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Entropy change during thermodynamic process


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Engineering Thermodynamics, Entropy

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Entropy change during thermodynamic process

  1. 1. 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
  2. 2. TopicTopic  Entropy Change During Thermodynamic Process Entropy Change For Pure Substances Third Law Of Thermodynamics (Nernst Law) 2
  3. 3. 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
  4. 4. 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
  5. 5.  Q = mC dT + pdvδ ᵥ 5
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  8. 8. 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
  9. 9. 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
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  11. 11. Heating A Gas At ConstantHeating A Gas At Constant PressurePressure Consider a m Kg of perfect gas being heated at a constant Pressure process. Heat supplied at constant Pressure 11
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  13. 13. Isothermal Process Or ConstantIsothermal Process Or Constant Temperature ProcessTemperature Process Consider a m Kg of perfect gas being heated at a constant temperature during expansion process. 13
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  15. 15. 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
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  17. 17. 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
  18. 18. 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
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  20. 20. Polytropic ProcessPolytropic Process Consider a m quantity of perfect gas being expanded by polytropic process. 20
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  27. 27. Entropy Change For PureEntropy Change For Pure SubstancesSubstances  Consider (M) Kg of ice is heated continuously at constant atmospheric pressure. 27
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  29. 29. 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
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33. 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
  34. 34. Process 5-6 Sensible Heating OfProcess 5-6 Sensible Heating Of SteamSteam Temperature of steam increases from Ts to Tsup . 34
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. Thank YouThank You 38