1Reversible & Irreversible Processes

Reversible Process

A process that, once having taken place, can be reversed and in doing so leaves no change in either the system or the surroundingsNet work and net heat transfer must be zeroA reversible process is an ideal that we never achieve all processes are irreversible

2Why do we study reversible processes?

They are ideals that we can shoot for.They are standards to which we can compare different processes.

Irreversibilities

Irreversibilities are what cause a process to be irreversible Friction Unrestrained expansion Heat transfer through a finite temperature

difference Mixing of different substances Others (chemical reactions, inelastic deformation

of solids, electric resistance, hysteresis effects)

3Internal vs. External Irreversibilities

External irreversibilities occur outside the system youre looking at.Internal irreversibilities occur inside the system youre looking at.A totally reversible system is internally & externally reversible.

Example 1

A gas in an insulated tank is separated from an evacuated region by a membrane. The membrane is removed and the gas expands.Internally or externally reversible? Or neither?

4Example 2

Which heat transfer process is irreversible? If the system is the piston, is the irreversibility internal or external?

Control Volume BoundaryTemperature=T

Carnot Cycle (1824)

A reversible cycle to which we compare all other cycles; an ideal to shoot forFour processes: Reversible isothermal expansion Reversible adiabatic expansion Reversible isothermal compression Reversible adiabatic compression

Will we ever achieve this cycle in real life?