In physics, an adiabatic process is a Thermodynamic process in which there is no heat or matter transfer into or out of a system and is generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat or matter transfer to take place.
On applying the first law of thermodynamics to the adiabatic process, we get:
Delta U = -W,
Delta U is the change in internal energy and W is work done by the system. We get following results from above relation:
- A system expanding under adiabatic conditions does positive work; hence its internal energy is decreased.
- A system contracting under adiabatic conditions does negative work; hence its internal energy is increased.
A common example of an adiabatic process is the compression and expansion stroke in an internal combustion engine. A negligible amount of heat is transferred outside of the system in an internal combustion engine; therefore it follows adiabatic compression and expansion.
Temperature change in gases under adiabatic processes-
The temperature of gases rises under adiabatic compression which is called adiabatic heating. However, in the expansion through adiabatic process against a pressure or a spring, the system causes a loss in temperature through a process called adiabatic cooling.
Adiabatic heating occurs when there is work done by the surroundings to pressurize the gas. This work done can be compared to a piston compression in a diesel engine cylinder. Moreover, the adiabatic heating can also occur naturally when the air mass is pressed down on the slope of a mountainous terrain, thereby increasing the temperature of the air because of work done on the air mass to decrease its volume against the land mass.
Adiabatic cooling occurs when the expansion takes place in an isolated system, rendering them to work in their surroundings. For example: when the air is depressurized by the lift in a wind current its volume gets spread out, thus reducing the temperature.
Adiabatic process and timescales-
The adiabatic process holds true for the long duration of time. However, it gets impossible for the mechanical processes of small time scales as there is no perfect insulator for the isolated system. In this, the heat is always lost when work is done.
Normally, an adiabatic process is the one where the net outcome of temperature remains unchanged. That doesn’t mean there is no transfer of heat in the process.
Types of adiabatic process-
An adiabatic process can be categorized into two processes namely reversible or irreversible.
- In the reversible adiabatic process, the entropy remains constant. It is also called an isentropic process.
- In the irreversible adiabatic process, the entropy doesn’t remain constant.
Consider m kg of air is heated adiabatically from point 1 to point 2, the Pressure-Volume diagram of the process is shown below:
Pressure, volume, and temperature at point 1 are P1, V1, and T1
Pressure, volume, and temperature at point 1 are P2, V2, and T2.
In the P-V diagram shown above, the gas in the piston cylinder is heated adiabatically.