# EnggPedia - The Engineering Encyclopedia

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# Basic Concepts of Thermodynamics

It is basically a physical science, based on the observations of physical phenomena followed by the experimental evidence which verify that it is correct. Finally, once the principles have been accepted then these observations are translated into mathematical formulation which provides mechanism by which these principles are applied to the engineering problems.

 Thermodynamics is defined as a study of energy and its relationship with the properties of matter. Energy could be defined as a capacity to produce change. Starting point for the study of Thermodynamics are the conservation of mass, the conservation of energy and second law of thermodynamics. Law of conservation of Mass & Energy are usually discussed in the introductory courses of physics. While the second law of thermodynamics is usually unique to a course in thermodynamics. Basic concept of second law of thermodynamics is developed from physical observation that heat transfer always takes place in the preferred direction without the external sources of energy.

In simple words heat always flows from high temperature region to low temperature region if, here the condition is “without the external sources of energy”. From this physical observation the concept of entropy can be formulated and used to predict whether the particular process occurs and to what extent the process will occur.

Thermodynamics provides important relationships among heat transfer, work interactions, kinetic and potential energy, and quantities that are called properties, which describe the condition of any substance. In fact, a major contribution of thermodynamics is the mathematical relationship between the amount of energy that is transferred to a substance and the change in the properties of that substance. The study of thermodynamics has emphasized applications to devices such as turbines, pumps, engines, compressors, air conditioners, solar collectors, rocket engines, fuel cells, wind and wave energy systems, and all systems that transform energy from one form to another to another.

System is a region enclosed by an imaginary boundary that may be rigid or flexible. Systems may be classified as being closed, open, or isolated. A closed system is one for which no mass crosses the boundary of the system. An example is for the gas with in the piston-cylinder assembly. Systems that permit the transfer of both mass and energy across the boundaries are called open systems. An example for an open system is described as a solar collector that uses the sun’s energy to heat water. A third type of system, called an isolated system, has neither mass nor energy crossing its boundary.

All thermodynamic systems consist of three basic elements: the imaginary surface that bounds the system, called the system boundary; the volume within the imaginary surface, called the system volume; and the surroundings. The surroundings are defined as every thing external to the system. If a system is isolated from its surrounding and the properties of the system do not change with respect to the time, the system is in thermodynamic equilibrium.

All energy or mass that enters or leaves a system must cross the surface area of the system boundary. When it does so, the properties inside the volume of the system may change. One of the objectives of our initial study of thermodynamics is to relate the amount of energy and mass that enter and leave the system to the changes in properties experienced by the mass with in the volume of the system.