Thermodynamic Processes | Isothermal, Adiabatic, Isobaric, Isochoric, Quasi-Static, Reversible Processes

Any change in the thermodynamic coordinates of a system is called a process. Isothermal, Adiabatic, Isobaric, Isochoric, Quasi-Static, Reversible

Thermodynamic Processes

Any change in the thermodynamic coordinates of a system is called a process. The following are familiar processes in the thermodynamics.

Isothermal Process

When a thermodynamic system undergoes a process under the condition that its temperature remains constant, then the process is said to be isothermal process. The essential condition for an isothermal process is that the system must be contained in a perfectly conducting chamber, so that the heat produced or absorbed during the process atonce goes out to, or comes in from outside. Hence, the temperatures will remain constant. But no perfect conductor is available. Therefore, a perfect isothermal process cannot be obtained.

For isothermal process,

ΔU = 0

from the first law of thermodynamics,

ΔU = Q-W

0 = Q-W

Q = W

Hence, for an ideal gas all heat is converted into work in isothermal process.

Example:- If a gas is filled in a brass cylinder is compressed very slowly by means of piston, the heat produced by compression gets enough time to go outside the cylinder through its walls. Hence, the temperature of the gas remains constant.

Adiabatic Process

When a thermodynamic system undergoes a process under the condition that no heat comes into or goes out of the system, then the process is said to be adiabatic process. Such a process can occur when a system is perfectly insulated from the surroundings. But since not perfect insulator is available, perfect adiabatic change is impossible.

For adiabatic process,

Q = 0

from the first law of thermodynamics,

ΔU = Q-W

ΔU = 0-W

ΔU = -W

Example:- If a gas is suddenly compressed, the heat of compression is added to its internal energy and its temperature rises. That is the reason by which the bicycle pump is heated when the air in it is suddenly compressed.

Isobaric Process

If the working substance is taken in expanding chamber in which the pressure is kept constant, the process is called isobaric process. In this process the gas either expands or shrinks to maintain a constant pressure and hence a net amount of work is done by the system or on the system.

Isochoric Process

If a substance undergoes a process in which the volume remains unchanged, the process is called an isochoric process. The increase of pressure and temperature produced by the heat supplied to a working substance contained in a non-expanding chamber is an example of isochoric process.

For isochoric process,

ΔV = 0,     W = PΔV,       W = 0

from the first law of thermodynamics,

ΔU = Q-W

ΔU = Q-0

ΔU = Q

Quasi Static Process

A quasi-static process is defined as the process in which the deviation from thermodynamics equilibrium is infinitesimal and all the states through which the system passes during quasi-static process may be treated as aquarium states. Thus it may be defined as a succession of equilibrium states.

Reversible Process

A reversible process is one which can be retraced in opposite order by slightly changing the external 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 occurring in the direct process are exactly repeated in the opposite order and inverse sense and no changes are left in any of the bodies participating in the process or in the surroundings.

For reversible process,

ΔU = 0

from the first law of thermodynamics,

ΔU = Q-W

0 = Q-W

Q = W

As an example of a reversible process consider a gas enclosed in a cylinder, made of a perfectly conducting material and immeresed in a large tank of water at a constant temperature. In fact, all isothermal and adiabatic operations are reversible when carried out very slowly.

Irreversible Process

Those process which can not be retraced in the opposite order by reversing the controlling factors are known as irreversible processes.

Example:- The conduction of heat from a hot body to colder is an example of irreversible process. Rapid isothermal and adiabatic changes are irreversible.

Thermodynamic System

A definite quantity of matter bounded by some closed surface is known as a system. A system might be the gas contained in a cylinders with a movable piston or it might be the air inside a tyre that is being inflated. It may have an enclosure made by means of physical walls or imagined to be made by a mathematical surface, which serves as the boundary of the system and comprises, everything of thermodynamic interest contained within the boundary. Thus the boundary of the system may be actual or imaginary.

General thermodynamic system are gases, vapour, steam, solid, liquid, surface films, stretched wires, electrical cells etc.

Open and Closed System

An open system is one which can exchange matter and energy with the surroundings while a closed system is one which can not exchange matter but can exchange energy with the surroundings.

Closed system in thermodynamics
Closed system in thermodynamics

Isolated system

If a system can not exchange matter or energy of any kind with its surroundings, it is called an isolated system. The boundary of the closed system must be impenetrable by matter while the boundary of an isolated system must not be crossed by matter, heat, radiation or other forms of energy. Thus an isolated system exchanges neither heat nor work with surroundings.

Thermodynamic Surroundings

Anything outside the thermodynamic system which can exchange energy with it and has a direct bearing on the behaviour of the system is called as its surroundings. The choice of the systems and the surrounding is however, quite arbitrary.

Read also

  1. Laws of Thermodynamics
  2. Vanderwaal's Equation
  3. Heat Engine

Frequently Asked Questions (FAQs)

Que:- What is free expansion?

Ans:- If a system expands in such a way that no heat is exchange and no work is done on and by the system then the expansion is called Free Expansion.

Que:- If hot air rises, why is it cooler at the top of mountain than near sea level?

Ans:- Hot air rises up. But as it rises, it expands also because pressure decreases with height and hence its temperature drops. Therefore, it is cooler at the top of a mountain than near sea-level.

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