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T H E R M O D Y N A M I C S

||| education & engineering |||

PREFACE:

When I joined IEC College of Engg & Technology in 1999, for the first time I heard of Richard Feynman & his style of writings. His three volume Lectures on Physics changed me permanently. The language was lucid and he told about the facts of Physics just like a thriller novel. I became a diehard fan of Physics and Feynman.

These piece of article on thermodynamic concept is an earnest try to tell about energy mechanics as a story.

"dedicated to the teaching methodology of Richard Feynman"

. . . . . . . . . . . .©sarpyl

CONCEPTUAL IDEAS :

If we want to analyze movement of energy over space, then we must define the space that would be used for the observation, we would call it as a SYSTEM, separated from the adjoining space that is known as "Surroundings", by a boundary that may be real or may be virtual depending upon the nature of the observation. The boundary is called as SYSTEM BOUNDARY. So, we shall define a system properly. A thermodynamics system refers to a three dimensional space occupied by a certain amount of matter known as ''Working Substance'', and it is the space under consideration. It must be bounded by an arbitrary surface which may be real or imaginary, may be at rest or in motion as well as it may chang its size and shape.

.......So we get a space of certain volume where ENERGY TRANSFER (movement of energy) is going on, what may or may not be real, and distinct, it may be virtual (in case of flow system ), again if real boundary exists, then it may be fixed (rigid boundary like constant volume system) or may be flexible (like cylinder-piston assembly). For a certain experiment the system and surroundings together is called UNIVERSE.

........The interface between the system & surroundings is called as "SYSTEM BOUNDARIES", which may be real & distinct in some cases where as some of them are virtual, but it may be real, solid & distinct.

.....ENERGY: Although we can't exactly define what is Energy, yet we can say how does Energy behave, even we can measure the change in energy of the system, we may say that Energy always posses the capability to do certain amount of work depending upon the form of Energy. We can also describe the different forms of Energy those can exist like potential energy, kinetic energy, chemical energy, binding energy, nuclear energy etc.

......Depending upon the capacity to do work, energy can be classified into different forms. If the energy is highly ordered then it is HIGH GRADE ENERGY, like Kinetic Energy, where as when Energy exists in a chaotic form we call it LOW GRADE ENERGY. Heat energy of a body arises due to random motions of the individual molecules. Hence we can say that HEAT ENERGY is related with the chaosness of the molecules, therefore, it is the most low grade energy.

HEAT TRANSFER :

.......Through the boundary, a system and its surroundings can exchange energy between them, if allowed by the boundary properties. There are three modes of energy interaction a system and surroundings. If the boundaries are permeable to allow heat flow across it, then the energy transfer mode is called HEAT TRANSFER.

........When a system absorbs heat energy from the surroundings due to the temperature difference between system & surroundings the transfer of energy is named as HEAT TRANSFER.

WORK TRANSFER :

....When a system has a flexible or movable boundary then energy can be transfer by virtue of workdone. If there exists a pressure gradient between a system and its surroundings then work exchange takes place between the system and the surroundings as the flexible boundary moves to destroy the pressure gradient that exists between the system and the surroundings. So we would say the Energy Transfer due to pressure gradient is named as WORK TRANSFER.

MASS TRANSFER :

For flow process in a open system, mass transfer takes place between system & surroundings which is the third type of Energy Transfer and named as Mass Transfer. Any open system has two passages for fluid flow. Through one passage, the mass of the working substance enters into the system and aptly named as INLET, while the second passage is used by the working fluid to flow out of the system and it is named as OUTLET. So, in open system mass flow occurs across the system and this phenomenon of mass inflow and outflow from the system is named as MASS TRANSFER between a system and it's boundaries.

CONCEPTS OF MASS :

What is mass? Or we can say what is it to be a substance? Here, again we face the fundamental difficulty to define Mass accurately, although we know how it does behave, we can measure its value even, but it is really not clear what is mass made of. When Einstein equates mass in terms of energy, it defines mass as a form of energy but they are bound within the mass which again consists of elementary but composite particles named electron, proton & neutron. Proton, neutrons are made of QUARKS, which are the most fundamental particles of nature. Although Quarks are already well researched, and we know the most possible reason of the “confinement” phenomenon, still there exists a large number of physical phenomenon which can be explained using the “standard model of particle physics”.

PROPERTIES OF A SYSTEM :

A system is characterized by the values of its properties. So the most logical question that would arise here would be about properties of a system. So what is a property of a system? It has been seen that every object that exists in this Universe possesses some physical & chemical characteristics, like size, shape, mass, energy, chemical composition, colors etc. Among these various characteristics, those are related with energy directly or indirectly are called as "thermodynamic functions". There are mainly two types of thermodynamic functions, which can be better described in mathematical terms as they are physical quantity and hence are measurable. Here we shall take a little hiatus (break) to know some facts about physical quantity.

Physical characteristics are of two types. Any physical characteristics can be represented by the mathematical quantity and it is thus represented by mathematical functions. There may be two types of mathematical functions. When expressed in differential form, some of the functions become Exact differential and some of them produces In-exact differential form. The exact differential functions are called as thermodynamic properties. They are also known as “Point Functions”. Where as the in-exact differentials are called “Path Functions”

EQUILIBRIUM CONDITIONS :

Every thermodynamic function are directly or indirectly measurable and when there is no energy transfer between the system and surroundings, then the value of the functions assume a certain value by which we can specify the state or condition of a thermodynamic system. So, the values are only measurable when they are not changing over a period of time. What does it implicate that the values of the thermodynamic functions are not continuously changing. When the values are not changing it indicates a stability of the state of the system over a certain periods of time. This stability of a system implies an Equilibrium condition. Each and every equilibrium states are distinct and they are specified by the distinct value of the properties of the system at that equilibrium conditions.

CONCEPTS OF A THERMODYNAMIC PLANE :

A thermodynamics system is a bi-variate function. It means that to specify a thermodynamic system we need to specify the values of any two properties. One can also describe a thermodynamic system has two degrees of freedom. So, mathematically, we can say that any thermodynamic system at a certain equilibrium condition can be represented as a point on a two dimensional plane. The plane thus formed plotting thermodynamic properties along X and Y axes of a Cartesian Coordinate system is known as thermodynamic plane.

A point on this plane represents a system at a thermodynamic equilibrium condition, which can be defined as a state which is time invariant when it is isolated from the surroundings. So, at equilibrium condition the values of different thermodynamic properties remain constant over a considerable amount of time.

External Disturbances and CHANGE OF STATE of a system.

We have already know that when the values of different thermodynamic properties become stable we get an equilibrium state where no values of the properties can be changed without application of any external influences. But, what happens, when an external agency tries to change the values of the properties of the system. Here, what does it mean by "external influences"? What does it mean in real life? We know from our daily life experiences that any kind of external influences can be at last reduced to any kind of force only and the use of external influences always lead to an exchange of energy between the system and the surroundings. To explain the phenomena we shall take a system at equilibrium with its surroundings. Hence, the pressure P and temperature T of both the system as well as the surroundings too. Now, to disturb the equilibrium condition of the system we must change either the pressure or the temperature of the system. Suppose we take a cylinder-piston assembly, whose temperature is T and pressure is P. Now suppose, we inject a small amount of energy very slowly into the system, can you tell, what type of change we should expect in this case.

Suppose we change the pressure from (P) to (P + dP) where (dP) is the change of pressure of the system, where as the pressure of the surroundings remains at (P). Then there exists a pressure difference of the flexible wall that separates our system from its surroundings. As a result a force will act on the flexible wall of the system, and the wall will move along the net force on it. Therefore, an amount of work done will be there due to the displacement of the boundary wall. There may be two type of cases, when (dP) is positive, the system does work on the surrounding as the volume of the system increases. As the volume increases from V to (V + dV) the pressure would drop to (P) from (P + dP). Hence due to this energy transfer from the system to surrounding again Equilibrium will be achieved.

VARIABLES IN THERMODYNAMICS

STATE and COMPOSITION of MATTER :

From our common sense we can say that matters are composed of mass, a fundamental form of energy. From our early experimentation with mass and nature, we could conclude a concept of mass as a continuous physical quantity, it implies that we can divide any quantity of mass, whatever small it may be. This view is essentially evolved on the basis of our macro world perception.

But within few years rapid growth of modern science shows that our perception of a continuous character of mass is an incorrect idea. Hence particle character was bestowed on mass that tells us that masses are made of tiny particles, that can independently exist in a stable condition and nicely named as molecules which are different for different materials. But it is not the fundamental particles of mass. There are more to come!

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