Introduction to Thermodynamics
.1 Definition
The word thermodynamics stems from the Greek words therme (heat) and dynamis (force/power),
which is most descriptive of the early efforts to convert heat into power: the capacity of hot bodies to
produce work.
Today, Thermodynamics is defined as the study of energy, its forms and transformations and the
interactions of energy with matter. Hence, thermodynamics is concered with
• the concept of energy
• the law that governs the conversion of one form of energy into another
• the properties of the working substances or the media used to obtain the energy conversion.
1.2 Value of Energy to Society
The availability of energy and people's ability to harness that energy in useful ways have transformed
our society. A few hundred years ago, the greatest fraction of the population struggled to subsist by
producing food for local consumption. Now, in many countries a small fraction of the total work force produces abundant food for the entire population, and much of the population is freed for other jobs.
We are able to travel great distances in short times by using a choice of transport means; we can
communicate instantaneously with persons anywhere on earth; and we control large amounts of
energy at our personal whim in the form of automobiles, electric tools and appliances, and comfort
conditioning in our dwellings.
It is very hard to imagine the present life without electricity and other forms of energy. The energy
available and consumed data exhibit the perspective picture of the economic condition and scope
and the level of advancement of living people's civilization.
1.3 Macroscopic versus Microscopic Viewpoint
It is well-known that a substance consists of a large number of particles called molecules. The
properties of the substance naturally depend on the behavior of these particles.
There are two points of view from which the behavior of matter can be studied: the macroscopic and
the microscopic approach.
a) Macroscopic Approach
This is the approach to the thermodynamics is concerned with gross or overall behavior. The
properties of the substance/matter is considered without taking into account the events
occurring at the molecular level.
For example, the pressure of a gas in a container is the result of momentum transfer between the
molecules and the walls of the container. However, one does not need to know the behavior of
the gas particles to determine the pressure in the container. It would be sufficient to attach a
pressure gauge to the container. Hence, instruments are used to find the value of the
thermodynamic properties.
This macroscopic approach to the study of thermodynamic that does not require knowledge of
the behavior of individual particles is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. The values of the properties of the systems are
their average values like pressure, temperature etc.
b) Microscopic approach
A more elaborate approach, based on the average behavior of large groups of individual particles
is called microscopic approach or statistical thermodynamics. The properties like velocity,
momentum, impulse etc. are studied by this approach. It required advanced statistical and
mathematical method since they are not easily measured by instruments.
1.4 Concept and Definitions
1.4.1 System and Surrounding
Universe: is defined as the totality of matter that exists.
A system and its surroundings together comprise a
universe.
System: is a quantity of matter or a region in space
selected for examination and analysis (study). The
system is a specified region wherein changes due to
transfers of mass and energy or both are to be studied.
It is not necessary that the volume or shape of the system should remain fixed.
Surrounding: It is the part of universe external to the system which strongly interacts with the
system under study.
Boundary: The real or imaginary surface that separates system from its surroundings is called
boundary. The boundary of a system can be fixed or movable. It is a contact surface shared by
both the system and surrounding.
Note: all transfers of mass and energy between the system and surroundings are evaluated at the
boundary.
The thermodynamic system may be classified into the following three groups: (a) Closed system;
(b) Open system; and (c) Isolated system.
a) Closed system
A system with fixed amount of matter (mass) i.e. no matter
(mass) can cross its boundary but the energy, in the form of
heat or work can cross the boundary. It is also referred to as
control mass (CM).
b) Isolated System
If neither mass nor energy is allowed to cross the boundary of a system is called an isolated
system. It is a special type of closed system that does not interact in any way with its
surrounding. E.g. any closed rigid insulated box.
c) Open system
If both mass and energy cross the boundary
of a system, it is called open system. An open
system permits both mass and energy cross
the boundaries and the mass within the
system may not remain constant. It is also
called control volume (CV).
Note: When the terms control mass and control
volume are used the system boundary is often referred to as a control surface.
1.4.2 Thermodynamic Property
The parameter to define the characteristic of system is called thermodynamic property. A
thermodynamic property is a macroscopic characteristic of a system such as mass, volume,
energy, pressure, and temperature to which a numerical value can be assigned at a given time
without knowledge of the previous behavior (history) of the system.
The thermodynamic properties of a system may be divided into the following two general classes.
a) Extensive properties
A property is called extensive if its value for an overall system is the sum of its values for the
parts into which the system is divided i.e. it is additives. The value of a property is
proportional to the mass of the system. Mass, volume, energy, and several other properties
introduced later are extensive. Extensive properties depend on the size or extent of a system.
The extensive properties of a system can change with time.
b) Intensive properties
If the value of a property is independent of the size or extent of a system i.e. independent of
the mass of the system, it is referred as intensive properties. Intensive properties are not
additive in the sense previously considered. It may vary from place to place within the system
at any moment. Thus, intensive properties may be functions of both position and time.
The ratio of an extensive property 'X' to the mass 'm' is called the specific value 'X/m' of that
property. Thus, v=V/m is the specific volume, specific total energy (e = E/m).
Generally, uppercase letters are used to denote extensive properties (with mass 'm' being a
major exception), and lowercase letters are used for intensive properties (with pressure 'P'
and temperature 'T' being the obvious exceptions).
1.4.3 Thermodynamic state and Thermodynamic Equilibrium
The word state refers to the condition of a system as described by its properties such as pressure,
volume, temperature, mass etc. At a given state, all the properties of a system have fixed values.
If the value of even one property changes, the state will change to a different one. Hence, each
unique condition of system is called a state. State is the condition of the system as all the
properties can be measured or calculated throughout the entire system, which gives us a set of
properties that completely describes the condition.
On the basis of the above discussion we can determine if a given variable is a property or not by
applying the following tests:
(a) A variable is a property if, and only if, it has a single value at each equilibrium state.
(b) A variable is a property if, and only if, the change in its value between any two prescribed
equilibrium states is single valued.
Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance.
In an equilibrium state there are no unbalanced potentials (or driving forces) within a system. For
a system, the properties describing the state will be constant if the system is not allowed to
interact with the surroundings or it the system is allowed to interact completely with unchanging
surroundings. Such a state is termed an equilibrium state and the properties are equilibrium
properties.
A system will be in a state of thermodynamic equilibrium, if the condition for the following three
types of equilibrium are satisfied
a) Mechanical equilibrium: a system is in mechanical equilibrium if there is no change in
pressure at any point of the system with time.
b) Thermal equilibrium: a system is in thermal equilibrium if the temperature is the same
throughout the entire system
c) Chemical equilibrium: a system is in chemical equilibrium if its chemical composition does
not change with time, that is, no chemical reactions occur
1.4.4 Thermodynamic Process and cycles:
Any change that a system undergoes from one
equilibrium state to another is called a process, and the
series of states through which a system passes during a
process is called the path of the process.
When a process proceeds in such a manner that the
system remains infinitesimally (very very) close to an
equilibrium state at all times, it is called a quasi-static,
or quasi- equilibrium, process. A quasi-equilibrium
process can be
viewed as a sufficiently slow process and also called
reversible process. It should be pointed out that a
quasi-equilibrium process is an idealized process and is
not a true representation of an actual process. But
many actual processes closely approximate it. It is
important because:
a) they are easy to analyze
b) serves as standards to which actual process can be compared.