20.10:
Isochoric and Isobaric Processes
A thermodynamic process that occurs at constant volume is called an isochoric process.
Consider evaporation occurring inside a sealed ecosystem. Since the water vapour does not escape the container, the volume remains constant, and the process is isochoric.
At constant volume, no work is done by the system. Now, applying the first law of thermodynamics, the heat added to the system equals the change in internal energy.
In a pV diagram, this process is represented by a vertical line parallel to the pressure axis.
A thermodynamic process occurring at constant pressure is an isobaric process. When water evaporates from a river, the water vapor escapes. However, the atmospheric pressure on the system remains constant, and hence this process is isobaric.
Here, the work done equals the product of pressure and volume change. Therefore, the heat added to the system is utilized to increase the system's internal energy and perform the work.
The pV diagram for an isobaric process is a horizontal line parallel to the volume axis.
20.10:
Isochoric and Isobaric Processes
A thermodynamic process that occurs at constant volume is called an isochoric process. According to the first law of thermodynamics, heat supplied or removed from the system is partially utilized to perform work and change the internal energy of the system. However, in an isochoric process, the volume remains constant. Hence, the work done by the system is zero. Therefore, the exchange of heat changes the internal energy of the system only.
Suppose 1000 g of water is heated from 40 degrees celsius to 70 degrees celsius at constant volume. Since the process is isochoric, the work done by the system is zero. The heat added to the system is the product of mass, change in temperature, and specific heat of the water. Since the temperature change is 30 K and the specific heat of water is 4184 J/kg.K, the heat added to the system is estimated to be 125.52 kJ.
A thermodynamic process that occurs at constant pressure is called an isobaric process. In this process, the heat added to the system is utilized to increase the system's internal energy and to perform the work. Therefore, the work done during this process equals the product of pressure and volume change. If the volume expands, the system does positive work on the surroundings. However, if the volume of the system contracts, work is done on the system and is negative.
The boiling of water into steam in an open container is an example of an isobaric process, where the atmospheric pressure remains constant.
The pV diagram for an isochoric process is a straight line parallel to the pressure axis. The pV diagram for an isobaric process is a straight line parallel to the volume axis.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp 635.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. pp 125 Retrieved from https://openstax.org/books/college-physics/pages/15-2-the-first-law-of-thermodynamics-and-some-simple-processes