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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:
  • air conditioners and refrigerators
  • turbo chargers and superchargers in automobile engines
  • steam turbines in power generation plants
  • jet engines used in aircraft
 
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.
 
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:


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

 
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.
 
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.
Vapor Compression Refrigeration Cycle
 
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.

Reversibility

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.
Reversible Systems
Some material excerpted from Van Wylen, Gordon J. and Sonntag, Richard E. Fundamentals of Classical Thermodynamics. 3rd Edition. New York: John Wiley & Sons, Inc., 1985.

Copyright © 1985, by John Wiley & Sons, Inc.

This material is used by permission of John Wiley & Sons, Inc
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