Thermodynamic Systems

1. ENGINEERING MECHANICS
AND
THERMODYNAMICS

2. UNIT III

3. Basic Concept and Defination
The field of Science, which deals with the energies possessed by gases and
vapors, is known as Thermodynamics.
It also includes the conversion of these energies in terms of heat and
mechanical work and there relationship with the properties of the system.
The machine which converts the heat into mechanical work and vise versa,
is known as Heat Engine.
Properties of a System:
The state of a system may be described by certain observable quantities
such as Volume, Temperature, Pressure and density etc.
All the quantities, which identify or describes the state of a system are
called Properties.

4. Thermodynamic System:
The Thermodynamic System may be classified into the following three
groups:
1. Closed System
2. Open system
3. Isolated System

5. Closed System:
 Closed System: This is a system of fixed mass and identity whose
boundaries are determined by the space of the matter ( working
substance ) occupied in it.
 A Closed System does not permit any mass transfer across the
boundary but it permits transfer of Energy.( Heat and Work)
 Example: Let us consider gas in a cylinder as a system.
If Heat is supplied to the cylinder from some external source, the
temperature of the gas will increase and the piston will rise.

6. Open system:
 In open system, the mass of the working substance crosses the
boundary of the system.
 Heat and work may also cross the boundary of the System.
 Open System permits both mass and energy (heat and work) transfer
across the boundaries and mass within the system may not be
constant.

7. Isolated System:
 A System which is completely uninfluenced by the surrounding is
called an isolated system.
 It is a system of fixed mass and no heat or work energy cross the
Boundary.
 An Isolated System does not have transfer of either mass or energy
( heat and work) with the surrounding.

8. Pressure:
The term “pressure” may be defined as the normal force per unit area.
The unit of pressure depends upon the units of force and area.
In S.I. system of units, the practical unit of pressure is N/mm², N/m²,
KN/m², MN/m² etc.
But sometimes a bigger unit of pressure (known as bar) is used, such
that,
1 bar = 1 x 10⁵ N/m²
= 0.1 x 10 ⁶ N/m²
= 0.1 MN/m²
1 Pa = 1 N/m²
and, 1 KPa = 1 KN/m²

9. Mathematical Expression of Gauge Pressure and
Atmospheric Pressure:
In case of Gauge pressure,
Absolute pressure = Atmosphere pressure + Gauge Pressure
In case of Vacuum Pressure,
Absolute Pressure = Atmospheric pressure – Vacuum Pressure
Absolute pressure
Gauge pressure
Atmosphere Pressure
Atmospheric pressure
Absolute pressure
Gauge pressure (-ve) or Vacuum Pressure
Relation between absolute , atmospheric and
gauge pressure Relation between absolute, atmosphere and
Vacuum Pressure
Gauge Pressure and Absolute Pressure:
All the pressure gauges read the difference between the (actual pressure in any system and the
atmosphere pressure).
The reading of the pressure gauge is known as gauge pressure, while the actual pressure is called
Absolute Pressure.

10. The standard value of atmospheric pressure is taken as 1.013 bar
(or 760 mm of Hg) at sea level.
We know that,
1 bar = 10⁵ N/ m²
Atmospheric pressure = 1.013 x 10 ⁵ = 1013 x 10² N/m²
We also know that atmosphere pressure
= 760 mm of Hg
1 mm of Hg = 1013 x 10² /760 = 133.3 N/m²
1 N/m² = 760 / 1013 x 10 ² = 7.5 x 10 ⁻³ mm of Hg.
Atmospheric Pressure:

11. Energy:
The energy is defined as the capacity to do work.
In other words, a system is said to possess energy when it is capable of
doing work.
The energy possessed by a system is of the following two types:
1. Stored energy
2. Transit energy
The stored energy is the energy possessed by a system within its
boundaries. The potential energy, kinetic energy and internal energy are
the examples of stored energy.
The transit energy (or energy in transition) is the energy possessed by a
System which is capable of crossing its boundaries.

12. Types of Stored Energy:
1. Potential Energy: It is the energy possessed by a Body or a system for doing work, by virtue of
its position above the ground level.
For example, a body raised to some height above the ground level possess potential energy
because it can do some work by falling on earth’s surfaces.
Potential Energy, P.E. = Wz = mgz
Where, W = Weight of the Body
m = mass of the Body
z = Distance through which the Body falls
g = Acceleration due to gravity = 9.81 m/s²
2. Kinetic energy: It is the energy possessed by a Body or a System, for doing work , by Virtue of its
mass and velocity of motion.
Let , m= Mass of the Body
V = Velocity of the Body
When m is in kg and V is in m/s, then kinetic energy will be in N-m, as discussed below:
We know Kinetic Energy ,
K.E. = ½ m V ² = kg x m²/s² = (kg- m)/ s² x m = N-m
3. Internal energy: It is the energy possessed by a body or a system due to its molecular
arrangement and motion of the molecules. It is represented by U.
In study of thermodynamics, we are mainly concerned with the change in Internal energy (dU)
which depends upon the change in temperature of the system.

13. Law of Conservation of Energy:
“The Energy can neither be created nor destroyed, though it can be transformed from one form to
any other form, in which the energy can exist”
The Heat is defined as the energy transferred without transfer of mass, across the boundary of a
System because of a temperature difference between the system and the surrounding .
It is usually represented by Q and is expressed in Joule (J) or Kilo-Joule (KJ).
The following points worth noting about heat:
1. The heat is transferred across a boundary from a system at a higher temperature.
2. The heat is a form of transit energy which can be identified only when it crosses the
boundary of a system . It exists only during transfer of energy into or out of a system.
3. The heat flowing into a system is considered as positive and heat flowing out of a system is
considered negative.
Heat:

14. Work:
In Mechanics, work is defined as the product of the force (F) and the distance
moved (x) in the direction of the force.
Mathematically,
Workdone,
W = F X x
The unit of Work depends upon the unit of force and the distance moved.
In S.I. system of Units, the practical unit of Work IS Newton – meter (N-m).
The work of 1 N-m is known as Joule (J)
1 N-m = 1 J

15. Zeroth Law of Thermodynamics:
“When two systems are each in thermal equilibrium with a third system, then the two systems are
also in thermal equilibrium with a third system, then the two systems are also in thermal
equilibrium with one another”
This law provides the basis of temperature measurement.
First Law of Thermodynamics:
1. The Heat and Mechanical work are mutually convertible.
2. The Energy can neither be created nor destroyed through it can be transferred from one
form to another.
Laws of Thermodynamics:
The following three laws of thermodynamics are important from the subject point of view:
1. Zeroth Laws of Thermodynamics
2. First Laws of Thermodynamics
3. Second Law of Thermodynamics

16. Limitations of First Law of Thermodynamics:
1. When a closed system undergoes a thermodynamic cycle, the net heat transfer is equal to
the net work transfer.: This statement does not specify the direction of flow and work (i.e.
whether the heat flows from a hot body of from a cold body to a hot body). It also does not
give any condition under these transfers take place.
1. The heat energy and mechanical work is Mutually convertible.:
Through the mechanical work can be fully converted into heat energy, but only a part of
heat energy can be converted into mechanical work. This means that the heat energy can be
converted into mechanical work.

17. Second Law of Thermodynamics:
The second law of thermodynamics may be defined in many ways, but the two common statements
according to Kelvin – Planck and Clausius are as follows:
1. Kelvin – Plank Statement:
According to Kelvien Planck “ It is impossible to construct on engine working on a cyclic process, whose
sole purpose is to convert heat energy from a single thermal reservoir into an environment amount of
Work.
1. Clausius Statement :
According to Clausius statement “ It is impossible for a self acting machine, working in a cyclic process, to
transfer heat from a Body at a lower temperature to a body at a higher without the aid of an external
agency.

18. Thermodynamic process:
When a system changes its state from one equilibrium state to another
equilibrium state, then the path of successive states through which the
system has passed is known as thermodynamic process.
Pressure
Volume
1
2
B
A
When a process or processes are performed on a system in such a way
that the final state is identical with the initial state, it is then known as
a thermodynamic cycle or cyclic process.