Thermodynamics
is the study of relationship between energy and entropy, which deals
with heat and work. It is a set of theories that correlate macroscopic
properties that we can measure (such as temperature, volume, and
pressure) to energy and its capability to deliver work. A thermodynamic
system is defined as a quantity of matter of fixed mass and identity.
Everything external to the system is the surroundings and the system is
separated from the surroundings by boundaries. Some thermodynamics
applications include the design of:
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- air conditioners and refrigerators
- turbo chargers and superchargers in automobile engines
- steam turbines in power generation plants
- jet engines used in aircraft
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| Zeroth Law of Thermodynamics |
The zeroth law of thermodynamics
states that when two bodies have equality of temperature with a third
body, they in turn have equality of temperature with each other. All
three bodies share a common property, which is the temperature. For
example: one block of copper is brought into contact with a thermometer
until equality of temperature is established, and is then removed. A
second block of copper is brought into contact with the same
thermometer. If there is no change in the mercury level of the
thermometer during this process, it can be said that both blocks are in
thermal equilibrium with the given thermometer. |
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| First Law of Thermodynamics |
The first law of thermodynamics
states that, as a system undergoes a change of state, energy may cross
the boundary as either heat or work, and each may be positive or
negative. The net change in the energy of the system will be equal to
the net energy that crosses the boundary of the system, which may change
in the form of internal energy, kinetic energy, or potential energy.
The first law of thermodynamics can be summarized in the equation:
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Where:
 is the heat transferred to the system during the process
 is the change in internal energy
 is the change in kinetic energy
 is the change in potential energy
 is the work done by the system during the process
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| Second Law of Thermodynamics |
The second law defines the
direction in which a specific thermal process can take place. The second
law of thermodynamics states that it impossible to construct a device
that operates in a cycle and produces no effect other than the transfer
of heat from a cooler body to a hotter body. The second law of
thermodynamics is sometimes called the law of entropy, as it introduces
the important property called entropy. Entropy can be thought of as a
measure of how close a system is to equilibrium; it can also be thought
of as a measure of the disorder in the system. |
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| Vapor Compression Refrigeration Cycle |
One of the applications that
involves thermodynamic principles is the refrigerator. The figure below
is a schematic diagram of the components found in a typical
refrigerator.
The refrigerant enters the compressor as a slightly
superheated vapor at a low pressure. It then leaves the compressor and
enters the condenser as a vapor at some elevated pressure, where the
refrigerant is condensed as a result of heat transfer to cooling water
or to the surroundings. The refrigerant then leaves the condenser as a
high-pressure liquid. The pressure of the liquid is decreased as it
flows through the expansion valve and, as a result, some of the liquid
flashes into vapor. The remaining liquid, now at a lower pressure, is
vaporized in the evaporator as a result of heat transfer from the
refrigerated space. This vapor then enters the compressor.
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| Reversibility |
A reversible process for a
system is defined as a process that, once having taken place, can be
reversed and leaves no change in either system or surroundings. The
difference between a reversible and an irreversible process can be
illustrated with the example below.
Suppose
a gas under pressure is contained in a cylinder fitted with a piston.
The piston is locked in place with a pin. If the pin is removed, the
piston is raised and forced abruptly against the stopper. Work is done
by the system during this process because the piston has been raised by a
certain amount. If the system has to be restored to its initial state,
force has to be exerted on the piston until the pin can be reinserted.
Since the pressure on the face of the piston is greater on the return
stroke than on the initial stroke, the work done on the gas is greater
on the return stroke than the work done by the gas in the initial
process. This caused an amount of heat to be transferred from the gas to
the surroundings in order that the system have the same internal
energy. The fact that work was required to force the piston down and
that heat was transferred to the surroundings during the reverse process
makes the system an irreversible process.
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Another system has a number of
weights loaded on the piston at the initial state. The weights are
removed from the piston one at a time, allowing gas to expand and do
work in raising the weight remaining. If the process is reversed, the
weight can be placed back onto the piston without any work requirement,
as for each level of the piston there will be a small weight that is
exactly at the level of the platform. Such a process is a reversible
process. There are many factors that render a process irreversible, such
as friction and unrestrained expansion.
Thus, to summarize,
reversible systems occur in situations when the system is essentially
in equilibrium during the transition and at each step, and only an
infinitesimal amount of work would be necessary to truly restore
equilibrium.
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