A “system” is a quantity of matter or a region in space for study. The “surroundings” or “environment” is everything that interacts with the system.
A boundary separates the system from the surroundings. They may be:
- Fixed or movable
- Real or imaginary
An isolated system has neither mass or energy cross the boundary.
A closed system has no mass exchange with surroundings, but energy is exchanged with the surroundings. Mass is fixed (and conserved), which is called a “control mass”.
An open system has both mass and energy exchange with surroundings.
- The mass is generally not fixed.
- Boundary is usually fixed – “control volume”.