B.4 · IB Physics
Heat engine cycle on a p–V diagram
Watch a fixed amount of ideal gas run round an Otto-like or a rectangular cycle. Change the compression ratio and temperatures to see how the enclosed work, the heat flows and the efficiency compare with the Carnot limit.
Heat in, Q_in
Heat out, Q_out
Net work, W
Efficiency, η
Carnot limit, η_Carnot
η / η_Carnot
Try this
- In the Otto-like cycle, move T_h from 1200 K to 2400 K: the loop and W grow, but does η change? Now change r instead.
- Set r = 8 and switch from diatomic to monatomic gas. Explain the change in η using η = 1 − r^(1−γ).
- Choose the rectangular cycle with r = 3, T_c = 300 K, T_h = 2500 K. How far below the Carnot limit is it, and which processes take in heat at temperatures well below T_h?
Exam tip: the net work done by the gas in one cycle equals the area enclosed by the loop (clockwise means the gas does net work), and W = Q_in − Q_out because ΔU = 0 over a complete cycle.