Thermodynamic Processes

  • Define a thermodynamic process.

  • Understand the difference between reversible and irreversible process.

  • State characteristics of common processes observed in thermodynamics and derive work done and heat transfer equations.

  • Generalize these common processes with the help of a single polytropic process.

  • Explain the differences among the simple thermodynamic processes—isobaric, isochoric, isothermal, and adiabatic.

A process occurs when the system undergoes a change in state due to energy transfer at a steady state. A process may be non-flow in which a fixed mass within the defined boundary is undergoing a change of state.
Example: A substance which being heated in a closed cylinder undergoes a non-flow process or closed system undergoes non-flow process.
A process may be a flow process in which mass is entering and leaving through boundary of an open system. In a steady flow process mass is crossing the boundary from surroundings at entry and an equal mass is crossing the boundary at the exit so that the total mass of the system remains constant.

Quasi-static process:

Quasi means almost. A quasi- static process is also called as reversible process. This process is a succession of equilibrium states and infinite slowness is its characteristic feature.

Reversible and Irreversible Processes:

Reversible process:

A reversible process (also sometimes known as quasi-static process) is one which can be stopped at any stage and reversed so that the system and surroundings are exactly restored to their initial states. This process has the following characteristics:

  • It must pass through the same states on the reversed path as were initially visited on the forward path.

  • This process when undone will leave no history of events in the surroundings.

  • It must pass through a continuous series of equilibrium states.

Examples: Some examples of nearly reversible processes are: Frictionless relative motion, Expansion and compression of spring, Frictionless adiabatic expansion or compression of fluid, Isothermal expansion or compression, Electrolysis.

Irreversible process:

An irreversible process is one in which heat is transferred through a finite temperature difference and usually represented by a dotted (or discontinuous) line joining the end states.
Examples: Relative motion with friction, Combustion, Free expansion, Diffusion, Electricity flow through resistance, Plastic deformation, Heat transfer. Irreversibilities are of two types:

  1. External Irreversibilities: These are associated with dissipating effect outside the working fluid. Example: Mechanical friction occurring during a process due to some external source.

  2. Internal Irreversibilities: These are associated with dissipating effects within the working fluid. Example: Unrestricted expansion of gas, viscosity and inertia of the gas.

Solved Examples

Solved Example:


In an open system, for maximum work, the process must be entirely

A reversible process gives the maximum work.

Correct Answer: B

Solved Example:


Which of the following is true for a closed system?

For a closed system mass does not change.

Correct Answer: B