System, Surroundings, Boundary, & Closed Systems

A fixed amount of mass. No mass can cross the boundary of the system.

The fluid within the cylinder of a piston-and-cylinder device is an example of a closed system with a moving boundary.

The system boundary is indicated on the diagram by the dashed, black line.

Closed Systems

Open Systems

Types of Systems:
Closed System -
a quantity of mass or a volume of space that
we have chosen to study and the surroundings are everything else in the universe. The system boundary separates the system from the surroundings.
System -

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Ch 1, Lesson C, Page 2 - System, Surroundings, Boundary, & Closed Systems

• In the simplest terms, a system is the stuff that we are studying and the surroundings are everything else in the universe.
• The system is the mass or volume that we are interested in and it is separated from the surroundings by the system boundary.
• The system boundary is not always fixed because our system can expand or contract.
• Once you understand what a system is, you need to know that there are two different types of systems: Closed Systems and Open Systems.
• First, lets look at closed systems.
• A closed system is a fixed amount of mass.  Therefore, no mass can cross the system boundary.
• The piston-and-cylinder device, shown here, contains a gas.
• The gas can expand or contract and the system boundary expands or contracts with it.  But no mass is allowed to enter or leave the system.
• Therefore, the gas inside the cylinder is a closed system.
• It is customary to show the system boundary with a dashed line like the black line shown here.
• Now, let’s take a look at an open system.