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…
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.
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Q1: What happens to work done in an isochoric process?
In an isochoric process, volume remains constant, so no work is done by the system. According to the first law of thermodynamics, all heat added to the system is converted entirely into internal energy change. For example, heating water in a sealed container increases only its internal energy without performing any work on surroundings.
Q2: How does an isobaric process differ from an isochoric process?
An isobaric process occurs at constant pressure, while an isochoric process occurs at constant volume. In isobaric processes, the system performs work as volume changes. Heat added is split between increasing internal energy and performing work. In isochoric processes, no work occurs and all heat increases internal energy only.
Q3: What does the pV diagram show for an isochoric process?
The pV diagram for an isochoric process is represented by a vertical line parallel to the pressure axis. This vertical line indicates that volume remains constant while pressure may change. The diagram visually demonstrates that no volume change occurs during the process, distinguishing it from other thermodynamic processes.
Q4: How is work calculated in an isobaric process?
In an isobaric process, work done equals the product of constant pressure and the change in volume. If volume expands, the system does positive work on surroundings. If volume contracts, work is done on the system and is negative. This work calculation is essential for understanding energy distribution in constant-pressure processes.
Q5: What is a real-world example of an isochoric process?
Evaporation occurring inside a sealed ecosystem is an isochoric process. Since water vapor cannot escape the sealed container, the volume remains constant. The heat added to the system increases only the internal energy of the water and air mixture without performing external work.
Q6: Why is boiling water in an open container an isobaric process?
Boiling water in an open container is an isobaric process because atmospheric pressure remains constant throughout. As water evaporates and converts to steam, the volume increases while pressure stays constant. The heat supplied performs work against atmospheric pressure while also increasing the internal energy of the system.
Q7: How do you calculate heat added in an isochoric heating process?
Heat added in an isochoric process equals the product of mass, specific heat, and temperature change. For example, heating 1000 g of water from 40°C to 70°C at constant volume involves a 30 K temperature change. Using water's specific heat of 4184 J/kg·K, the heat added is approximately 125.52 kJ, all converting to internal energy.