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What Is an Isobaric Process?
An isobaric process is a thermodynamic process taking place at constant pressure. The term isobaric has been derived from the Greek words “iso” and “baros”, which means equal pressure. As such, the constant pressure is obtained when the volume is expanded or contracted. This basically neutralises any pressure change due to the transfer of heat.
In an isobaric process, when the heat is transferred to the system, some work is done. However, there is also a change in the internal energy of the system. This further means that no quantities, as in the first law of thermodynamics, become zero.
Constant Pressure Process
An isobaric process is also called a constant pressure process.
The figure below shows one method of changing the state of gas while keeping the pressure constant.
A cylinder of gas has a tight-fitting, massless piston that can slide up and down, but the container is closed so that no atoms enter or escape. The mass on top applies a constant downward force Mg on the piston. The atmosphere also presses down on the piston. In equilibrium, the upward force PA on the piston by the gas in the cylinder, where A is the piston’s area, exactly balances the downward force Mg plus the air-pressure force Patmos A. Consequently, the gas pressure inside the cylinder is,
This pressure is independent of the temperature of the gas or the height of the piston, so it stays constant as long as M is unchanged.
If the cylinder is warmed, the gas will expand and push the piston up. But the pressure, determined by mass M, will not change. This process is shown on the PV diagram as the horizontal line 1 → 2. We call this an isobaric expansion. As isobaric compression occurs if the gas is cooled, lowering the piston. An isobaric process appears on a PV diagram as a horizontal line.
Isobaric Process Example
An example of the isobaric process includes the boiling of water to steam or the freezing of water to ice. In the process, a gas either expands or contracts to maintain constant pressure, and hence the net amount of work is done by the system or on the system. The amount of heat dQ is partly used in increasing the temperature dT and partly used in doing external work. One of the perfect scenarios of the isobaric process is the reversible expansion of an ideal gas.
Work Done by a Gas in an Isobaric Process
The work done is found from the equation,
Since the pressure is constant in an isobaric process, the integral becomes,
If the gas expands,
When the gas is compressed,
The work done by the gas is represented on the PV diagram by the rectangular area under the isobaric path. Whether the area is positive or negative depends on whether the gas expands or compresses.
Also Read: Thermodynamics
What Is Thermodynamic Process?
A process occurs when the system changes from one state (one set of values of its physical properties) to another state. The system returns to its original state when all of its macroscopic physical properties resume their original values.
Heat transfer and work are two processes that change the state of thermodynamic equilibrium. A quasi-static process is one in which the system changes slowly enough so that each succeeding state through which it passes is essentially in equilibrium. All the reversible processes are quasi-static (occur very slowly). An equilibrium state is a static state. During a reversible process, the system can deviate from equilibrium by only an infinitesimal amount.
There are other thermodynamic processes in equilibrium thermodynamics. The processes are characterised by the thermodynamic variable that is kept constant.
- Isothermal process: A process is isothermal when the temperature of the system is kept constant.
- Isochoric process: A process is isochoric when the volume of the system is constant.
- Adiabatic process: A process is adiabatic when no heat is transferred to or from the system.
Read More about the Thermodynamic Processes Below:
| Isothermal process |
| Isochoric process |
| Adiabatic process |
Frequently Asked Questions on Isobaric Process
Define isobaric process.
An isobaric process is a thermodynamic process that takes place under constant pressure. Temperature, volume, and internal energy are not constant in this process, even though the pressure is constant.
Is there a change in volume during an isobaric process?
During an isobaric process, there is an increase in volume.
Is there heat transfer during the isobaric process?
Heat enters the system during an isobaric expansion process. The system uses some of the heat to work on the environment, while the rest of the heat is used to increase the internal energy.
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