Surroundings and environment

The surroundings (or environment) are everything outside a thermodynamic system . Together, “system + surroundings” form the remainder of the universe relevant to the thermodynamic description, with interactions occurring only through the system boundary .

Physical interpretation

In thermodynamics, the surroundings represent the “rest of the world” that can impose constraints and exchange quantities with the system:

• If the boundary permits, energy may be transferred as heat (driven by temperature differences) or work (via generalized forces and boundary displacements).
• If the boundary is permeable, matter exchange can occur, changing particle number or composition inside the system.

A common idealization is to treat part of the surroundings as a reservoir: a very large system whose intensive variables are effectively unchanged by exchanges. For example, a thermal reservoir maintains an approximately constant temperature , and a work reservoir can supply or absorb work without appreciable change in its own macroscopic state.

Key properties and relations

• The distinction between “system” and “surroundings” is a modeling boundary choice; moving the boundary reassigns what is counted as internal energy versus energy transfer.
• For a system treated as isolated , the surroundings are thermodynamically irrelevant because no exchange is permitted across the boundary; in practice, “isolated” means interactions with the environment are negligible on the timescale of interest.
• When the surroundings can be idealized as fixed-$T$ or fixed-$p$ reservoirs, they provide the simplest physical route to defining controlled processes such as contact at fixed pressure or temperature.