adiabatic

Meaning: In an adiabatic process, no heat crosses the boundary of the system. This happens when the system is well insulated or when the process is too fast for heat to flow. Energy can still enter or leave as work, so the internal energy change equals the work exchanged. For an ideal gas, internal energy depends on temperature. A gas that expands adiabatically does work on its surroundings, loses internal energy, and cools. A gas that is compressed adiabatically gains internal energy and warms.

Equation: ΔU = Q − W = −W, because Q = 0. ΔU is the change in internal energy (J), Q is heat added to the system (J), and W is work done by the system (J).

On the MCAT: Passages use PV diagrams. An adiabatic curve through a given starting point is steeper than the isotherm through that point. Questions ask for the sign of ΔU, W, or the temperature change, or ask you to spot the process from clues such as "insulated container" or "rapid compression." Thermodynamic cycles, such as the Carnot cycle, include adiabatic steps.

Common confusion: Adiabatic does not mean isothermal. An isothermal process holds T constant, which for an ideal gas means ΔU = 0, and it needs heat flow to do so. An adiabatic process has Q = 0 and the temperature usually changes. The exception is free expansion of an ideal gas into a vacuum. It is adiabatic, but W = 0, so ΔU = 0 and T stays the same.