Entropy change during thermodynamic process

Engineering Thermodynamics
(2131905)
Guided by :
Prof. Nita R. Patel
Prepared By :
Name : Shah Preet .P.
Enroll. No. : 160410119117
Batch : C
Dept : Mechanical
Topic : Entropy
1

TopicTopic
 Entropy Change During
Thermodynamic Process
Entropy Change For Pure Substances
Third Law Of Thermodynamics (Nernst
Law)
2

Entropy Change DuringEntropy Change During
Thermodynamic ProcessThermodynamic Process
Let m Kg of gas at a pressure P , volumeV ,₁ ₁
absolute temperature T and entropy S , be₁ ₁
heated by any thermodynamic process.
Its final pressure, volume, temperature and
entropy are P ,V ,T and S respectively.₂ ₂ ₂ ₂
3

Law Of Conservation Of EnergyLaw Of Conservation Of Energy
 Q = dU + W,δ δ
Where,
 Q = small change in heatδ
dU = small change in internal energy
 W = small change of work doneδ
dT = small change in temperature
dv = small change in volume
4

Change Of Entropy Of DifferentChange Of Entropy Of Different
ProcessesProcesses
1. Constant Volume Process
2. Constant Pressure Process
3. Constant Temperature Process
4. Reversible Adiabatic Process
5. Irreversible Adiabatic Process
6. Polytropic Process
8

Heating A Gas At ConstantHeating A Gas At Constant
Volume ProcessVolume Process
Consider a m Kg of perfect gas being
heated at a constant volume process.
Heat supplied at constant volume
9

Heating A Gas At ConstantHeating A Gas At Constant
PressurePressure
heated at a constant Pressure process.
Heat supplied at constant Pressure
11

Isothermal Process Or ConstantIsothermal Process Or Constant
Temperature ProcessTemperature Process
heated at a constant temperature during
expansion process.
13

Reversible Adiabatic ProcessReversible Adiabatic Process
 In the adiabatic process, heat is neither supplied to system or
rejected by the system.
 This shows that the change of entropy during a reversible
adiabatic process is zero, the T-S graph is shown by line 1-2.
 The entropy of the gas remains constant during reversible
adiabatic expansion or compression of the gas, this process is
said to be isentropic process.
 It is frictionless adiabatic process.
 In this, change in internal energy is equal to the work done by
the gas during expansion .
15

Irreversible Adiabatic ProcessIrreversible Adiabatic Process
 If adiabatic process with internal friction, it is known as irreversible adiabatic
process.
If the irreversible adiabatic expansion process takes place between same
temperature limits T₁ and T₂ then due to internal friction, the internal energy of
the gas at the end of expansion will be more than at point 2 of reversible process.
Suppose, δQ’ heat gain by gas due to internal friction then the increase of
entropy will be given by,
17

It means that higher entropy at the end of expansion of
irreversible process, therefore work done by gas will be
less than that of reversible process.
Mathematically,
U₁ - U₂ = W, dS = 0 for reversible adiabatic process
U₁ - U’₂ = W’ dS > 0 for irreversible adiabatic process
Where,
U’₂ = internal energy of gas at the end of irreversible
expansion
U₂ = internal energy of gas at the end of reversible
expansion
18

Polytropic ProcessPolytropic Process
Consider a m
quantity of
perfect gas being
expanded by
polytropic
process.
20

Entropy Change For PureEntropy Change For Pure
SubstancesSubstances
 Consider (M) Kg of ice is
heated continuously at
constant atmospheric
pressure.
27

T₁ = initial temperature of ice
T₂ = melting temperature of ice = 0C =
273 K
T = boiling temperature of water =100C
T = superheated steam temperatureᵤ
Cᵨ = specific heat of iceᵢ
Cᵨ = specific heat of waterᵥᵥ
Cᵨ = specific heat of steam
29

Process 1-2 Sensible Heating Of IceProcess 1-2 Sensible Heating Of Ice
Temperature of ice increase from T to₁
T . Change of entropy during 1-2₂
30

Process 2-3 Melting Of IceProcess 2-3 Melting Of Ice
 On further heating of ice is converted
into water at constant temperature T₂
(0C).
 The heat supplied is utilized to change
phase called latent heat of fusion of ice
(h ).ᵢ
 Therefore Q = mhᵢ
 Change of entropy during process
31

Process 3-4 Sensible Heating OfProcess 3-4 Sensible Heating Of
WaterWater
Water from T is heated to water at T₂ .
Change of entropy during process
32

Process 4-5 Boiling Of WaterProcess 4-5 Boiling Of Water
On further heating of water, is
converted into steam at constant
temperature T .
The heat supplied is utilized to
change phase called latent heat of
evaporation.
Change of entropy during process.
33

Process 5-6 Sensible Heating OfProcess 5-6 Sensible Heating Of
SteamSteam
Temperature of steam increases from Ts
to Tsup .
34

Third Law Of ThermodynamicsThird Law Of Thermodynamics
(Nernst Law)(Nernst Law)
Entropy is a measure of molecular randomness
At absolute zero temperature molecules
become completely motionless.
The entropy of a pure crystalline substance at
absolute zero temperature is zero since there is
no uncertainly about the state of the molecules at
that instant.
35

This is third law of thermodynamics which states that
“ The entropy of all perfect crystalline substance
(Solid) is zero at absolute zero temperature”.
 In equilibrium crystalline state, its atoms are arranged in a
pattern that represents the maximum degree of order, and
if it also at absolute zero temperature there must be a
minimum of disordering thermal motion.
Mathematically,
 The third law of thermodynamics provides an absolute
reference point for the determination of entropy.
 The entropy determined relative to this reference point
is called absolute entropy.
36

ApplicationApplication
1. Provides an absolute reference point for the
determination of entropy.
2. Explaining the behaviour of solids at very low
temperature.
3. Measurement of action of chemical forces of the
reacting substances.
4. Analysing the chemical and phase equilibrium.
37

Entropy change during thermodynamic process

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