Entropy in Thermodynamics

1. Subject: Engineering Thermodynamics
Topic: Entropy
Name: Nilrajsinh Vasandia
Branch: Mechanical
En No.:- 130990119060

2. Entropy
• Entropy is a function of quantity of heat which shows the
possibility of conversion of that heat into work.
• The increase in the entropy is lower when heat is added at a
high temperature and the increase in entropy is higher when
heat is added at a low temperature
• The maximum entropy means , there is minimum availability
for conversion into work and the minimum entropy means ,
there is maximum availability for conversion into work.

3. • Consider the heating of working substance by reversible
process.
• Let heat supplied to
the working substance
in process which will
increase the entropy by
dS.
• Heat supplied to
reversible process
Q
TdsQ 
Where, T = absolute temperature, dS = Increse in entropy

4. Total heat supplied or absorbed
Also, change in entropy
dsTQ  
2
1
2
1

T
Q
ds


 
2
1
2
1
T
Q
ds


2
1
12
T
Q
SS


5. Entropy & Disorder
• Entropy can be considered to be a measure of microscopic
disorder.
• Work interaction can be considered ordered molecular
energy. While heat interaction can be considered disordered
molecular energy.
• All irreversible processes are associated with a transition
from ordered molecular energy to disordered molecular
energy.

6. • Let us consider a box with a partition dividing it into two
sections contains only O2 molecules in one section and only
N2 molecules in the other section.
• Now partition be withdrawn, allowing the two gases to mix.
This process is irreversible ( since system can not be returned
to its original state without taking work from surroundings
).This process is accompanied by an increase in entropy.

7. • The mixing process has resulted in a more random
arrangement of the system, hence an increase in entropy can
be associated with a change from a more ordered to less
ordered, or more random state. The more a system
approaches equilibrium, the greater is the disorder and
irreversibility and when equilibrium is reached, it has the
maximum disorder.
• In other word, “entropy is a measure of the disorder of the
system.”
• A measure of molecular disorder can be found in a molecular
mean free path. Thus, whenever there is an increase in the
volume in the microscopic motion of the individual particles
of matter, the entropy increases.

9. Characteristics of entropy
• Entropy is property of system.
• For reversible process change in entropy is
• Entropy is point function, independent on path
of the process.
for reversible process
for irreversible process
T
Q
SS
rev


2
1
12
0  gsurroundinsystem SS
0  gsurroundinsystem SS

10. • Entropy is extensive property.
• It is expressed as energy units per unit degree temperature
KJ/K.