Thermodynamics: Adiabatic Process

In physics, an adiabatic process is a thermodynamic process in which there is no heat 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 transfer to take place.

Applying the first law of thermodynamics to an adiabatic process, we obtain:

delta-Since delta-U is the change in internal energy and W is the work done by the system, what we see the following possible outcomes. A system that expands under adiabatic conditions does positive work, so the internal energy decreases, and a system that contracts under adiabatic conditions does negative work, so the internal energy increases.

The compression and expansion strokes in an internal-combustion engine are both approximately adiabatic processes—what little heat transfers outside of the system is negligible and virtually all of the energy change goes into moving the piston.

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What Is a Thermodynamic Process?

By Andrew Zimmerman Jones

Adiabatic and Temperature Fluctuations in Gas

When gas is compressed through adiabatic processes, it causes the temperature of the gas to rise through a process known as adiabatic heating; however, expansion through adiabatic processes against a spring or pressure causes a drop in temperature through a process called adiabatic cooling.

Adiabatic heating happens when gas is pressurized by the work done on it by its surroundings like the piston compression in a diesel engine's fuel cylinder. This can also occur naturally like when air masses in the Earth's atmosphere press down on a surface like a slope on a mountain range, causing temperatures to rise because of the work done on the mass of air to decrease its volume against the land mass.

Adiabatic cooling, on the other hand, happens when expansion occurs on isolated systems, which force them to do work on their surrounding areas. In the example of air flow, when that mass of air is depressurized by a lift in a wind current, its volume is allowed to spread back out, reducing the temperature.

Time Scales and the Adiabatic Process

Although the theory of adiabatic process holds up when observed over long periods of time, smaller time scales render adiabatic impossible in mechanical processes—since there are no perfect insulators for isolated systems, heat is always lost when work is done.

In general, adiabatic processes are assumed to be those where the net outcome of temperature remains unaffected, though that does not necessarily mean that heat is not transferred throughout the process. Smaller time scales can reveal the minute transfer of heat over the system boundaries, which ultimately balance out over the course of work.

Factors such as the process of interest, the rate of heat dissipation, how much work is down, and the amount of heat lost through imperfect insulation can affect the outcome of heat transfer in the overall process, and for this reason, the assumption that a process is adiabatic relies on the observation of the heat transfer process as a whole instead of its smaller parts.