Adiabatic Process

p-V graph of adiabatic process
By Yuta Aoki (Original) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Definition: An adiabatic process is a thermodynamic process in which there is no heat transfer (Q) into or out of the system. In other words Q = 0.

An adiabatic process 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-U = -W
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.
  • A system that contracts under adiabatic conditions does negative work, so the internal energy increases.
There is often, though not always, a change in temperature associated with the change in internal energy.

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.