Lecture on thermodynamics

INTRODUCTION:
 Thermodynamics is that branch of
Engineering science which deals with energies
possessed by gasses and Vapors
 Thermodynamics deals with the energies in
terms of Heat and Mechanical work and their
relationship with the properties of the system

Types of Thermodynamic System:
 Closed System: In closed system heat and
work cross the Boundary of the System, there
is no loss of original mass
 Open System: In open system working
substance crosses the boundary of the
system. The heat and work may also cross
the Boundary of the system
 Isolated System: It is a system of fixed mass
and no work cross its Boundary

 Extensive Property : The properties of the system
whose value for the entire system is equal to the
sum of their values for the individual parts of the
system are called Extensive Property.
Example: Total Volume, Total Mass, Total Energy is
extensive properties of the system
 Intensive Property: The properties of the system,
whose value for the entire system is not equal to
the sum of their value of individual parts of the
system , are called intensive properties.
Example: Temperature, pressure and density of a
system are intensive properties.

 Zeroth Law of Thermodynamics:
The Law states that when two Bodies are in Thermal
equilibrium with a Third Body, they are also in thermal
equilibrium with each other.
 First Law of Thermodynamics:
This law states that Heat and Mechanical work are
mutually convertible. According to this law, a definite
amount of mechanical work is needed to produce a
definite amount of heat and vice versa.
This law also states that the energy can neither be
created nor destroyed, though it can be transformed
from one form to another
Q = W + E

 Second Law of Thermodynamics:
This Law states that there is a definite limit to the
amount of mechanical energy, which can be
obtained from a given quantity of heat energy
According to the Claussius, this law may be states
as “ It is impossible for a self – acting machine
working in a cyclic process, to transfer heat from
a body at lower temperature to a body at a higher
temperature without the aid of an external agency”
The second law of thermodynamics has also been
stated by Kelvin- Plank as “ It is impossible to
construct an engine working on a cyclic process,
whose sole purpose is to convert heat energy into
work”

 Boyle’s Law : The absolute pressure of a give volume of a
perfect gas inversely as its volume, when the temperature remains
constant
p=1/ v or pv = constant
P1v1 = p2v2 = p3v3 = constant
 Charles Law : The Volume of a given mass of a perfect
gas varies directly as its absolute temperature, when the
absolute pressure remains constant”
v α T or v/T = Constant
v1/T1 = v2/T2 = v3/T3= constant
 Gay – Lussac Law:“The absolute pressure of a given
mass of a perfect gas varies directly as its absolute
temperature, when the volume remains constant”
p α T or p/T = Constant

Characteristics equation of a Gas: It is a modified
form of general gas equation . If the voume (v) in
the general gas equation is taken as that of 1 kg
of gas equation ) is represented by another
constant R (in the characteristics equation of
gas)
The Characteristics Gas Equation is written as:
pv = mRT
Where m = mass of the Gas
R = characteristics gas constant

Avogadro’s Law: Equal volume of all gases , at the same
temperature and pressure, contain equal number of
molecules
Acc to the Law:
1m3 of O2 will contain the same number of molecules as
1m3 of hydrogen (H2)

Specific heat at constant volume:
It is the amount of heat required to raise the temperature
of a unit mass of a gas through 1⁰, when it is heated at
constant pressure. It is generally denoted by Cp.
m = Mass of the gas
T1 = Initial temperature of the gas
v1 = Initial volume of the gas,
T2, V2 = Corresponding values for the final
Total heat of gas supplied at constant pressure,
Q = Mass x Sp. Heat at constant pressure x
Rise in temperature
= m x Cp x ( T2-T1)

Relation between Specific Heats:
The following relations between the two
specific heats (i.e. Cp and Cv) are impotant:
1. The difference of two specific heats is equal
to gas constant (R) , i.e.
Cp – Cv = R
2. The ratio of two specific heats (i.e. Cp/ Cp)
is known as adiabatic index and it is
represented by ‫ץ‬
Cp – Cv = R
or, Cp/ Cv = 1 + R / Cv

1.Constant volume process:
When the gas is heated at Constant volume,its temperature
and pressure will increase.
Since there is no change in its volume , its temperature
and pressure will increase.It is governed by Gay–Lussac
Law. Heat Supplied, Q 1-2 = (U2-U1) = m. cv ( T2 –T1)
2.Constant pressure process or Isobaric
When the gas is heated at Constant pressure, its temperature
and volume will increase.
Since there is a change in its volume, the heat supplied is
utilized in increasing the internal energy of the gas, and also
for doing some external work. This process is governed by
Charles Law.
Heat Supplied to gas at constant pressure,
Q 1-2 = m. cp ( T2 –T1)

4. Constant temperature process:
A process in which the temperature of the working substance
remains constant during the expansion or compression, is called
a constant temperature process or Isothermal process.
Charecteristics of Isothermal process:
1. there is no change in temperature
2. there is no change in internal energy
3. Hyperbolic process:
A process , in which the gas is heated or expanded in such
away that the product of its pressure and volume
( i.e. p x v) remains constant , is called a Hyperbolic
process.
This process is governed by Boyle’s Law, p.v. = constant
If we plot the graf for the pressure and volume , during the
process , we get a rectaangular hyperbola and hence this
process is known as hyperbolic process.

5. Adiabatic Process or Isentropic Process:
A process in which the working substance neither receives
nor gives out heat to its surroundings, during its expansion
or compression is called an Adiabatic process. This will
happen when the working substance remains thermally
insulated, so that no heat enters or leaves it during the
process.
Thus an adiabatics process can be summarized as follows:
1. No heat leaves or enter the gas
2. the temperature of the gas changes, as the work is done
at the cost of internal work is done
Hence the total heat of the fluid remains constant.
Q 1-2 =0, W1-2= 0 and du =0
6. Constant temperature process:
A process in which the temperature of the working substance
remains constant during the expansion or compression, is
called a constant temperature process or Isothermal process.
Characteristics of Isothermal process:
1. there is no change in temperature
2. there is no change in internal energy

8. Throttling process:
When a perfect gas is expanded through an aperture of
minute dimensions, such as a narrow throat or a slightly
opened valve, the process is termed as throttling process.
Q 1-2 = 0, W1-2 = 0 , du =0

Lecture on thermodynamics

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Lecture on thermodynamics