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Understanding Isobaric Process

Understanding Isobaric Process

An isobaric process is a thermodynamic process in which the pressure of the system remains constant, while volume and temperature may change. The word: Iso in the Greek language means ‘equal’ and “bar” is from baros (weight/pressure), which shows the nature of constant pressure of the system.

Let us understand how the isobaric process happens. Read on to learn its characteristics and equations in detail.

What is an Isobaric Process?

When a system undergoes a process at constant external pressure, it is called an isobaric process.

  • The First Law of Thermodynamics states that:

Q = ΔU + W

In which,

  • Q = Heat supplied to the system
  • ΔU = Change in internal energy of the system
  • At constant pressure, W = PΔV = Work done by the system

Breakdown of Isobaric Process

Here is a complete understanding of an isobaric process:

Condition Requirements

  • Constant pressure → P = constant
  • Temperature and volume → can change.

Work Done in Isobaric Process

W = P(V₂ − V₁)

→ V₁ is initial volume
→ V₂ is final volume

Heat Transfer in Isobaric Process

Q = nCpΔT

→ Cp is the molar heat capacity at constant pressure

Change in Internal Energy

ΔU = nCvΔT

Graphical Representation

In a P–V diagram, an isobaric process is represented as a horizontal line, because pressure remains constant while volume changes. Slope = 0 because pressure is constant.

Screenshot 2025-12-24 120411_0.png

 PV diagram of isobaric process

Example: Heating of Gas in a Cylinder with a Movable Piston

When a gas in a piston-cylinder arrangement is heated at constant pressure, the piston moves upward, increasing the volume. This is an example of isobaric expansion.

Details At A Glance

Property Details
Process type Isobaric (constant pressure)
Pressure Constant
Work done W = PΔV
Heat absorbed Q = nCpΔT
Change in Internal Energy ΔU = nCvΔT
PV diagram Horizontal line
Example Gas heated in piston-cylinder

Formal Expression in Isobaric Process

By using the First Law:

Q = ΔU + W

We can substitute the values of:

Q = nCvΔT + PΔV

And also since, PΔV = nRΔT

Q = nCvΔT + nRΔT = nCpΔT

Hence, the Mayer’s relation is proved -

Cp − Cv = R

Summing Up

An isobaric process is one which occurs at constant pressure. Heat when supplied is partly used to raise internal energy and then partly to do work. And when we look at the PV diagram, it is a horizontal line.

Work done in an isobaric process depends on change in volume: W = P(V₂ − V₁)

And we also saw that the heat absorbed by the system → Q = nCpΔT

Frequently Asked Questions

Q1. What is meant by an isobaric process?

An isobaric process is a thermodynamic process at constant pressure.

Q2. How is work done calculated in an isobaric process?

We use the formula: W = P(V₂ − V₁).

Q3. What is the PV diagram of an isobaric process?

A horizontal line parallel to the volume axis.

Q4. What is an example of an isobaric process?

Heating or cooling of a gas in a piston-cylinder arrangement at constant pressure.

Q5. Why is Cp used in isobaric processes?

Since the heat is supplied at constant pressure, the molar specific heat at constant pressure (Cp) is applicable.

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