SYSTEM AND CONTROL VOLUME
A system is defined as a quantity of matter or a region in space chosen for
study. The mass or region outside the system is called the surroundings.
The real or imaginary surface that separates the system from its surroundings
is called the boundary (Fig. 1–23). The boundary of a system can be
fixed or movable. Note that the boundary is the contact surface shared by
both the system and the surroundings. Mathematically speaking, the boundary
has zero thickness, and thus it can neither contain any mass nor occupy
any volume in space.
Systems may be considered to be closed or open, depending on whether a
fixed mass or a volume in space is chosen for study. A closed system (also
known as a control mass) consists of a fixed amount of mass, and no mass
can cross its boundary. But energy, in the form of heat or work, can cross
the boundary, and the volume of a closed system does not have to be fixed.
If, as a special case, even energy is not allowed to cross the boundary, that
system is called an isolated system.
Consider the piston–cylinder device shown in Fig. 1–24. Let us say that
we would like to find out what happens to the enclosed gas when it is
heated. Since we are focusing our attention on the gas, it is our system. The
inner surfaces of the piston and the cylinder form the boundary, and since
no mass is crossing this boundary, it is a closed system. Notice that energy
may cross the boundary, and part of the boundary (the inner surface of the
piston, in this case) may move. Everything outside the gas, including the
piston and the cylinder, is the surroundings.
An open system, or a control volume, as it is often called, is a properly
selected region in space. It usually encloses a device that involves mass flow
such as a compressor, turbine, or nozzle. Flow through these devices is best
studied by selecting the region within the device as the control volume.
Both mass and energy can cross the boundary of a control volume.
A large number of engineering problems involve mass flow in and out of
a system and, therefore, are modeled as control volumes. A water heater, a
car radiator, a turbine, and a compressor all involve mass flow and should
be analyzed as control volumes (open systems) instead of as control masses
(closed systems). In general, any arbitrary region in space can be selected
as a control volume. There are no concrete rules for the selection of control
volumes, but the proper choice certainly makes the analysis much easier. If
we were to analyze the flow of air through a nozzle, for example, a good
choice for the control volume would be the region within the nozzle.
A control volume can be fixed in size and shape, as in the case of a nozzle,
or it may involve a moving boundary, as shown in Fig. 1–25. Most control
volumes, however, have fixed boundaries and thus do not involve any