Adiabatic process

An adiabatic process is one in which no energy is transferred into or out of a system by thermal processes. With no heating or cooling, the change in internal energy equals the work done on the system.

Essential knowledge 9.4.B.3 defines it by what is missing: a process in which no energy is transferred to or from the system through thermal processes. So $Q = 0$ , and the sheet's first law reduces to

\Delta U = W

with $W = -P\Delta V$ the work done ON the gas.

Adiabatic is not isothermal. This is the confusion the term is tested on. Both feel like "nothing is being added", but they zero out opposite terms.

Process	What is zero	Consequence
Adiabatic	$Q$	$\Delta U = W$ , so the temperature moves
Isothermal	$\Delta U$	$Q = -W$ , so energy flows through

Compress a gas adiabatically and $\Delta V < 0$ , so $W > 0$ , so $\Delta U > 0$ and the temperature rises even though nothing was heated. The tip of a bicycle pump gets hot for exactly this reason. Let a gas expand adiabatically and it cools.

There are two ways to arrange one. Insulate the system so no thermal transfer is possible, or run the process fast enough that there is no time for one. The second is why a rapid compression is treated as adiabatic even in an uninsulated cylinder.

On a pressure-volume diagram an adiabatic path crosses isotherms rather than following one, moving onto cooler isotherms as the gas expands and hotter ones as it is compressed.

Combining paths and reading areas is the PV diagrams guide's job.

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