Introduction

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Introduction of Thermodynamics

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Introduction

  1. 1. IntroductionIntroductionIntroductionIntroduction
  2. 2. Thermodynamic SystemsThermodynamic Systems A thermodynamic system is a collection of matter which has distinct boundaries. OR A real or imaginary portion of universe whish has distinct boundaries is called system. OR A thermodynamic system is that part of universe which is under thermodynamic study.
  3. 3. Thermodynamic systemsThermodynamic systems Thermodynamic system Thermodynamic system is any collection of objects that is convenient to regard as a unit, and that may have the potential to exchange energy with its surroundings. A process in which there are changes in the state of a thermodynamic system is called a thermodynamic process. Signs for heat and work in thermodynamics Q is + if heat is added to system Q is – if heat is lost by system W is + if work is done by the system. W is – if work is done on the system. Q is + if heat is added to system Q is – if heat is lost by system W is + if work is done by the system. W is – if work is done on the system.
  4. 4. Work done during volume changes Work done Piston in a cylinder moves by dl due to expansion of gas at pressure p Force on piston: F = p·A Incremental work done: dW = F·dl = p·A dl = p·dV ∫ ∫== f i V V pdVdWW:doneworkTotal
  5. 5. Work and pV diagram ∫ == B A V V pdVW curveV-punderarea Work done depends on the path taken • Process A B : If VB>VA , W > 0 • Process B A : If VB>VA , W < 0 • Process A B : If VA=VB , W = 0 Work done during volume changes (cont’d)
  6. 6. Work and pV diagram (cont’d) Work done depends on the path taken.. C A C : W = 0 C B : W = pB(VB-VA) B C : W = pB(VA-VB) D A C B : W = pB(VB-VA) A D : W = pA(VB-VA) D B : W = 0 A D B : W = pA(VB-VA) Work done during volume changes (cont’d)
  7. 7. V P 1 2 34 V P W = P∆V (>0) 1 2 34 ∆V > 0 V P W = P∆V = 0 1 2 34 ∆V = 0 V P W = P∆V (<0) 1 2 34 ∆V < 0 V W = P∆V = 0 1 2 34 P ∆V = 0 V P 1 2 34 If we go the other way then Wtot < 0 V P 1 2 34 Wtot > 0 Wtot = ??Isobaric Process: P=constant Isochoric Process: V=constant
  8. 8. V P 1 2 3 Now try this: What is the total work done by system when going from state 1 to state 2 to state 3 and back to state 1 ? V1 V2 P1 P3 Area = (V2-V1)x(P1-P3)/2
  9. 9. Concept Question Shown in the picture below are the pressure versus volume graphs for two thermal processes, in each case moving a system from state A to state B along the straight line shown. In which case is the work done by the system the biggest? 1. Case 1 2. Case 2 3. Same A B4 2 3 9 V(m3 ) Case 1 A B 4 2 3 9 V(m3 ) P(atm) Case 2 P(atm) correct Net Work = area under P-V curve Area the same in both cases!
  10. 10. Internal energy and the first law of thermodynamicsInternal energy and the first law of thermodynamics Internal energy Tentative definition:Tentative definition: The internal energy U of a system is the sum of the kinetic energies of all of its constituent particles, plus the sum of all the potential energies of interactions among these particles. The first law of thermodynamicsThe first law of thermodynamics WQUUU −=∆=− 12 Now define the change in internal energy using the first law of thermodynamics. It is known from experiments that while Q and W depend on the path, DU does not depend on the path.
  11. 11. Internal energy and the first law of thermodynamicsInternal energy and the first law of thermodynamics A cyclic process A B pA pB VA VB A process in which the initial and final states are the same. upper AB lower BA upper AB lower BA WWWW →→→→ <<> ,0,0 0<+= →→→→ upper AB lower BAABA WWW If the directions of the arrows are reversed, then 0>+= →→→→ lower AB upper BAABA WWW
  12. 12. Internal energy and the first law of thermodynamicsInternal energy and the first law of thermodynamics The first law of thermodynamics: dWdQdU −= For the system we will discuss, the work dW is given by pdV pdVdQdU −=
  13. 13. Kinds of thermodynamics processKinds of thermodynamics process
  14. 14. Boyle’s LawBoyle’s Law 2 12 1 V V P P = T = const P1 V1 P2 V2
  15. 15. CharlesCharles’ LawLaw 1 2 1 2 T T V V =T1 V1 T2 V2 P = const
  16. 16. BalloonBalloon ExampleExample • A balloon is filled with air to a pressure of 1.1 atm. • The filled balloon has a diameter of 0.3 m. • A diver takes the balloon underwater to a depth where the pressure in the balloon is 2.3 atm. • If the temperature of the balloon does not change, what is the new diameter of the balloon?
  17. 17. ( ) 33 2 1 12 2 13 1 3 2 3 1 3 1 1 3 2 3 2 2 2 1 12 2 1 1 2 atm3.2 atm1.1 m3.0 23 4 23 4 == = =      = =      = =⇒= P P DD P P kDkD kD D V kD D V P P VV P P V V π π = 0.235 m SolutionSolution P1 = 1.1 atm D1 = 0.3 m P2 = 2.3 atm D2 = ? Sphere V=4/3 p r3
  18. 18. Heat Capacity for Constant Volume Processes (Cv) • Heat is added to a substance of mass m in a fixed volume enclosure, which causes a change in internal energy, U. Thus, Q = U2 - U1 = ∆U = m Cv ∆T The v subscript implies constant volume Heat, Q addedm m ∆Tinsulation
  19. 19. Heat Capacity for Constant Pressure Processes (Cp) • Heat is added to a substance of mass m held at a fixed pressure, which causes a change in internal energy, U, AND some PV work. Heat,Heat, QQ addedadded ∆T m m ∆x
  20. 20. CCpp DefinedDefined • Thus, Q = DU + PDV = DH = m Cp DT The p subscript implies constant pressure H, enthalpy. is defined as U + PV, Experimentally, it is easier to add heat at constant pressure than constant volume, thus you will typically see tables reporting Cp for various materials (Table 21.1 in your text).
  21. 21. Diagram of a hot reservoir
  22. 22. Heat Engine
  23. 23. Heat EngineHeat Engine • A heat engine is a cyclic device that converts thermal energy into work output • It is a device that takes heat from a high-T reservoir, converts some of to (useful) work, and transfers the rest to the surroundings (a low-T reservoir) • Examples: steam engines; internal combustion engines (car engines) • Thermal efficiency (“what you get out/what you put in”): • No heat engine operating in a cycle can convert all of its heat input completely to work work out 1 heat in out c th in h W Q e Q Q = = = −
  24. 24. IntercoolingIntercooling
  25. 25. Skillcruise.com

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