Thermodynamic Processes
A thermodynamic process is a path through state space. Constraints such as constant pressure, volume, or temperature shape the path and determine how energy crosses the boundary as work and heat.
Why does the path between thermodynamic states matter?
State variables describe the endpoints. Heat and work describe energy transfer along the process. Boundary work depends on how pressure changes while volume changes, so different paths can transfer different amounts of work.
On a pressure-volume diagram, reversible boundary work is the signed area under the process curve.
Move one idealized gas process through pressure–volume space.
Heat and work are not state variables. Their values depend on the process connecting the endpoints.
Pressure, volume, temperature, and internal energy characterize thermodynamic states. A state does not remember the path used to reach it.
A horizontal path on a pressure-volume diagram. Expansion creates a rectangular positive work area; compression gives negative work by the gas.
A vertical path. Volume does not change, so there is no boundary work even though pressure, temperature, and internal energy can change.
For an ideal gas at constant temperature, internal energy stays constant, so the first law requires heat transfer to balance the boundary work.
A closed loop in state space can convert recurring heat transfers into net work.
When a system returns to its initial thermodynamic state, its net change in internal energy over the cycle is zero. The enclosed pressure-volume area represents net boundary work, so the first law connects that work to net heat transferred over the cycle.