Thermodynamics (chapter-1 introduction)

[R Gnyawali/ P Timilsina] Page 1
Chapter 1: Introduction
Thermodynamics is defined as the study of energy, its forms and transformations, and the
interactions of energy with matter. Hence, thermodynamics is concerned with the concept of
energy, the laws that govern the conversion of one form of energy into another, and the
properties of the working substance or the media used to obtain the energy conversion.
In engineering or applied thermodynamics, the scope is restricted to the study to heat and work
and the conversion of one into another. Thermodynamics laws are applied to work producing and
work absorbing devices in order to understand their functioning and improve their performance.
Value of Energy to Society:
Energy means ability to do work. Life is not possible without the energy. The ability of people to
harness the energy into useful forms has changed our society. The amount of energy needed for
people is not limited. Primitive people needed energy as only in the form of food. After finding
fire, they started cooking and keeping warm themselves. Wood Energy is usually used to get heat
energy which is one of the useful forms of energy. Then, they stared to cultivate land. They keep
domestic animals to work for them. Similarly, wind energy is used for ship sailing, driving wind
mills, and water energy is utilized to rotate the water wheels. These forms of energy have
simplified the life style of people. After the invention of steam engine (18th
Century), there was
industrial revolution. Later development of IC engine with use of fossil fuels (diesel, petrol,
kerosene) has changed our society greatly. Therefore, the amount of energy needed for society
has increased with time.
Due to use of energy with uncontrolled way, however, it has created different problems to our
environment like air pollution, water pollution and sound pollution.
One of the major problems faced nowadays is global warming. Global warming is due to green
house effect occurring due to increased in CO2, N2.
Fossil Fuels are rapidly consuming so these are coming to an end. Therefore, people are utilizing
and harnessing renewable forms of energy. These renewable energies are Solar energy, Wind
energy, Geothermal Energy and Nuclear Energy.
Concept and Definitions:
Thermodynamics is the science of energy transfer and its effect on the physical properties of
substances. It describes the state and changes in state of physical systems. It is based upon
observation of common experience which has been formulated into thermodynamics laws.
These laws govern the principle of energy conversion. Thermodynamics generally explain the
four laws known as Zeroth, First, Second and Third law of thermodynamics. These laws are
based on experimental observations and have no mathematical proof.
Thermodynamic System: A system is defined as a particular quantity of matter or a particular
region of space upon which attention is focused for the study of energy transfer.
Surroundings: The region outside a system that has a direct bearing (effect) on the system is
called its surroundings.
Boundary: Any real or imaginary surface that separates the system from its surroundings is
called the boundary. The boundary of a system can be either stationary or movable. Energy
transfer and mass transfer in and out of the system always take place through boundary.
Example: Real Boundary like wall of a vessel, Imaginary Boundary like pipe where fluid flows
through this imaginary boundary.

Types of System
a) Closed System: A system which can exchange
energy with its surroundings but there is no
mass transfer across the system is known as
closed system. In this type of system,
boundary is real and it can be fixed or
movable. Since the mass remains constant, this
type of system is also called Control Mass
(CM) System.
Example: cylinder fitted with movable piston, refrigerator unit, pressure cooker etc.
b) Open System: An open system can exchange mass
with surroundings along with transfer of energy in
the form of heat and work. The mass within the
system doesn’t necessarily remain constant; it may
change depending upon mass inflow and mass
outflow. In this type of system, the boundary is
normally imaginary. An open system can be called
as Control Volume (CV) System. The boundary of
the CV system is called control surface. Example:
Pump, compressor, boiler etc.
c) Isolated System: It is a special case of a closed system. It is of fixed mass and energy. It
exchanges neither mass nor energy with the surroundings. An isolated system has no
interaction with the surroundings. When a system and its surroundings are taken together,
they constitute an isolated system. Example: The universe can be considered as an
isolated system. Thermos flask can be considered an isolated system to some extent.
Homogeneous and Heterogeneous System: A system consisting of a single phase is called
homogeneous, while a system consisting of more than one phase is known as heterogeneous
system.
Macroscopic Vs Microscopic Viewpoint
There are two points of view from which the behavior of the matter can be studied namely
macroscopic and Microscopic.
Macroscopic View point Microscopic Viewpoint
1. A certain quantity of matter is
considered, without the events occurring at
the molecular level being taken into
account. In other words, the approach to
thermodynamics is concerned with gross or
overall behavior.
2. This is also known as classical
thermodynamics.
3. It is only concerned with the effects of
1. The approach considers that matters is
composed of very large numbers of
molecules. These molecules have different
velocities and energies. The behavior of
system is described by summing up the
behavior of each molecule.
2. Such study approach to thermodynamics
is also known as Statistical
Thermodynamics.
System
Energy
transfer
possible
No mass
transfer
Weight
Boundary
System Energy
transfer
possible
Mass
in
Mass
out

the action of many molecules and these
effects can be perceived by human senses.
For example, the macroscopic quantity, the
pressure, is the average rate of change of
momentum due to all the molecular
collisions made on a unit area. These
observations can be measured by pressure
gauge.
4. The analysis of this approach requires
simple mathematical methods.
3. The properties like velocities, positions,
accelerations, momentum which describes
the molecular cannot be easily measured
by instruments. Hence, our human senses
cannot feel the effects.
4. The behavior of system is found by
using statistical methods since large
numbers of particles is involved. Therefore,
advanced statistical mathematical methods
are needed to explain the changes in
system.
Thermodynamic Properties
Thermodynamic property of a system is any characteristics associated with a system which we
can measure or observe. Any thermodynamic system which contains many parameters that
describe system’s state (or Condition) is property of that system. The properties are mass,
elevation, pressure, temperature, viscosity, specific volume, energy (internal energy, enthalpy),
entropy, quality, moisture content.
Many properties have no significance in thermodynamics as electrical resistance and will not be
considered.
Intensive properties: If the value of the property is independent of the mass of the system, it is
called an intensive property. Such properties remain constant no matter how many times you
divide the system. Eg: temperature, pressure, density, viscosity.
Extensive properties: If the value of the property is dependent (or proportional ) to the mass of
the system, it is called extensive property. Eg. volume, surface area, energies (K.E., P.E. and
enthalpy).
Specific Property: An extensive property expressed per unit mass of the system is called
extensive property. This way, the extensive properties are converted into intensive property.
Sp. Volume v= V/m ; Sp. Energy e = E/m
Properties are dependent only on the state of the systems. It does not depend upon the path the
system follows to attain that particular state. Properties have definite unique values when the
system is in a particular state.
State: Each unique condition of a system is called a state. All the properties of a system which
are not undergoing any change can be computed or measured. These give us a set of properties
that completely describes the condition of a system, this condition is known as state. Every
equilibrium condition of a system is regarded as state and it is defined by a set of properties.
In graphical form, each state is denoted by a point.

T
T1
S
S1
Process: A thermodynamic process may be defined as a change of a system from one
equilibrium state to another and the series of states through which a system passes during a
process is called the path of the process. The path is represented by a line in a diagram. The line
may be straight or curved or dashed line.
Cycle: If a number of processes in sequence bring back the system to
its original state, the system is said to execute a cycle. In
thermodynamic cycle, the initial state and final state is identical. As the
system is restored after a cycle, the change in value of any property of
the system for a cycle/cyclic process is zero.
∫ = 0dp i.e. Change in any property (Px,Py) = 0
Pfinal (x,y)- Pinitial (x,y) = 0
Thermodynamic Equilibrium:
For a system, the properties describing the state of a system will be constant if the system is not
allowed to interact with the surroundings or if the system is allowed to interact completely with
unchanging. Such a state is termed as equilibrium state, and the properties are equilibrium
properties.
A system is said to be in equilibrium when it involves no change in property with time. A system
is said be in thermodynamic equilibrium if it satisfies the following conditions:
1) Mechanical Equilibrium: Uniformity of pressure or no unbalanced forces (Constant P).
2) Thermal Equilibrium: Uniformity of temperature (constant T).
3) Chemical Equilibrium: No chemical reactions; chemical composition remains same
throughout the system will time.
4) Phase Equilibrium: Mass of each phase reaches an equilibrium level and stays there.
Quasi-equilibrium/Quasi-static Process:
If a system is altered so that the state of the system moves along the surface from on equilibrium
position to another equilibrium position, the process is termed as quasi-equilibrium process. This
process proceeds in such a manner that the system remains infinitesimally close to an
equilibrium states at all times. A quasi-equilibrium process is a sufficiently slow process which
allows the system to adjust itself internally so that all properties in one part of the system do not
change any faster than those at other parts. Many actual processes closely approach a quasi-static
process.
P
V
1
4
3
2

Let us consider a system of gas contained in a cylinder fitted with a piston upon which are placed
very small pieces of weights. The system is initially in equilibrium state identified by pressure P1,
volume V1 and Temperature T1. If all the weights are removed at once the piston will rise rapidly
until mechanical equilibrium is again stored. This would be a non-equilibrium process and the
system would not be in equilibrium any time during this
change of state.
If the weights on the piston are taken off one by one, then the
total process can be described by a path of intermediate
equilibrium states. This process is considered as quasi-
equilibrium process.
A quasi-static process is also called a reversible process,
because it is possible to return to original position through
the same path of equilibrium states.
Reversible and Irreversible Process
A reversible process for a system is defined as a process that once having taken place can be
reversed so that the system and surroundings can be restored to their initial state.
An irreversible process is a process which once having taken place can’t be restored to its initial
state.
The reversible process is an idealization, and all the actual processes are normally irreversible
process.
Types of irreversibility
1. Mechanical and Thermal Irreversibility
2. Internal and External Irreversibility
P
V
1
2

Mechanical irreversibility is associated with the fluid friction among the molecules and the
mechanical friction between surfaces. Thermal irreversibility is associated with heat transfer with
finite temperature difference between the parts of system or between a system and surroundings.
Internal irreversibility is associated with fluid friction and temperature variation within the fluid.
Combustion and diffusion also cause internal irreversibility. External irreversibility is associated
with friction between the atmosphere and rotating members. All these absorb some work
developed by the system. External irreversibility occurs outside the boundary of the system.
Factors that makes process irreversible
1. Friction
2. Unrestricted Expansion
3. Heat transfer through a finite temperature difference
4. Mixing of two different fluids
Examples
Reversible Processes Irreversible Processes
1. Frictionless adiabatic expansion or
compression
2. Friction less isothermal expansion
or compression
3. Condensation or boiling of liquid
1. Free Unrestricted Expansion
2. Combustion, diffusion
3. Electric current flow through
resistor
4. Mixing of two fluids
5. Heat transfer from high temperature
to low temperature
4. Process involving friction
For a process to be reversible, it should obey following conditions.
1. The process should not involve friction of any kind.
2. Heat transfer should not take place with finite temperature difference.
3. There should be no mixing.
4. There should be no free or unrestricted expansion.
The process must proceed in a series of equilibrium states. It should move at an infinitely slow
pace.
Some common properties
1. Pressure:
It is the normal force exerted per unit area on a real or imaginary surface within the system.
A
F
A
f
P =
∂
∂
= Units: In SI; 1 Pa= 1 N/m2
1 bar = 105
Pa
1 atm = 101325 Pa = 760 mm of Hg

Pressure Measurement:
1) Barometer: For measuring atmospheric pressure
2) Manometer: Gauge pressure/Vacuum
Pressure
3) Bourden Pressure Gauge: Gauge Pressure
Problem: Find PA, if the fluid has a density ρ.
2. Volume:
Specific Volume: Specific volume of a substance is defined as the volume per unit mass and is
given by symbol v.
ρ
1
==
m
V
v Unit: m3
/kg
3. Temperature:
Temperature is a thermal state of a body which distinguishes a hot body from a cold body. The
temperature of a body is proportional to the stored molecular energy.
Two systems are equal in temperature if no change in any property occurs when they are brought
into contact, then it is called as equality of temperature.
Zeroth Law of Thermodynamics
The 'zeroth law' states that 'if two bodies (say A and B) are in thermal equilibrium with another
body C, then the bodies A and B will also be in thermal equilibrium with each other'. This law is
important as it helps in understanding the concept of temperature. It is fundamental law for
measurement of temperature.
This means that when two bodies have equality of temperature separately with the third body,
they in turn will have equality of temperature with each other.
H
PA
Patm

Measurement of temperature
1. Celcius (0-100)
2. Fahrenheit (32-212)
3. Kelvin (273.16-373.16)
4. Rankine (491.69-671.67)
Relation between temperature scales is given by;
Measuring Techniques
1. Change in volume: Mercury Thermometer
2. Change in Pressure: Constant Volume Gas Thermometer (T∝ P at constant volume)
3. Electrical Resistivity: Platinum resistant thermometer, Thermistor
4. Electrical Potential: Thermocouple
Ideal Gas
An ideal gas is defined as a gas having no forces of intermolecular attraction. The gases which
follow the gas laws at all ranges of pressure and temperature are considered an ideal gas.
However, real gases follow these laws at low pressure or high temperature or both. This is
because the forces of attraction between molecules tend to be very small at reduced pressures
and elevated temperatures.
Boyle’s Law:
P
V
1
∝ (T = Constant)
Charles Law: TV ∝ (P = Constant)
Ideal Gas Equation: TRnPV = ( R = Universal Gas Constant = 8.314 kJ/kg.K)
Also,
mRTPV
T
M
R
mPV
TR
M
m
PV
=
=
=
Here, R = Specific Gas Constant (R = 287 J/kg.K for air)

Thermodynamics (chapter-1 introduction)

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