A thermodynamic process is a chemical or physical process that takes place in a thermodynamic system, which changes the system from an initial state to a final state. There are different forms of thermodynamic processes. Isobaric and isochoric processes are two such processes.
What is Isobaric Process?
An isobaric process is a chemical process that takes place in a thermodynamic system under constant pressure. Therefore, the change in pressure or ∆P is zero. Usually, the system keeps the pressure constant via allowing the volume of the system to change; it can be either an expansion or a contraction. This change in volume can neutralize the changes in pressure that may occur due to heat transfer between the system and the surrounding.
Typically, in an isobaric process, the internal energy (U) changes. Therefore, the work (W) is done by the system during the heat transfer. We can calculate the work at constant pressure using the following equation.
W = P*∆V
Here, W is work, P is pressure and ∆V is the change in volume. Thus, if the heat transfer causes the volume of the system to expand, then the system does a positive work while if the heat transfer causes the volume of the system to contract, then the system does negative work.
What is Isochoric Process?
Isochoric process is a chemical process that takes place in a thermodynamic system under a constant volume. Therefore, there is no change in volume; ∆V is zero. Since the volume remains constant, the work done by the system is zero; thus the system does not work. Most of the times, this is the easiest thermodynamic variable to control. The process occurs in a sealed container that neither expands or contracts.
The internal energy of the thermodynamic system changes according to the heat transfer. However, all the heat transferred either increase or decrease the internal energy. Since ∆V is zero, the work done by the system (or the work done on the system) is zero as well. If U is the internal energy and Q is the heat transferred;
∆U = Q
What is the Difference Between Isobaric and Isochoric Process?
Isobaric process is a chemical process that takes place in a thermodynamic system under constant pressure while the isochoric process is a chemical process that takes place in a thermodynamic system under a constant volume. This is the key difference between isobaric and isochoric process. It means that the pressure of the thermodynamic system remains unchanged during an isobaric process whereas the pressure changes accordingly in an isochoric process. Besides this, the volume of the thermodynamic system changes during an isobaric process while the volume remains constant during an isochoric process. However, in both processes, the internal energy of the system changes. But unlike in the isobaric process, during an isochoric process, all the heat that is transferred either convert into internal energy or comes from internal energy.
The below infographic presents more details on the difference between isobaric and isochoric process in tabular form.
Summary – Isobaric vs Isochoric Process
Both isobaric and isochoric processes are thermodynamic processes that take place in thermodynamic systems while keeping a parameter constant. Therefore, the difference between isobaric and isochoric process is that isobaric process occurs at constant pressure whereas isochoric process occurs at constant volume.
1. Jones, Andrew Zimmerman. “The Isochoric Process.” ThoughtCo, Jun. 22, 2018. Available here
2. Jones, Andrew Zimmerman. “Under Constant Pressure: Understanding Isobaric Processes.” ThoughtCo, ThoughtCo. Available here