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Chapter: Mechanical - Engineering Thermodynamics - Basic Concepts And Definitions

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Pure Substances and Concept of continuum

Substances of fixed chemical composition throughout are known as pure substances.

Pure Substances

 

Substances of fixed chemical composition throughout are known as pure substances.

 

That is, pure substances have homogenous and invariable chemical composition irrespective of the phase or phases in which they exist.

 

Example

 

a.Atmosphere air

 

b.                  Water

 

c. Nitrogen

 

d.                  Water-steam mixture

 

e. Product of combustion.

 

Though, mixture of water and steam is considered a pure substance, air and liquid air cannot be, since, the chemical composition of liquid air differs from that of gaseous air.

 

The Ideal Gas

 

Based on the experimental work carried out by Boyle, Charles and Gay-Lussac, pressure, temperature and specific volume of many gases at low pressure and moderate temperature are related by the following equation.

 

pv = RT where R=

 

This equation is known as equation of state of an ideal gas. The term R is known as characteristic gas constant and Ru universal gas constant. In SI unit Ru= 8.314 kJ/kgmol.K.


Concept of continuum

 

In microscopic approach the substance is assumed to be continuously distributed, ignoring the space between the molecules. This is known as continuum hypothesis.

 

Since the matter is treated as continuous, the density at a point can be defined as


Where vis the smallest volume for which a definite value of the ratio exists. Below the limiting value of v, the fluctuation in average density will be high and a definite value for the ratio becomes impossible, with the mean free path* of the molecules approaching the order of magnitude of the dimension of the vessel



BASIC CONCEPTS AND DEFINITIONS

 

 

Thermodynamics is the science of energy transfer which deals with the relations among heat, work and properties of systems.

 

The name thermodynamics is derived from the Greek words therme, meaning heat and dynamis meaning power. Thus, thermodynamics is basically the study of heat and power.

 

Application Area of Thermodynamics

 

Energy transfer is present in almost all the engineering activities. Hence, the principles of thermodynamics are playing vital role in designing all the engineering equipments such as internal combustion engines, rockets, jet engines, thermal and nuclear power plants, refrigerators etc.


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