Thermodynamics - System Models

System Models

An important concept in thermodynamics is the thermodynamic system, a precisely defined region of the universe under study. Everything in the universe except the system is known as the surroundings. A system is separated from the remainder of the universe by a boundary, which may be notional or not, but by convention delimits a finite volume. Exchanges of work, heat, or matter between the system and the surroundings take place across this boundary.

The boundary is simply a surface around the volume of interest. Anything that passes across the boundary that effects a change in the internal energy needs to be accounted for in the energy balance equation. The volume can be the region surrounding a single atom resonating energy, as Max Planck defined in 1900; it can be a body of steam or air in a steam engine, such as Sadi Carnot defined in 1824; it can be the body of a tropical cyclone, such as Kerry Emanuel theorized in 1986 in the field of atmospheric thermodynamics; it could also be just one nuclide (i.e. a system of quarks) as hypothesized in quantum thermodynamics.

Boundaries are of four types: fixed, moveable, real, and imaginary. For example, in an engine, a fixed boundary means the piston is locked at its position; then a constant volume process occurs, no work being permitted. In that same engine, a moveable boundary allows the piston to move in and out, permitting work. For closed systems, boundaries are real while for open system boundaries are often imaginary.

Thermodynamics sometimes distinguishes five classes of systems, defined in terms of what is allowed to cross their boundaries. There is no mechanical boundary for the whole earth including its atmosphere, and so roughly speaking, no external work is done on or by the whole earth system. Such a system is sometimes said to be diabatically heated or cooled by radiation. A process in which no work is transferred is sometimes called adynamic. In the table, ± denotes a permitted type of transfer, and 0 denotes a non-permitted type of transfer.

Engineering and natural processes are often described as composites of many different component simple systems, sometimes with unchanging or changing partitions between them.