3$, following the straight-line path from A to B. If $500\,\text{kJ}$ heat is absorbed in this process, $\Delta U$ from A to B is