Adiabatic wall

An adiabatic wall is an idealized system boundary that does not allow heat transfer between a system and its surroundings . Equivalently, the net heat flow across an adiabatic wall is taken to be zero, so the heat term described by the inexact differential of heat satisfies

for energy exchange across that boundary.

Physical interpretation.
An adiabatic wall models perfect thermal insulation: even if there is a temperature difference, no energy crosses the boundary as heat. Real-world approximations include vacuum insulation, multilayer reflective shields, or very short time scales where there is negligible time for heat conduction.

What an adiabatic wall does not specify.
“Adiabatic” only constrains heat transfer. It does not, by itself, determine whether the system is closed or open (matter transfer is a separate idealization), nor whether the boundary permits mechanical motion and hence work transfer .

Connection to the first law.
For a closed system separated from its environment by an adiabatic wall, the first law reduces to a pure work balance:

• heat exchange vanishes across the wall, so changes in internal energy are accounted for entirely by work transfer (with the sign determined by the work sign convention used).

This is one reason adiabatic walls are conceptually important: they isolate work effects and help distinguish state functions (like internal energy) from path functions (like heat and work).

Contrast.
The complementary idealization is a diathermal wall , which allows heat exchange and enables thermal equilibration.