A thermodynamic process is a path through a sequence of states that takes a system from an initial state to a final state. In a cyclic process, the system returns to its initial state, so the changes in state properties and state functions (ΔT, Δp, ΔV, ΔU, ΔH) over one complete cycle are zero. However, heat and work transfers can still occur during the cycle, and the net heat and net work over the cycle need not be zero.
A reversible process occurs when the system is infinitesimally close to equilibrium and can restore itself and its surroundings to the initial states with an infinitesimal change. The work done in any reversible process is dwrev = -pdV. However, this requires the absence of friction or finite changes in composition, pressure, or temperature. Conversely, the processes that do not proceed infinitesimally slowly are considered irreversible. During an isothermal process, the temperature of the system remains constant while other properties, like volume, may change. This can be achieved by placing the system in a thermal bath at a fixed temperature.
An adiabatic process experiences no heat transfer between the system and its surroundings, so the net heat (dq) transferred over the process is zero. This is achieved using adiabatic walls. In a constant-volume process, the system volume is fixed, so the work done is zero. A constant volume process is accomplished using rigid walls. A constant-pressure process maintains a constant pressure, often using a cylinder with a movable piston set at a fixed external pressure.
In conclusion, the common process types are defined by what remains constant: T (isothermal), q (adiabatic: zero heat transfer), V (constant-volume, so pV work is zero), or p (constant-pressure).
A thermodynamic system undergoes a change of state when one or more properties defining its state change.
Processes that go from the same initial state to the same final state via different paths are considered different processes.
In a cyclic process, the system returns to its initial state, so state functions like temperature, pressure, and volume return to their original values. However, the heat and work exchanged during the cycle can be nonzero.
In a reversible process, the system is constantly close to equilibrium, and any infinitesimal change can re-establish both the system and surroundings to their original states. Work in such a process is given by dwrev = -p dV, and heat dq, when divided by temperature, corresponds to the change in entropy.
An isothermal process keeps temperature constant throughout, while an adiabatic process involves no heat exchange, so q and dq are equal to zero.
An isochoric process holds volume constant, while an isobaric process holds pressure steady during the entire process. For a constant-pressure process, it's found that ΔH = qp.
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