B.Sc. Sem-III Phy-I Unit-II ( Second and Third Laws of Thermodynamics)

1. Department of Physics
Topic –2nd Law of Thermodynamics
Prof. B.V. Tupte
Head, Department of Physics
Shri Govindrao Munghate Arts and Science
College Kurkheda.

2. • 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!
Limitation of “FIRST LAW”

3. • 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.
Limitation of “FIRST LAW”

4. Limitation of “FIRST LAW”

6. Processes that are usually idealized as reversible include:
•Frictionless movement
•Restrained compression or expansion
•Energy transfer as heat due to infinitesimal temperature non-uniformity
•Electric current flow through a zero resistance
•Restrained chemical reaction
•Mixing of two samples of the same substance at the same state.
Processes that are irreversible include:
•Movement with friction
•Unrestrained expansion
•Energy transfer as heat due to large temperature non uniformities
•Electric current flow through a non zero resistance
•Spontaneous chemical reaction
•Mixing of matter of different composition or state.

7. • The second law 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 of thermodynamics introduces the notion of entropy (S), which
is a measure of system disorder (messiness)
• 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….
SECOND LAW OF THERMODYNAMICS
Note:
U is the quantity of a system’s energy, S is the quality of a system’s energy.

8. The 2nd Law helps determine the preferred direction of a
Process
• A reversible process is one which can change state and
then return to the original state
• This is an idealized condition
• ( all real processes are irreversible)

9. The 2nd law of thermodynamics

10.  Entropy is an extensive thermodynamic property.
Entropy is a physical quantity that controls the direction of irreversible processes.
It is a property of the state of a system; like T, P, V, U.
Entropy principle: “If an irreversible process occurs in a closed system, the entropy
of that system always increases; it never decreases.”
Another statement of the second law of thermodynamics:
(The total entropy of an isolated system never decreases).
Entropy is a measure of the disorder of a system. This gives us yet another
statement of the second law:
Natural processes tend to move toward a state of greater disorder.
Entropy (S )

11. Entropy (S )
The greater the number of possible arrangements, the greater the
entropy of a system, i.e., there is a large positional probability.
The positional probability or the entropy increases as a solid
changes from a liquid or as a liquid changes to a gas

12. Second Law of Thermodynamics: Kelvin Plank’s Statement
“It is impossible for any system to operate in a thermodynamic cycle and deliver a net
amount of work to its surroundings while receiving an energy transfer by heat from a
single thermal reservoir”
It is impossible for any device that operates on a cycle to
receive heat from a single reservoir and produce a net
amount of work
In other words, no heat engine can have a thermal
efficiency of 100%
A heat engine that violates the Kelvin-Planck
statement of the second law cannot be built

13. Second Law of Thermodynamics: Clausius Statement

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

15. • Heat Engine is a device which working in a cycle converts energy in form of
heat into work
- Heat engines convert heat to work
• There are several types of heat engines, but they are characterized by the
following:
- They all receive heat from a high-temperature source (oil furnace, nuclear
reactor, etc.)
- They convert part of this heat to work
- They reject the remaining waste heat to a low-temperature sink
- They operate in a cycle
Heat Engine

16. • The cycle was first suggested by Sadi Carnot, in 1824, which works on reversible cycle
• Any fluid may be used to operate the Carnot cycle, which is performed in an engine cylinder the head of
which is supposed alternatively to be perfect conductor or a perfect insulator of a heat
• Heat is caused to flow into the cylinder by the application of high temperature energy source to the
cylinder head during expansion, and to flow from the cylinder by the application of a lower temperature
energy source to the head during compression
Carnot Cycle

17. The assumptions made for describing the working of the Carnot engine are as follows :
1. The piston moving in a cylinder does not develop any friction during motion
2. The walls of piston and cylinder are considered as perfect insulators of heat
3. The cylinder head is so arranged that it can be a perfect heat conductor or perfect
heat insulator
4. The transfer of heat does not affect the temperature of source or sink
5. Working medium is a perfect gas and has constant specific heat
6. Compression and expansion are reversible

18.  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)
Carnot Cycle
• Idealized thermodynamic cycle consisting of four reversible processes (working fluid
can be any substance):
• The four steps for a Carnot Heat Engine are:

23. 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”
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.”
Corollary of Carnot Theorem: