Fig 5Piston and cylinderTap rack, stack, knobp 100 kPa · V 1.00 L · T 300 K Q +0 · W +0 · ΔU +0 J
Gas in the window, thermometer column, glowing coilPiston, rod and four slotted weightsMonatomic ideal gas, N k = 0.333 J/K, so pV = NkT and U = (3/2)NkT (L17). Each weight adds 50 kPa to the 100 kPa of the open top. ΔU = Q + W with W the work done on the gas, W = −p_ext ΔV (L14). A heated gas expands against a steady p_ext: Q > 0, W < 0. A weight added compresses it in an irreversible step: W > 0. The lagging leaks a little heat to the room, so Q turns negative when the gas sits hot.