adiabatic process
/ ad-ee-uh-BAT-ik /
An adiabatic process is a change in which no heat enters or leaves — the system is sealed off thermally, as if wrapped in a perfect blanket. Any energy that changes hands does so purely as work. Pump up a bicycle tyre quickly and the pump warms in your hand: you compressed the air faster than its heat could leak away, so the work you did went straight into raising its temperature. That is adiabatic compression.
Because q = 0, the first law shrinks to a tidy statement: the change in internal energy equals the work alone, ΔU = w. Do work on the system by compressing it and its internal energy — and usually its temperature — rises. Let it do work by expanding and it cools, paying for that work out of its own internal energy. No outside heat comes to the rescue.
Perfect adiabatic isolation is an idealization — no blanket is flawless — but it is an excellent approximation whenever a change happens too fast for heat to flow, or inside well-insulated walls. It explains why air cools as it rises and expands into thinner sky, giving us clouds, and why a diesel engine can ignite fuel by compression alone.
A diesel engine has no spark plug. It compresses air so swiftly and so hard — adiabatically — that the air's temperature soars high enough to ignite the injected fuel all on its own.
Adiabatic compression heats a gas; adiabatic expansion cools it.
Don't confuse adiabatic with isothermal. Adiabatic means no heat crosses the boundary, so the temperature is usually free to change. Isothermal means the temperature is held fixed, which generally requires heat to flow in or out. They are almost opposites: one blocks heat, the other deliberately lets heat pass to keep temperature steady.