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Adiabatic Process


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.

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