Thermodynamics
Adiabatic Process
An adiabatic process is a change where no heat enters or leaves the system, so Q = 0, either because it is insulated or because it happens too fast for heat to flow.
Because no heat is exchanged, any work the gas does has to come out of its own internal energy. So when a gas expands adiabatically it cools, and when it is compressed adiabatically it heats up.
For an ideal gas, an adiabatic change follows PVᵞ = constant, where γ (gamma) is the ratio of specific heats. On a PV graph this curve is steeper than the isothermal curve PV = constant, because pressure and temperature change together.
Where students slip up
The trap is thinking that no heat exchange means the temperature stays the same. That is the isothermal case, not adiabatic. In an adiabatic process the temperature does change, precisely because no heat flows in to make up for the work done. No heat in or out is not the same as no temperature change.
Hold onto this
Adiabatic: no heat flow (Q = 0), but the temperature still changes.
Why it helps in the exam: know PVᵞ = constant and that the adiabatic curve is steeper than the isothermal one. Common line: a gas expands adiabatically, so it cools.
Where this connects
Adiabatic changes explain everyday things that happen fast enough to beat heat flow: air cooling as it rises to form clouds, and the warmth you feel when a bicycle pump is compressed quickly. Speed is what makes a process adiabatic.
Quick check
A gas expands adiabatically (Q = 0). What happens to its temperature?
- AStays the same
- BRises
- CFalls
- DDepends on the container