Heat and Thermodynamics

We observe the following:
• For the path B→C→D, the volume (V) decreases; ΔW < 0.
ΔU < 0 ⇒ ΔQ < 0
• For the process A→B→C, ΔW is the area of the semicircle and ΔW ≠ 0.
• For the process A→B (semicircle), it cannot be an isothermal process (whose PV-diagram is a rectangular hyperbola).
• For (clockwise) process A→B→C→D→A, ΔW is the area enclosed, which is positive.