In thermodynamics, all measured
space is represented by the universe. However, considering the entire
universe every time, a calculation is needed and that will be not very
convenient. Therefore, the universe is broken down into the region of
interest, the system, and the rest of the universe, the surroundings.
The system is typically
chosen so that it contains the substance of interest, excluding the physical
apparatus itself. Its volume may be fixed, or its volume may change with time.
Similarly, its composition may be fixed, or the composition may change due to
mass flow or chemical reaction.
The system is separated from the surroundings
by its boundary, which may be a real and physical entity or an imaginary
construct. There are instances when a great deal of computational effort is
saved by judiciously choosing the system and its boundary.
The term "control
volume" is employed to describe the region of space inside the
system's boundaries. Often, primary interest lies in the material and energy
crossing the system's boundaries. For example, in designing a heat engine,
consideration is given to how much work is produced (energy leaving the
system), and understanding how the turbine affects the material flowing through
it involves examining the differences between the material entering and leaving
the system.
An open system can have
both matter and energy entering and/or leaving it. A lake serves as an
open system, where material can enter as rain and leave as evaporation.
Additionally, rivers or streams may flow into and out of the lake. Energy
transfer, exemplified by sunlight, can affect the system, causing the lake
water to warm, even when no water enters or leaves.
A closed system has no
matter entering or leaving, although energy transfer is still possible. A
sealed aluminum can is an example of a closed system; material cannot enter or
leave, but its contents can be heated or cooled.
The Rankine cycle provides
examples of both closed and open systems. If the turbine is defined as
the system, it is open, as steam continuously flows both into and out of it.
Similarly, the pump, evaporator, and condenser are individual open systems.
However, defining the system to include all four unit operations makes it a
closed system. Water, in liquid or vapor form, circulates continuously inside
the system, but no material crosses its boundaries, making it a closed system.
An adiabatic system
experiences no heat entering or leaving. It's possible for either closed
or open systems to be adiabatic. For example, a section of pipe in a house may
have water continuously flowing into and out of it, making it open, but it may
be insulated enough to be considered adiabatic.
In an isolated system,
there is neither entry nor exit of matter or energy. If a thermos is
closed and sufficiently insulated to be conceptualized as perfectly insulated,
then it is deemed an isolated system. In a system that is both closed and
adiabatic, work can be added or removed (e.g., the "expansion work"),
but an isolated system remains unaffected by the surroundings in any manner.
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