The document discusses the first and second laws of thermodynamics and the concept of entropy. It defines thermodynamics as the flow of heat and explains that the laws are based on observable macroscopic properties like pressure, temperature, and volume. The second law states that heat cannot be completely converted to work and the efficiency of any energy conversion is less than unity. Entropy represents the disorder or randomness of a system and increases as a system moves towards equilibrium. Spontaneous processes result in an increase in unavailable thermal energy and entropy.
3. •The word thermodynamics literally means flow
of heat.
•Thermodynamics is based on three laws.
•First, second, and third law.
•These laws are however based on human
experience and have never been proved
experimentally in direct way.
•The laws of thermodynamics applies on
macroscopic observable properties of matter such
as Pressure, Temperature and Volume.
4.
5. THE SECOND LAW OF THERMODYNAMICS
•First law of thermodynamics states that one form of
energy gets converted into another form of energy.
•It is silent about the extent to which it takes place.
•The second law of thermodynamics states that, while
all other form of energy can be completely converted
to heat, the complete conversion of heat into any form
of energy cannot take place without leaving some
change in the system.
6. •In other words, heat energy can never be converted
into work and it is isothermally unavailable for doing
work.
•The fraction of heat that a machine can convert into
work is called efficiency.
Let us consider a steam engine operating reversibly ,
where
-T2 is the upper temperature,
- T1 is the lower temperature,
-Q2 is the heat absorbed from the source,
-Q1 is the heat returned to the cold body,
- W is the work done.
7. Efficiency = W/Q2
= (Q2-Q1)/Q2
Since the flow of heat follows the temperature
gradient, the heat absorbed (Q2) and heat rejected
(Q1) can be related directly to temperature T2 and
T1 respectively as:
Q2/Q1 = T2/T1
By combining the above equations we can write:
Efficiency = (Q2-Q1)/Q2 = (T2-T1)/T2
Contd…
8. From the above equation, it is clear that the efficiency
of the engine becomes greater when T2 becomes
significantly greater than T1. when T1 becomes
absolute zero on the kelvin scale, the efficiency
becomes unity and the reversible heat engine converts
heat completely into work.
Efficiency = (T2-0)/T2
= 1
Contd…
9. However, since absolute zero is not possible, the
efficiency is always less than inity and hence heat
can never be converted completely into work.
When T2 becomes equal to T1, the process is
isothermal and the efficiency becomes zero.
Contd…
11. ENTROPY
•Entropy serves as a criterion of spontaneity of a
process.
•Entropy of a system is also described as the
randomness or disorderliness of a system.
•Thus, the solid state of a substance represents the
highest degree of orderliness of molecules. As the
temperature is increased, the solid state gets
transformed to the liquid state and the liquid to
vapour state resulting in progressive increase in the
disorderliness of the molecules.
Contd…
12. •The impossibility of converting all heat energy into
work is due to disorderliness of the molecules in the
system.
•In terms of entropy change, the second law of
thermodynamics states that a spontaneous reaction
involving a system and its surrounding occurs in the
direction of increased entropy.
•When the system reaches equilibrium, the net entropy
change undergone by the system and its surroundings
equals to zero.
ENTROPYContd…
13. ENTROPY
Since the total amount of energy in the universe is
believed to be constant, all physical and chemical
processes that take place spontaneously result in an
increase in the proportion of energy that is unavailable
for doing work.
Entropy may thus be defined as the capacity factor of
thermal energy that is unavailable for doing work and
the entropy change ΔS for the absorption of heat in a
system at absolute temperature T in any step of a
reversible process is given by:
Contd…
14. ΔS = Q (rev)/T
The unit of entropy change is cal/deg.
For reversible cyclic process, the entropy change of
the system as well as surrounding is equal to zero.
ΔS (total system) = ΔS(syst.) + ΔS (surr.) = 0.
For irreversible process, the entropy change of the
system as well as surrounding is always increasing.
ΔS (total system) ˃ 0.
Contd…
ENTROPY
