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Thermodynamics
Thermodynamics
Engineering is nothing but a problem solving tools based on the knowledge of science and mathematics
Thermodynamics is science which deals with energy and its transformations
It is the science of the relations between heat, Work and the properties of the systems
The word thermodynamics comes from two Greek words thermo (heat) and dynamics (work)
Examples of energy transformations were the study of thermodynamics is useful
Thermal energy producing systems like boilers, heat exchangers etc
Power producing devices like Internal combustion engines, steam turbine, gas turbine etc
Power absorbing devices like compressors, pumps etc
The science of thermodynamics is based on four laws known as zeroth, first, second and third law of thermodynamics
This law based on observations and does not have mathematical proof
Classical thermodynamics and Statistical thermodynamics
Classical thermodynamics uses macroscopic approach while statistical thermodynamics uses microscopic approach for analysis
In statistical thermodynamics the average behavior of large groups of individual molecules is considered
Classical thermodynamics approach does not require knowledge of behavior of individual molecules
In our study we use classical thermodynamics approach which is direct and easy way to the solution
Thermodynamic System
A thermodynamic system is defined as quality of matter or region in space chosen for thermodynamic study
The thermodynamic system is analogous to the free body diagram to which we apply the laws of mechanics, (i.e. Newton’s Laws of Motion)
The system is a macroscopically identifiable collection of matter on which we focus our attention(e.g.: the water kettle or the aircraft engine)
The mass or region outside the thermodynamic system is called as surroundings
The real or imaginary surface that separates the system from its surroundings is called the system boundary, It can be fixed or movable
The system and surrounding together is called as universe
Universe = System + Surroundings
Classification of Thermodynamics Systems
Closed System - It is defined as the system with fixed mass and where only energy crosses system boundary
It is also known as control mass
We do permit heat and work to enter or leave but not mass
Open System
It is defined as the system in which not only mass but also energy crosses system boundary
It is also known as control Volume
A surface that confines the control volume is called control surface
Control surface is nothing but system boundary
Most of the engineering devices are open system (examples - water heater, car radiator, turbine, compressor, and boiler)
Isolated System
It is defined as a neither system where neither mass nor energy crosses its boundaries
An universe can be considered to be an isolated system
Thermos Flask can be considered to be an isolated system
Choice of the System and Boundaries Are at Our Convenience
We must choose the system for each and every problem we work on, so as to obtain best possible information on how it behaves
In some cases the choice of the system will be obvious and in some cases not so obvious
Important: you must be clear in defining what constitutes your system and make that choice explicit to anyone else who may be reviewing your work (e.g.: In the exam paper or to your supervisor in the work place later)
The boundaries may be real physical surfaces or they may be imaginary for the convenience of analysis
e.g.: If the air in this room is the system, the floor, ceiling and walls constitutes real boundaries
The plane at the open doorway constitutes an imaginary boundary
Units and Dimensions
The fundamental dimensions are the time, length, mass and force
A unit is standard by which a dimensions is to be measured
S. I. system is used
Time
The fundamental unit of time is second (s)
It is defined as 1/886400 part of mean solar day
Length
The fundamental unit of length is meter (m)
It is distance between two marks on a platinum-Iridium rod at 00C, kept in a vault at International Bureau of weights and measures at Sevres, France
Mass
The fundamental unit of mass is the kilogram (kg)
It is defined as the mass of lump of platinum iridium also kept at International Bureau of weights and measures at Sevres, France
Force
The fundamental unit of force is Newton (N)
It is defined by Newtons second law, force is directly proportional to the product of mass and acceleration
F  m. a
F = 1/gc m. a
Where F = force, m= mass, a= acceleration, gc = proportionality constant
In S I units gc = 1
Thus
F = m. a
Weight
Weight of body is the force exerted on its mass due to gravity
The mass of body remains constant but value of weight is changing from place to place due to different gravitational force
F = m. g
Where g= gravitational acceleration
Unit of weight is N
Density ()
The density of a system is ratio of mass to the volume
Unit of density is kg/m3
Specific Weight (w)
It is the weight per unit volume
Unit of specific weight is N/m3
Specific Volume (v)
It is the volume per unit mass
Unit of specific volume is m3/kg
Specific Gravity
Specific gravity of a substance is defined as the ratio of density of substance to the density of standard substance