UNIT- I:
Fundamental Concepts and Definitions; Terminology, Definition and Scope;
Microscopic and Macroscopic Approaches; Engineering Thermodynamics; it's definition and practical applications;
Systems and Control volumes; Characteristics of System boundary and Control Surfaces; Surroundings and fixed, moving and imaginary boundaries; Thermodynamic States, state point; identification of a state through properties; definitions and units; extensive, intensive and specific properties, Thermodynamic planes and coordinate systems using properties; Change of state, path and processes; Quasi-static processes; Reversible processes, Restrained and unrestrained processes; Thermodynamic Equilibrium; diathermic wall, Zeroth Law of thermodynamics, Temperature as an important properties.
Q.1) What is the meaning of Thermodynamics?
Ans:) The branch of science that deals with energy and its movements in the space is generally known as Thermodynamics. The study of this science is based upon experimental values and common experiences and the laws are empirical in thermodynamics.
¤ Introduction:
The most of general sense of thermodynamics is the study of energy and its relationship to the properties of matter. All activities in nature involve some interaction between energy and matter. Thermodynamics is a science that governs the following:
- (i) Energy and its transformation
- (ii) Feasibility of a process involving transformation of energy
- (iii) Feasibility of a process involving transfer of energy
- (iv) Equilibrium processes
More specifically, thermodynamics deals with energy conversion, energy exchange and the direction of exchange.
¤ Areas of Application of Thermodynamics:
All natural processes are governed by the principles of thermodynamics. However, the following engineering devices are typically designed based on the principles of thermodynamics.
Automotive engines, Turbines, Compressors, Pumps, Fossil and Nuclear Power Plants, Propulsion systems for the Aircrafts, Separation and Liquefaction Plant, Refrigeration, Air-conditioning and Heating Devices.
The principles of thermodynamics are summarized in the form of a set of axioms. These axioms are known as four thermodynamic laws:
- Zeroth law of thermodynamics,
- First law of thermodynamics,
- Second law of thermodynamics, and
- Third law of thermodynamics.
The Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures.
The First Law deals with the conservation of energy and introduces the concept of internal energy.
The Second Law of thermodynamics provides with the guidelines on the conversion of internal energy of matter into work. It also introduces the concept of entropy.
The Third Law of thermodynamics defines the absolute zero of entropy. The entropy of a pure crystalline substance at absolute zero temperature is zero.
¤ Different Approaches in the Study of Thermodynamics:
There are two ways through which the subject of thermodynamics can be studied
- Macroscopic Approach
- Microscopic Approach
¤ Macroscopic Approach:
Consider a certain amount of gas in a cylindrical container. The volume (V) can be measured by measuring the diameter and the height of the cylinder. The pressure (P) of the gas can be measured by a pressure gauge. The temperature (T) of the gas can be measured using a thermometer. The state of the gas can be specified by the measured P, V and T . The values of these variables are space averaged characteristics of the properties of the gas under consideration. In classical thermodynamics, we often use this macroscopic approach. The macroscopic approach has the following features.
- The structure of the matter is not considered.
- A few variables are used to describe the state of the matter under consideration. The values of these variables are measurable following the available techniques of experimental physics.
¤ Microscopic Approach:
On the other hand, the gas can be considered as assemblage of a large number of particles each of which moves randomly with independent velocity. The state of each particle can be specified in terms of position coordinates ( x
i , y
i , z
i ) and the momentum components ( p
xi , p
yi , p
zi ). If we consider a gas occupying a volume of 1 cm
3 at ambient temperature and pressure, the number of particles present in it is of the order of 10
20. The same number of position coordinates and momentum components are needed to specify the state of the gas. The microscopic approach can be summarized as:
- A knowledge of the molecular structure of matter under consideration is essential.
- A large number of variables are needed for a complete specification of the state of the matter.
¤ Zeroth Law of Thermodynamics:
This is one of the most fundamental laws of thermodynamics. It is the basis of temperature and heat transfer between two systems. Suppose we take three thermodynamic system named System A, System B and System C. Now let that system A is in thermal equilibrium with system B. By thermal equilibrium we mean that there is no heat transfer between system A and system B when they are brought in contact with each other. Now, suppose system A is in thermal equilibrium with system C too and there is no contact between system B and system C. It implies that although system B and C are isolated from each other, they will remain at thermal equilibrium to each other. It means that there will be no heat transfer between system B and C, when they are brought in contact with each other. This is called the Zeroth Law of thermodynamics.
¤ Basis of Temperature:
When two bodies are kept at contact with each other and if there is no heat transfer between them we say that their body temperatures are same. It means that temperature is the property of a system which decides whether there will be any heat transfer between two different bodies. Heat transfer always occur from a higher temperature body to a lower temperature body. Further whenever there is any heat inflow to a body, it raises its temperature and conversely, if heat outflow occurs from a system it lowers its temperature.
Suppose we take two bodies one of which is at higher temperature than the other. Now when we bring the bodies at contact, heat will be transformed from a higher temperature body to that of lower temperature. Then what will be its effect, we may ask as a result of this heat transfer? Is this heat transfer a perpetual process? Our common life experiences tell us that it will not be the case. Although, at first heat transfer will take place, but its amount will be gradually decreased and after some time, a situation will come when there will be no heat transfer between the bodies or the bodies will come to a state of thermal equilibrium with each other. So, what is the reason for that? Can we justify the situation?
Yes, we can justify it as the hotter body releases heat to the colder body, the temperature of the hotter body decreases where as the temperature of the colder body increases and after sufficient time both the bodies will have equal temperature and a state of thermal equilibrium will be achieved.
¤ Temperature Measurement:
We know the temperature of a body can be measured with a thermometer. How can we actually calculate the temperature of a body with the help of thermodynamics?
¤ Thermometer:
A thermometer is a temperature measuring instrument. It is made of a thin capillary glass tube, one end is closed and the other end is fitted with metallic bulb full of mercury. The mercury is in thermal equilibrium with the metallic bulb. Therefore, the temperature of the mercury is equal to the temperature of the metallic bulb.
Mercury has a good coefficient of volume expansion and it means that as the temperature of the mercury increases, its volume increases too and as a result mercury column inside the capillary rises up.
The capillary tube has been graduated with the help of calibrating with standard temperature sources. Therefore, the temperature of the mercury can be measured from the height of mercury column as the tube is finely graduated.
Whenever we want to measure the temperature of a body, we kept the body in contact with the metallic bulb of the thermometer. When thermal equilibrium is established between the body and the metallic bulb of the thermometer, the temperature of both the body will be equal again the metallic bulb is in thermal equilibrium with mercury then the temperature of the mercury will be equal to the temperature of the metallic bulb and the temperature of the object.
As we can measure the temperature of the mercury from the column height, hence we can also determine the temperature of the object as they are equal to each other.
DISCUSSION:
Microscopic basis of temperature and pressure:
Here we shall try to discuss the basis of temperature and pressure only qualitatively, without any mathematical expression.
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Thermodynamic Systems:
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 now 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 change its size and shape. All thermodynamic systems contain three basic elements:
•
System boundary: The imaginary surface that bounds the system.
•
System volume: The volume within the imaginary surface.
•
The surroundings: The surroundings are everything external to the system.
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 and surroundings is called as "System boundary", which may be real and distinct in some cases where as some of them are virtual, but it may be real, solid and distinct. 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.
Classification of Thermodynamic Systems:
Systems can be classified as being
(i) closed, (ii) open, or (iii) isolated.
(i) Closed System:
A thermodynamic system may exchange mass and energy with its surroundings. There are systems which allow only energy transfer with surroundings in the form of either heat transfer or work transfer or both heat and work transfer between a system and its surroundings. In these types of system, any sorts of mass transfer between the system and its surroundings are prohibited. These types of systems are classified as closed system. Examples of closed thermodynamic systems include a fluid being compressed by a piston inside a cylinder, a bomb calorimeter. In a closed system although energy content may vary over a period of time, but the system will always contain the same amount of matter.
(ii) Open System or Control Volume:
An open system is a region in space defined by a boundary across which matter may flow in addition to work and heat exchange between the system and the surroundings. So, in an open system, the boundaries must have one or more opening through which mass transfer may take place in addition to work and heat transfer. Most of the engineering devices are examples of open system. Some examples are (a) a gas expanding from a container through a nozzle, (b) steam flowing through a turbine, and (c) water entering a boiler and leaving as steam. The boundary of an open system may be real or imaginary and it is called as control surface. The space inside an open system is called as control volume.
(iii) Isolated System:
In an isolated system, there is no interaction between a system and its surroundings. Hence, the quantities of mass and energy in these types of system doesn’t change with time or we can say mass and energy remain constant in an isolated system. If there is no change in energy of a system, it indicates that there is neither any kind of heat transfer nor any kind of work transfer. Our universe as a whole can be regarded as an isolated system.
Property, Equilibrium and State:
A property is any measurable characteristic of a system. The common properties include:
• pressure (P)
• temperature (T)
• volume (V)
• velocity (v)
• mass (m)
• enthalpy (H)
• entropy (S)
Properties can be intensive or extensive. Intensive properties are those whose values are independent of the mass possessed by the system, such as pressure, temperature, and velocity. Extensive properties are those whose values are dependent of the mass possessed by the system, such as volume, enthalpy, and entropy.
Extensive properties are denoted by uppercase letters, such as volume (V), enthalpy (H) and entropy (S). Per unit mass of extensive properties are called specific properties and denoted by lowercase letters. For example, specific volume v = V/m, specific enthalpy h = H/m and specific entropy s = S/m
*Note that work and heat are not properties. They are dependent of the process from one state to another state.
When the properties of a system are assumed constant from point to point and there is no change over time, the system is in a thermodynamic equilibrium.
The state of a system is its condition as described by giving values to its properties at a particular instant. For example, gas is in a tank. At state 1, its mass is 2 kg, temperature is 160°C, and volume is 0.1 m3. At state 2, its mass is 1 kg, temperature is 80°C, and volume is 0.2 m3..
A system is said to be at steady state if none of its properties changes with time.
State:
It is the condition of a system as defined by the values of all its properties. It gives a complete description of the system. Any operation in which one or more properties of a system change is called a change of state.
Phase:
It is a quantity of mass that is homogeneous throughout in chemical composition and physical structure. Examples of phase are solid, liquid, vapour, gas. Phase consisting of more than one phase is known as heterogenous system, where as if it consists of only one phase, it is called as homogenous system.
Process, Path and Cycle:
The changes that a system undergoes from one equilibrium state to another are called a process. The series of states through which a system passes during a process is called path.
In thermodynamics the concept of quasi-equilibrium processes is used. It is a sufficiently slow process that allows the system to adjust itself internally so that its properties in one part of the system do not change any faster than those at other parts.
When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. For example, the piston of car engine undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back to Intake again. It is a cycle.
Quasi-static Processes:
Although the processes can be restrained or unrestrained, in practical purpose we need restrained processes.
A quasi-static process is one in which,
• The deviation from thermodynamic equilibrium is infinitesimal.
• All states of the system passes through are equilibrium states.
In a cylinder-piston assembly, several small weights are placed on the piston as shown in the figure. If we remove a weight, the pressure on the enclosed gas will be reduced by an infinitesimal amount. If we remove these weights one by one very slowly, then the pressure on the gas will be reduced by very small amount very slowly. Every time we remove a weight, the equilibrium state will be changed to a new equilibrium state at a very slow rate, such that the system will be appeared at a static condition as the change is infinitesimally small and the rate of change is also very small. The path of the change will be a series of quasi-equilibrium states. These types of processes are known as quasi-static processes.
Equilibrium States:
A system is said to be in an equilibrium state if its properties will not be changed without some perceivable effect in the surroundings.
• Equilibrium generally requires all properties to be uniform throughout the system.
• There are mechanical, thermal, phase, and chemical equilibrium.
Nature has a preferred way of directing changes. As examples, we can say,
• Water flows from a higher to a lower level
• Electricity flows from a higher potential to a lower one
• Heat flows from a body at higher temperature to the one at a lower temperature
• Momentum transfer occurs from a point of higher pressure to a lower one.
• Mass transfer occurs from higher concentration to a lower one
Equilibrium state will be achieved when there will not be any change of the values of the properties of a system. Neither the system will exchange
Heat Energy nor any Work exchange nor any kind of mass exchange with its surroundings. There are mainly three kind of Equilibrium and they are as follows.
* Thermal Equilibrium
* Mechanical Equilibrium
* Chemical Equilibrium
Thermal Equilibrium:
When two bodies are in contact, there will be heat exchange between the bodies if and only there exists a temperature difference (ΔT) between the bodies.
Due to the temperature difference between the bodies, heat will flow from the high temperature body to the low temperature body.
As a result of this heat transfer, the temperature of the hot body will be decreased and the temperature of the cold body will be increased.
When the temperature of both the bodies becomes equal to each others, the flow of heat stops. This equilibrium condition is known as the Thermal Equilibrium.
Mechanical Eqiilibrium :
If there exists a pressure gradient (ΔP) inside a system, between two systems or between a system and its surroundings, then the interface surface will experience a net force not equal to zero and due to which work transfer will happen where the system having higher pressure will do work against the lower pressure system.
Due to this work transfer, pressure of the high pressure system will be decreased as energy has flown out of the system. On the other hand, the pressure in the low pressure system will be increased. When the pressure becomes equal in both sides, the work energy flow will be stopped and this state is known as the state of Mechanical Equilibrium.;
Chemical Equilibrium:
If there exists a chemical potential (Δμ) within the components of the system or between the system and surroundings, then there will be a spontaneous chemical reaction which will try to neutralize the chemical potential, after sometimes when the chemical potential becomes zero, the reaction stops and then there will not be any more changes in chemical properties of the system. This condition is called Chemical Equilibrium.
When a system attains thermal, mechanical and chemical equilibrium simultaneously, the state of the system is called in a "THERMODYNAMIC EQUILIBRIUM".
