Closedsystem

Closedsystem, commonly written as closed system, is a term used in physics, chemistry, and engineering to describe a system that does not exchange matter with its surroundings, though it may exchange energy in the form of heat or work.

In thermodynamics, a closed system has fixed mass: matter cannot enter or leave the system boundaries. Energy transfer across the boundary is allowed, such as heat transfer or work performed by or on the system. The first law of thermodynamics for a closed system states that the change in internal energy ΔU equals the heat added to the system Q minus the work done by the system W (ΔU = Q − W, with the common convention that W is work done by the system). Different disciplines may adopt slightly different sign conventions, but the core idea remains energy conservation with conserved mass.

Closedsystem is often contrasted with an open system, which can exchange both matter and energy with its surroundings, and with an isolated system, which exchanges neither matter nor energy. In an isolated system, the total internal energy remains constant. Examples of closed systems include a gas in a sealed, rigid, but thermally conducting container, a chemical reaction in a sealed bottle that does not allow mass transfer, or a piston-cylinder device where the piston moves but no material passes through the boundary.

While the term closedsystem is widely understood, the preferred terminology in most modern texts is "closed system." The concatenated form is typically encountered in file names, software variables, or nonstandard usages rather than formal scientific writing.