BASIC CONCEPTS AND RELATED TERMINOLOGIES
Thermodynamics is the science of energy transfer which deals with the relations among heat, work and properties of systems..
Thermodynamic Systems and Surroundings
A Thermodynamic system is defined as a quantity of matter or a region in space whose behavior is being investigated.Everything external to the system is defined as surroundings.
Boundary is the surface which separates the system from its surroundings. It may be fixed or moving and real or imaginary.
Fig.1 Thermodynamic System, boundary, surroundings
Types of Thermodynamic Systems
There are three types of thermodynamic systems:- Closed System
- Open System and
- Isolated System
In closed system, attention is focused on a fixed mass. Energy in the form of heat and work (The terms heat and work will be defined in the chapter 2.) can cross the boundary of the system. But there is no mass flow across the boundary.
Hence, the possibility of change in volume is always there in the closed systems.
In open system, both matter and energy can cross the boundary. Here, the behavior of a fixed region in space called control volume is investigated and hence, there is no change in volume. The surface of the control volume is known as control surface.
Fig 3 Open system
A system that exchanges neither energy nor matter with its surroundings is known as an isolated system.
Fig 4 Isolated system:
THERMODYNAMIC PROPERTIES
Thermodynamic properties are classified into two categories:1. intensive
2. extensive
Intensive properties are independent of quantity of matter or mass .
(Example: pressure,temperature,density, specific energy,etc)
Extensive properties are dependent on mass.
(Example: volume, total energy, mass etc)
Thermodynamic State and Equilibrium
When a system does not undergo any change, all the properties have fixed values. This condition is known as a thermodynamic state.The factors that cause a change without any interactions with its surroundings are:
- Pressure difference
- Temperature difference
- Chemical reaction
If a system is balanced in all respects, it is in a state of thermodynamic equilibrium. Balanced in all respects means :
Thermal equilibrium: There should not be any temperature difference within the
system, so that the system is thermally balanced.
Mechanical equilibrium: No pressure difference exists between any two points within the
system (Neglecting gravitational effects) and between the
system and surroundings, so that it is mechanically balanced.
Chemical equilibrium: No chemical reaction is taking place, so that it is chemically
balanced.
No phase change: If two phases are involved, mass of each phase remains constant
so that phase equilibrium is achieved.
Hence, for a system in a state of thermodynamic equilibrium, there is no change in any macroscopic property.
Point and Path Functions
Thermodynamic functions are classified into two categories namely point and path functions. Point functions are those for which the change depends on only the end states and not on the path followed. Hence point functions are inexact differentialsPath functions are those for which the change depends not only on the end states but also on the path followed. Hence path functions are exact differentials
In can be observed the change in any property during a process depends only on end states. Therefore all the properties are point functions.
To demonstrate path and point functions, let us consider two stations A and B on a hill as shown in the Fig.1.6. While moving from station A to station B, let the distance traveled and increase in height from the mean sea level are observed. Distance traveled in path 1 is different from that in path 2. Hence it may be regarded as path function. But the change in height is same in both path 1 and path 2, therefore it is a point function.
Fig 6 Illustration of point and path functions
Temperature and Zeroth Law
Maxwell defined the temperature of a system as its Thermal state considered with reference to its ability to communicate heat to other bodies.When a hot body is brought into contact with a cold body, the hot body becomes cooler and the cold body becomes hotter. After sufficient time, the temperature of both the bodies will be equal. At that point, the two bodies are said to have reached thermal equilibrium.
Consider three bodies A, B and C. If the bodies A and B are in thermal equilibrium with C when brought into contact separately, they are also in thermal equilibrium with each other. This concept is known as zeroth law of thermodynamics.
Several properties of materials are found to be varying with temperature in a predictable way. This variation is used to measure temperature. In mercury thermometers, expansion of mercury with temperature is used for temperature measurement.
Temperature Scales
Freezing point of water known as ice point and boiling point of water known as steam point are taken as the reference states for all types of temperature scales.The various types as temperature scales in use are :
Celsius scale
Fahrenheit scale
Kelvin scale
Rankine scale
Reference state Celsius Kelvin Fahrenheit Rankine
Steam point 100 373 212 672
Ice point 0 273 32 492
Absolute Zero -273 0 - 460 0
Homogeneous and Heterogeneous Systems
Matter can exist in any one of the three phases namely solid, liquid and gas. A system consisting of a single phase is known as homogeneous systems. If the matter exists in more than one phase, the system is known as heterogeneous system.http://www.mediafire.com/view/57yaay9rd3a67lh/THERMO%201.docx
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