# The most detailed single article on Reversible/Irreversible processes

Practically speaking there is no such cycle which is reversible in nature. A reversible process is defined as the process wherein the substance is brought back to its initial stage without leaving any impact on the surrounding. We can by any means bring back our system to the initial stage but it will definitely affect the surroundings. Reversible processes are idealizations or models of real processes. A process must be quasi-static (quasi-equilibrium) to be reversible.

This means that the following effects must be absent or negligible:

• Free (unrestrained) expansion.
• Heat transfer through a finite temperature difference
• Mixing of two fluids
• Electric resistance
• Inelastic deformation of solids
• Magnetic hysteresis of materials
• Combustion
• Irregular stirring of the viscous liquid
• Sudden change of phase
• Diffusion of two gases of different composition

Processes that are not reversible are called irreversible process.

The concept of reversible processes leads to the definition of the second law efficiency for actual processes, which is the degree of approximation to the corresponding reversible processes. The factors that cause a process to be irreversible are called irreversibilities. They include friction, unrestrained expansion, heat transfer across a finite temperature difference, inelastic deformation of solids, and chemical reactions. The presence of any of these effects renders the process as irreversible processes.
Now we will speak about the type of reversible processes.

Types of reversible processes:
• Internally reversible process
• Externally reversible process

• Internally reversible process:

A process is called internally reversible if no irreversibilities occur within the boundaries of the system during the process. During an internally reversible process, a system proceeds through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state. That is, the paths of the forward and reverse processes coincide for an internally reversible process. The quasi-equilibrium process is an example of an internally reversible process.

• Externally reversible process

A process is called externally reversible if no irreversibilities occur outside the system boundaries during the process. Heat transfer between a reservoir and a system is an externally reversible process if the outer surface of the system is at the temperature of the reservoir.

A process is called totally reversible, or simply reversible, if it involves no irreversibilities within the system or its surroundings. A totally reversible process involves no heat transfer through a finite temperature difference, no non quasi-equilibrium changes, and no friction or other dissipative effects.

Types of irreversible processes
Mechanical irreversibility- the dissipation of the mechanical work into internal energy either in a system or surrounding may be due to the phenomenon of electrical resistance viscous effects inelasticity magnetic hysteresis etc

Thermal irreversibility- This arises due to the heat transfer between two bodies at finite temperature difference.

Internal irreversibility- The major causes are internal fluid friction combustion and diffusion.

External irreversibility- This is caused by the external friction occurring at supports and at bearing ends and try to absorb some work out of the work producing devices.

Irreversibility due to dissipative effects- Whenever the work done on a system instead of increasing the high grade energy (P.E and K.E) increases the internal energy or heat at molecular levels the temperature of the system increases and the work is lost. These dissipative effects may be internal or external to the system.

Points to ponder upon

Ques- Why is the work done equal to zero in the free expansion?

• In this problem, the system is everything inside the rigid container. There is no change in volume, no “dV” so no work done on the surroundings. Pieces of the gas might be expanding, pushing on other parts of the gas, and doing work locally inside the container (and other pieces might be compressed and thus receive work) during the free expansion process, but we are considering the system as a whole, and there is no net work done.

Ques- Is heat transfer across a finite temperature difference only irreversible if no device is present between the two to harvest the potential difference?

• If we have two heat reservoirs at different temperatures, the irreversibility associated with the transfer of heat from one to the other is indeed dependent on what is between them. If there is a copper bar between them, all the heat that comes out of the high temperature reservoir goes into the low temperature reservoir. If there were a Carnot cycle between them, some (not all) heat from the high temperature reservoir would be passed on to the low temperature reservoir, the process would be reversible, and work would be done. The extent to which the process is irreversible for any device can be assessed by computing the total entropy change (device plus surroundings) associated with the heat transfer.

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