System boundary

A system boundary is the surface (or more generally, the interface region) that separates a thermodynamic system from its surroundings . It may be a physical wall (container, membrane, piston) or an imaginary surface drawn through a continuum (as in control-volume descriptions).

The boundary specification determines which exchanges are allowed:

• Energy exchange as heat and/or work .
• Matter exchange (permeable vs impermeable).

Physical interpretation

The boundary is where “interaction with the outside world” happens. In thermodynamic bookkeeping, changes of internal energy are attributed either to transfers across this boundary or to changes within the system’s state.

Two especially important boundary idealizations are:

• A diathermal wall that permits heat exchange (thermal contact).
• An adiabatic wall that forbids heat exchange.

Similarly, the boundary may be:

• Rigid or movable, controlling whether mechanical boundary work can occur.
• Permeable or impermeable, controlling whether the system can be open (matter exchange) or remain closed (no matter exchange).

Key relations (energy transfer at the boundary)

• Heat and work are boundary-crossing energy transfers and are therefore path dependent rather than state properties. Their differentials are typically written as $\delta Q$ and $\delta W$ to emphasize they are not exact differentials (see heat as an inexact differential and work as an inexact differential ).
• For a simple compressible system with a moving boundary, mechanical interaction often appears as pressure–volume work of magnitude $p\,dV$. The sign depends on the chosen convention (see the pressure–volume work sign convention and the more general work sign convention ).

Boundary choice and system type

By design, the boundary choice determines whether the same physical material is modeled as an isolated system (no exchange), a closed system (energy exchange only), or an open system (matter exchange allowed).