in thermodynamics

8/8/2019 In Thermodynamics

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In thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a processthat can be "reversed" by means of infinitesimal changes in some property of the system without

loss or dissipation of energy.[1]

Due to these infinitesimal changes, the system is inthermodynamic equilibrium throughout the entire process. Since it would take an infinite amount

of time for the reversible process to finish, perfectly reversible processes are impossible.

However, if the system undergoing the changes responds much faster than the applied change,the deviation from reversibility may be negligible. In a reversible cycle, the system and itssurroundings will be exactly the same after each cycle.

[2]

An alternative definition of a reversible process is a process that, after it has taken place, can be

reversed and causes no change in either the system or its surroundings. In thermodynamic terms,a process "taking place" would refer to its transition from its initial state to its final state

There are two main types of thermodynamic processes: reversible process

and th irreversible process. The reversible process is the ideal process

which never occurs, the irrversible process is the natural process which is

commonly found in the nature.

What is Reversible Process?

The process in which the system and surroundings can be restored to the initial state

from the final state without producing any changes in the thermodynamics properties of the universe is called as the reversible process. In figure below, let us suppose that thesystem has undergone change from state A to state B. If the system can be restoredfrom state B to state A, and there is no change in the universe, then the process is saidto be reversible process. The reversible process can be reversed completely and thereis no trace left to show that the system had undergone thermodynamic change.

For the system to undergo reversible change, it should occur infinitely slowly due toinfinitesimal gradient. During reversible process all the changes in state occurred in thesystem are in thermodynamic equilibrium with each other.

Thus there are two important conditions for the reversible process to occur. Firstly, theprocess should occur in infinitesimally small time and secondly all the initial and finalstate of the system should be in equilibrium with each other.

If during the reversible process the heat content of the system remains constant i.e. it isadiabatic process, then the process is also isentropic process i.e. the entropy of thesystem remains constant.



The phenomenon of undergoing reversible change is also called as reversibility. Inactual practice the reversible process never occurs, thus it is an ideal or hypotheticalprocess.

Reversible Process

What is Irreversible Process?

The irreversible process is also called as natural process because all the processesoccurring in the nature are irreversible processes. The natural process occurs due tofinite gradient between the two states of the system. For instance heat flow between twobodies occurs due to temperature gradient between the two bodies; this is in fact thenatural flow of heat. Similarly, water flows from high level to low level, current movesfrom high potential to low potential etc.

Here are some important points about the irreversible process:

1) In the irreversible process the initial state of the system and surroundings cannot berestored from the final state.

2) During irreversible process the various states of the system on the path of changefrom initial state to final state are not in equilibrium with each other.

3) During the reversible process the entropy of the system increases decisively and itcannot be reduced back to its initial value.

4) The phenomenon of system undergoing irreversible process is called asirreversibility.

REVERSIBLE PROCESS

"An alternative definition of a reversible process is a process that, after it has taken place, can be

reversed and causes no change in either the system or its surroundings"

IRREVERSIBLE PROCESS"In an irreversible process, finite changes are made; therefore the system is



not at equilibrium throughout the process. At the same point in an irreversible cycle, the system will be in

the same state, but the surroundings are permanently changed after each cycle."

Now suppose we have an adiabatic cylinder filled with an ideal gas and a light frictionless piston attached

to it.

If Suppose I compress the piston all of a sudden, then initially the pressure and tempertaure will be

different at different points in the gas. I did some work on the gas that led to this change in state of the

gas.

After this, if i stop pushing it further and leave the gas to itself, due to the increased pressure, the gas will

tend to expand back to i ts initial state where the internal pressure equals the external pressure.

So when the gas expands back, it does the same amount of work back on me (does it or does it not??).

So the final result that we have is the gas coming back to its initial state.

How is this process not reversible?? We didn't carry out the process infinetely slowly but still it comes

back to it's initial state. And what "permanent" changes to the surroundings, as the wikipedia definition

says, has the gas caused in this case that it wouldn't have if we had carried out the process infinetely

slowly???

Introduction:

Reversible process is universe thermodynamic properties which have

system and surroundings without changes to initial state to final state. It is restored

process. During infinitesimal gradient, reversible process will be changed. The nature

has all processes that is irreversible process. It is also called natural process. During

finite gradient, the states are in natural process.

Explanation for Reversible and Irreversible Processes

Reversible process:

A reversible processes is changing in the extreme conditions. The working

substance in the reverse process passes through all the stages as in the direct process

in such a way that all changes of heat and work occurring in the direct process are

exactly repeated in the reverse order and the system and surroundings attain their initial

states.

If heat is absorbed by the substance in the direct process, the same quantity will

be given out by it in the reverse process and if work is done by the substance in thedirect process, an equal amount of work. There is no wastage of energy at all in the

reversible process.

For example, consider a gas enclosed in a cylinder, made of a perfectly conducting

material and immersed in a large tank of water at a constant temperature. Let the gas

be compressed very slowly such that its temperature remains unchanged throughout. If



reducing volume of the gas, it is allowed to expand isothermally and the expansions be

just as infinitely small as compressions with similar pauses in between, heat will flow

from the enclosing water to compensate the loss during expansion and keep the

temperature of the gas unchanged. If the reversible process slow means the isothermal

and adiabatic reactions are included.

Irreversible process:

Those processes which cannot be retraced in the opposite order by reversing

the controlling factors known as irreversible processes. For example, conduction of heat

from a hot body to colder one. Productions of heat by friction or by the passage of

current through an electrical resistance are also irreversible process. The mixing of milk

into a cup of tea by a spoon is also an example of irreversible process.

Conditions of reversibility:

y The substances undergoing a reversible change must not lose heat by conduction,

convection or radiation or in overcoming friction.

y The changes in the pressure and volume of the working substance must take place at an

infinitely slow rate

These conditions are never strictly realized in practive because no mechanical

process is frictionless and no insulator or conductor is perfect

Please give examples of reversible and irreversible process.?

Best Answer - Chosen by Voters

Reversible -- dissolution of a salt into water, reaction of O2 and H2 to form water, phase changes like

freezing or boiling of water

Irreversible -- hydrocarbon combustion like the burning of wood or oil, radioactive decay

It depends somewhat on context. Some processes are for all practical purposes irreversible, but in

theory are reversible, like spilling a glass of milk on a rug for example.

in thermodynamics

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