Isobaric Process

Under Constant Pressure: Understanding Isobaric Processes

An isobaric process is a thermodynamic process in which the pressure remains constant. This is usually obtained by allowing the volume to expand or contract in such a way to neutralize any pressure changes that would be caused by heat transfer.

The term isobaric comes from Greek iso, meaning equal, and baros, meaning weight.

In an isobaric process, there are typically internal energy changes. Work is done by the system, and heat is transferred, so none of the quantities in the first law of thermodynamics readily reduce to zero.

However, the work at a constant pressure can be fairly easily calculated with the equation:

W = p * Δ V

Since W is the work, p is the pressure (always positive) and ΔV is the change in volume, we can see that there are two possible outcomes to an isobaric process:

  • If the system expands (ΔV is positive), then the system does positive work (and vice versa)
  • If the system contracts (ΔV is negative), then the system does negative work (and vice versa)

Examples of Isobaric Processes

If you have a cylinder with a weighted piston and you heat the gas in it, the gas expands due to the increase in energy. This is in accordance with Charles' law - the volume of a gas is proportional to its temperature. The weighted piston keeps the pressure constant. You can calculate the amount of work done by knowing the change of volume of the gas and the pressure. The piston is displaced by the change in volume of the gas while the pressure remains constant.

If the piston was fixed and didn't move as the gas was heated, the pressure would rise rather than the volume of the gas. This would not be an isobaric process, as the pressure was not constant. The gas could not produce work to displace the piston.

If you remove the heat source from the cylinder, or even place it into a freezer so it lost heat to the environment, the gas would shrink in volume and draw the weighted piston down with it as it maintained constant pressure.

This is negative work, the system contracts.

Isobaric Process and Phase Diagrams

In a phase diagram, an isobaric process would show up as a horizontal line, since it takes place along a constant pressure. This diagram would show you at what temperatures a substance is solid, liquid, or vapor for a range of atmospheric pressures.

Thermodynamic Processes

In thermodynamic processes, a system has a change in energy and that results in changes in pressure, volume, internal energy, temperature, or heat transfer. In natural processes, often more than one of these types are at work at the same time. Also, natural systems most of these processes have a preferred direction and are not easily reversible.