ABSTRACT
Systems are separated from the rest of the universe by walls that allow them to interact with their surroundings. An isolated system is separated from its surroundings by walls that do not allow exchange of heat or matter. A closed system exchanges heat with its surroundings, but does not exchange matter, and an open system can exchange heat and matter with its surroundings. Energy exchanged between a system and its surroundings is always counted from the perspective of the system. A negative value means energy flows from the system into the surroundings: the energy of the system is reduced. A positive value means that energy flows into the system, and the energy of the system is increased.
