Transformation of energy

Isobaric process. Definition and examples

Isobaric process. Definition and examples

An isobaric process is a thermodynamic process change in the state of a certain amount of matter in which the pressure remains constant. What it may change is one or more of its state variables. If heat is transferred to the system, work is done and the internal energy of the system also changes.

In a pressure-volume diagram, it drives a horizontal line according to the ideal gas law.

The isobaric process is governed by Charles's law. According to Charles's law, for a fixed mass of ideal gas at constant pressure, the volume is directly proportional to the Kelvin temperature.

Isobaric processes are regulated by the first law of thermodynamics. In these processes, the increase in energy is equal to the increase in enthalpy minus the pressure multiplied by the increase in volume:ΔE = ΔH - P · ΔV.

Not to be confused with isothermal processes, which are carried out at constant pressure or with adiabatic processes, which do not exchange heat. In these processes, a pressure change can occur. When the process is done in a constant volume it is called an isochoric process.

Isobaric process examples

To better understand this thermodynamic process, it will help us to see a couple of examples.

  • Expansion phase of the cylinder of an engine.

  • Boil water in an open container.

  • Heating of a globe due to the effects of solar radiation.

  • Hot air balloons experiment isobaric and isochronic process.

Heating the air of a balloon

Isobaric process. Definition and examplesThe change in volume that a balloon experiences as the sun's rays strike it is an example of an isobaric process. While the sun is increasing the temperature, the volume of the gas (air) experiments an isobaric expansion. 

At the beginning of the morning it presents a certain pressure, volume and temperature, as the air inside heats up, the pressure increases, but this does not vary due to the increase in its volume.

Cylinder expansion phase of a heat engine

The cylinder in a thermal engine can be expanded or contracted depending on the phase of the cycle. The expansion of air in a cylinder with a movable piston to which heat is supplied is carried out by an isobaric process. In the same way, during the compression, the volume is reduced isobarically.

The volume will increase in proportion to its temperature and the pressure will remain constant. This is in accordance with Charles Law.

Boiling water in an open container

An everyday example of an isobaric process is boiling water in an open container. By supplying heat energy to the water, it rises in temperature and turns into steam.

The steam obtained has a higher temperature and occupies a greater volume, however, the pressure remains constant. From the beginning the pressure is equal to atmospheric pressure.

Heating a hot air balloon

A hot air balloon is an example of the isobaric process. 

Hot air balloons work because hot air rises. By heating the air inside the balloon with the burner, it becomes lighter than the cooler air on the outside. This causes the balloon to float upwards, as if it were in water.

The pressure inside the balloon is the same as the atmospheric one. When the pilot injects heat into the air the temperature rise. It makes decreases the density of the air and because of the difference between its density and that the air, the balloon goes up.

Thermodynamically, part of the heat is converted into work making the hot air balloon rising up. Part of this heat is released outside the system because of thermodynamic contact of external air and because of the loss of hot air when it is expanded.


1-2 = P ( V 2 - V 1 )
1-2 = n R ( T 2 - T 1 )
1-2 = m c p ( T 2 - T 1 )
1-2 = ( k / ( k -1)) P ( V 2 - V 1 )



  • W 1-2 amount of work done by state change

  • Q 1-2 the amount of heat supplied or removed

  • P pressure

  • V the volume

  • T the absolute temperature

  • n the amount of dust (usually expressed in moles)

  • m the mass of the substance

  • cp the specific heat of the substance at constant pressure

  • k is a ratio equal to the quotient of the specific heat at constant pressure and constant volume, respectively

From the first equation we can see that if the system expands (ΔV is positive), then the system does positive work. On the contrary, if the volume increase is negative, the system contracts and the work is negative.

Equation of state of an ideal gas

The equation of state of an ideal gas (sometimes the Mendeleev  - Clapeyron equation or the Clapeyron equation) is a formula that establishes the relationship between pressure, molar volume, and the absolute temperature of an ideal gas. The equation is:

pV = nRT


p - pressure,
V- gas volume,
n- the amount of gas,
R - universal gas constant , R ≈ 8.314  J / (mol⋅K),
T - thermodynamic temperature, K kelvin.

Publication Date: April 24, 2018
Last Revision: August 24, 2020