A system (often called a closed system) is any collectionof matter within a prescribed boundary, but no matter may enter or leave the system.
1. That the boundary may be real or imaginary and theshape of the boundary can change E.g.
2. Different parts of the system may be at different temperatures, pressures and velocities and may contain different sorts of matter.
3. If a fluid is passing steadily through a reservoir so thatthe mass in the reservoir is constant, we still can notconsider the reservoir as a system. Matter is crossing the boundary.
4. A system only has two kinds of interaction with itssurroundings. Work and Heat.
Positive WORK is done by a system, during a given operation, when the sole effect external to the systemcould be reduced to the rise of a weight.