Reversible and Irreversible Processes

By Rajat Shukla |Updated : January 17th, 2017


Isothermal Process



Process in which the temperature remains constant

achieved with help of constant temperature heal reservoir.

The gas obeys BOYL E's LAW, during an isothermal process.

I law becomes: ΔQ = W

If the gas expands, it tends to cool down, but the heat reservoir gives heat input to keep temperature constant does positive work.

If the gas is compressed isothermally, it gives heat to the reservoir and the work done is negative, i.e., in the equation ΔQ = W, both ΔQ and W are negative.

W = 2.303 nRT

Adiabatic Process

In this process, no heat enters or leaves the system.

Achieved by heavily insulating the system.

From Ist Law becomes: ΔU + W = 0

Obeys the relation PVy = constant             (γ = Cp/Cv)

the adiabatic curve in P-V diagram is much steeper than the isothermal curve.

Due to expansion molecules lose speed after colliding with a receding (moving away) piston and its internal energy decreases.  

ΔU + W = 0. ΔU is negative and W is positive.


(Internal energy of the gas is being converted into work done)


Isochoric Process

A process in which the volume remains constant.

Achieved using a container with rigid walls so that neither expansion nor compression occurs.

The gas obeys the relation: P α T

from Ist Law of thermodynamics ΔQ = ΔU 

If heat is given to the gas, its temperature, pressure and internal energy rises. ΔU = ΔQ are both positive.


Isobaric Process

A process in which the pressure is kept constant.

Achieved by keeping external pressure on the piston is kept constant.  

The gas obeys CHARLES' LAW: V α T

 W = P (V2 – V1) = nR (T2 – T1)

If heat is given to the gas, isobaric expansion occurs. The volume and the temperature both rise. The gas expands doing positive work.

ΔU = ΔQ + W, all the three terms are positive in the isobaric expansion

all the three terms ΔQ, ΔU and W are negative in the isobaric compression.


Carnot Cycle

The Carnot cycle when acting as a heat engine consists of the following steps:

During step A to B the expanding gas makes the piston work on the surroundings. The gas expansion is propelled by absorption of quantity Q1 of heat from the high temperature reservoir.

For step B to C Isentropic (reversible adiabatic) expansion occurs. The gas continues to expand, working on the surroundings. The gas expansion causes it to cool to the "cold" temperature, TC.

From C to D reversible isothermal compression occurs, causing quantity Q2 of heat to flow out of the gas to the low temperature reservoir.

In D to A isentropic compression of the gas takes place causing the surroundings to work on the gas, compressing it and causing the temperature to rise to TH. bringing the system to the same state as at the start of step 1.


Efficiency and coefficient of performance

Efficiency= h = W/QH = (QH – QC)/QH = (TH – TC) / TH

TH and TC are absolute temperatures.

The effectiveness of a heat pump is sometimes called the coefficient of performance(COP).

The equation is: COP = QH / QH – QC = QH/W


Q H is the heat supplied to the hot reservoir

QC is the work consumed by the heat pump.

It determines whether we need an engine to obtain heat, or if fuel must be burned directly to obtain heat. As soon as the COP is less than about 3, it would be cheaper to burn the original fuel directly for the heat, rather than generate electricity to operate a heat pump.


write a comment
siddhant sinha
I have a dought  from B to C temperature increase or decrease 
Aryan Omar

Aryan OmarFeb 10, 2017

Temp remains const bro

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