(Basic concepts & Statements)

1. Engineering Thermodynamic
Topic:~ Second law of thermodynamic
(Basic concepts & Statements)
Mechanical Department
B_2
Prepared By:
Kushal Panchal

2. Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain
energy flow takes place when a system undergoes a
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each
other.
• But from experience this is NOT TRUE!

3. Limitation of “FIRST LAW”
• First law does not help
to predict whether the
certain process is
possible or not.
• The first law does not
give info about
Direction.
• It does not provide and
specify sufficient
condition to process
take place.
Work(W) System Heat(Q)
Hot Reservoir T1
Cold Reservoir T2
Heat
Flow
Hot Reservoir T1
Cold Reservoir T2
Heat
Flow
Not possible

4. The “SECOND LAW” of
thermodynamic:
• The second low of thermodynamic gives more
information about thermodynamic processes.
• Second law may be defined as
– “Heat can not flow itself from colder body to a
hotter body”.
• The Second law is also used to determine the
theoretical limits for the performance of
mostly used engineering systems like heat
engines and heat pump….

5. “Kelvin-Plank” statement:
• The Kelvin-Plank statement of the second law of
thermodynamic is states that
– “It is impossible to for any devise as heat engine that
operates on a cycle to receive heat from a single
reservoir and produce net amount of work”.
• This statement means that only part of total heat
absorbed by heat engine from a high
temperature is converted to work, the remaining
heat must be rejected at a low temperature.

6. Continued…..
• The heat engine as shown
in fig.1is converting 100 %
of heat into 100% work.
• This system is not
satisfying second law.
• So, this statement can be
also expressed as “No
heat engine has a 100 %
of thermal efficiency”.
Q=100 kj
w=100 kj
Q=100 kj
w=40kj
Q2=60 kj
Heat Source
Heat
Engine
Heat Source
T1
Heat
Engine
Sink T2
1

7. “Clausius” statement
• Clausius statement of second law of
thermodynamic is as below
– “It is impossible to construct a device as heat
pump that operates in a cycle and produces no
effect other than the transfer of heat from lower
temperature to higher temperature body”.
• This statement means that heat cannot flow
from cold body to hot body without any work
input.

8. Heat Source T1 Heat Source T1
Heat sink T2
Heat sink T2
Heat
pump
Heat
pump
Q1=100 kj
Q2=100 kj
Q2=60 kj
Q1=100 kj
No external
work
supplied
1
W=40 kj
(external
work
supplied)
2

9. Continued:
• The cop for 2 is 100/40=2.5.
• So, in other simple words this statement can
be defined as
– “Heat cannot itself flow from a colder body to a
hot body”.

10. COMPARISON
Kelvin-Plank Statement
• It is applied to ‘Heat Engine’.
• It is negative statement.
• It is based on experimental
observations and no
mathematical proof.
Clausius Statement
• It is applied to ‘Heat Pump’
and ‘Refrigeration’.
• It is also negative
statement.
• It is based on experimental
observations and no
mathematical proof.

11. Perpetual Motion Machine
PMM 2
• If the engine exchange
heat only single thermal
reservoir, in which heat
is supplied is completely
converted into an
equivalent amount of
work and its efficiency
becomes 100 %.
• This kind of machine is
known as “PMM 2”.
Thermal Reservoir
PMM 2
W=Q1
Q1

12. Continued:
• The PMM 2 violates the second law of
thermodynamic.
• Practically its IMPOSSIBLE to construct.
• The efficiency PMM 2 is
• W=Q
• N is 100 % for PMM 2.

13. Reversible Processes and
Irreversibility's
• A reversible process is one that can be executed in the reverse direction
with no net change in the system or the surroundings.
• At the end of a forwards and backwards reversible process, both
system and the surroundings are returned to their initial states.
• No real processes are reversible.
• However, reversible processes are theoretically the most efficient
processes.
• All real processes are irreversible due to irreversibilities. Hence, real
processes are less efficient than reversible processes.
Common Sources of Irreversibility:
• Friction
• Sudden Expansion and compression
• Heat Transfer between bodies with a finite temperature difference.
• A quasi-equilibrium process, e.g. very slow, frictionless expansion or
compression is a reversible process.

14. Carnot cycle and Carnot heat engine:
• A carnot cycle is hypothetical cycle consist of
four process
– Two reversible isothermal process and
– Two reversible adiabatic process.
• Heat is caused to flow in working fluid by
application of high temperature energy source
during expansion, and flow out of fluid by the
application of lower temperature energy sink
during compression

15. Assumptions made in Carnot Cycle:
I. Piston moving in the cylinder does not produce any
friction.
II. The cylinder head is arranged in such a way that it
can be perfect heat conductor or heat insulator.
III. The walls of cylinder and piston are consider as
perfect insulator of heat.
IV. Heat transfer does not affect temperature source or
sink.
V. Compression and expansion are reversible.
VI. Working is perfect gas and has constant specific heat.

16. Continued:
• Idealized thermodynamic cycle consisting of four reversible processes
(working fluid can be any substance):
• The four steps for a Carnot Heat Engine are:
 Reversible isothermal expansion (1-2, TH= constant)
 Reversible adiabatic expansion (2-3, Q = 0, THTL)
 Reversible isothermal compression (3-4, TL=constant)
 Reversible adiabatic compression (4-1, Q=0, TLTH)
1-2 2-3 3-4 4-1

17. The Carnot Cycle (cont’d)
Work done by the gas =  PdV, i.e. area under
the process curve 1-2-3.
1
2
3
3
4
1
Work done on gas = PdV, area under the
process curve 3-4-1
subtract
Net work
1
2
34
dV>0 from 1-2-3
PdV>0
Since dV<0
PdV<0

18. Carnot’s theorem:
“It is impossible to construct an engine
operating between two constant
temperature reservoir can be more efficient
then reversible engine operating between
the same reservoir”

19. Corollary of Carnot Theorem:
1) “All reversible engine operating between
the two constant temperature thermal
reservoir have the same efficiency.”
2) “The efficiency of any reversible heat engine
operating between two thermal reservoir
does not depend on nature of working fluid
and depends only on the temperature of the
reservoir.”

20. 20
The Thermodynamic
Temperature Scale:
A temperature scale that is independent
of the properties of the substances that
are used to measure temperature is called
a thermodynamic temperature scale.
That is the Kelvin scale, and the
temperatures on this scale are called
absolute temperatures.
L
H
revL
H
T
T
Q
Q






cyclesreversibleFor
The second Carnot principle state that
the thermal efficiencies of all reversible
heat engines operating between the
same two reservoirs are the same.
hth, rev = f (TH,TL)

21. • Thank You..!
• T H E R M O D Y N A M I C S……..;)