Interactive thermodynamics
Select any curve to follow the gas through that process. Watch the piston, particles, and energy balance change together.
Pressure–volume diagram
Moving right means the gas expands. Moving up means pressure increases. The thin black curves are constant-temperature paths: TH is the hot isotherm and TC is the cold isotherm. The enclosed area is the net work produced in one complete cycle.
Molecular view
State 1 → State 2
The gas absorbs energy from the hot reservoir and expands, lifting the piston. Its temperature stays constant, so every joule of heat entering becomes work done on the surroundings.
Positive work means work is done by the gas.