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A thermodynamic process describes how a system transitions between equilibrium states, typically represented on a P-V (pressure-volume) diagram. The four idealized processes are: isothermal (constant T), adiabatic (Q = 0), isobaric (constant P), and isochoric or isovolumetric (constant V). Each traces a distinct curve on the PV diagram with a specific relationship between Q, W, and ΔU. Real engines and refrigerators approximate combinations of these idealized processes.
Sketch all four process types on a single PV diagram starting from the same state. For each, identify which terms in ΔU = Q − W are zero and compute the others. This visual fluency on PV diagrams is essential for analyzing heat engines.
You already know the first law of thermodynamics — ΔU = Q − W — and the ideal gas law PV = nRT. Thermodynamic processes are where these two equations meet: they describe specific paths a gas can take as it changes state, and each path has a different signature for how Q, W, and ΔU relate to one another.
The PV diagram is the essential visual tool. Plot pressure on the vertical axis and volume on the horizontal axis. Every equilibrium state of a gas is a point on this diagram, and every quasi-static process is a curve connecting two points. The area under the curve on a PV diagram equals the work done by the gas during that process — a geometric fact you should internalize, because it makes comparing processes visual and immediate.
The four idealized processes each constrain one variable. In an isochoric (constant volume) process, the curve is a vertical line; no volume change means no work done, so ΔU = Q entirely. In an isobaric (constant pressure) process, the curve is horizontal; work W = PΔV, and both Q and ΔU are generally nonzero. In an isothermal process, temperature is fixed, so for an ideal gas ΔU = 0 and Q = W — heat flows in and is entirely converted to work. In an adiabatic process, no heat is exchanged (Q = 0), so any work done comes at the cost of internal energy: ΔU = −W. A gas expanding adiabatically cools, which is why diesel engines ignite fuel without a spark — the adiabatic compression heats the air enough to combust.
A common trap: students hear "isothermal" and assume no heat flows, or hear "adiabatic" and assume constant temperature. The names are clues: "iso-thermal" means same temperature; "a-diabatic" means no heat passage (from the Greek for "not passable"). Keep these definitions sharp. On a PV diagram, the adiabatic curve through any point is always steeper than the isothermal curve through the same point, because in an adiabatic expansion the temperature falls, causing pressure to drop faster than it would at constant temperature.
Real engines — the Carnot cycle, diesel engines, refrigerators — are sequences of these four idealized processes stitched together. The enclosed area on a PV diagram for a complete cycle equals the net work output of the engine. Building fluency with individual process types now is what makes heat engine analysis tractable later.