raised or lowered. The confined gas is the system, and the piston and heat reservoir
are the surroundings.
The state of the gas is given once its pressure, volume, and temperature are known,
and the equation that connects these state variables is the ideal gas law, PV = nRT.
We’ll imagine performing different experiments with the gas, such as heating it or
allowing it to cool, increasing or decreasing the weight on the piston, and so on,
and study the energy transfers (work and heat) and the changes in the state
variables. If each process is carried out such that at each moment, the system and its
surroundings are in thermal equilibrium, we can plot the pressure (P) versus the