Thermodynamics lecture 2


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Thermodynamics lecture 2

  1. 1. BITS Pil i Pilani Pilani CampusLecture 2: PL t 2 Processes; Th Thermal equilibrium and l ilib i dTemperature; Phase behavior of pure substance
  2. 2. State Postulate• Phase – Spatially uniform (in chemical composition andphysical properties), mechanically separable part of system• Homogeneous – single phase, else heterogeneous• Pure substance – one of unvarying chemical constitution• Simple, compressible substance – only form of work thatof volume change, no magnetic, electrical, effects etc., Alsowe will normally ignore surface effects• Postulate – two intensive properties suffice to determine allothers (ie., determine the equilibrium state) of a singlephase, pure simple compressible substance. If i addition, h i l ibl b t If, in dditithe mass is known then so are all other extensive properties• Also applies to a mixture of fixed composition such as airin a single phase BITSPilani, Pilani Campus
  3. 3. Equilibrium surface• Properties are also called state functions• Set of all equilibrium states constitutes a surface inthe space of independent intensive variables BITSPilani, Pilani Campus
  4. 4. Process• Process – system goes from state i to state f. In so doing,in general it will interact with the surroundings• Quasistatic process – intervening states are all equilibriumstates – slow and controlled• Isobaric isochoric, isothermal processes Isobaric, isochoric•If intervening states not equilibrium states, then showndashed• Cycle – Initial and final states are the same BITSPilani, Pilani Campus
  5. 5. Thermal Equilibrium• Diathermal material – One which allows two systems incontact across a rigid wall of such a material to influence geach other’s state, eg., copper. (It is a thermal conductor)• Adiabatic material – One which does not permit such aninteraction as above when in the form of a rigid wallseparating two systems, ie., it is a thermal insulator•S t Systems separated by a di th t db diathermal wall are i th l ll in thermal lcontact, and will reach thermal equilibrium• Zeroth Law of Thermodynamics – If A and B are separatelyin thermal equilibrium with C, then A and B will be in thermalequilibrium with one another• An experiment with gases – equation of state BITSPilani, Pilani Campus
  6. 6. Temperature and Thermometry• States in thermal equilibrium with one another havethe same temperature T• Equation of state: A relationship between P, v, and T,characteristic of a substance• T can be used as one of the variables t characterize th b d f th i bl to h t i thestate, ie., v= v(P,T)• Thermometry: Such a relationship that enables one todetermine the temperature from a measurement of aproperty for eg., height of mercury in capillary, resistance ofa wire pressure of a fixed volume of a gas wire,• T also determines as we all know the direction in whichheat transfer occurs, though we will introduce the concept of g pheat formally a little later in this course. BITSPilani, Pilani Campus
  7. 7. Pressure• Pressure: Normal force exerted by a fluid per unit areaP = δFn/δASI unit 1 Pascal (Pa) = 1 N/m21 bar = 105 Pa = 0.1MPa = 100kPa1 bar = 105 Pa = 0 1MPa = 100kPa1 atm = 101325 Pa = 101.325kPa1 Torr = 1mm of Hg = 133.3224Pa g• Absolute Pressure and Gauge Pressure• Hydrostatic Pressure – due to a columnof fluid of height h in gravitational field∆P = ρgh is the pressure difference ρg p BITSPilani, Pilani Campus
  8. 8. Slide headings hereBarometer measuring  Manometer measuring pressure  pabsolute pressure relative to atmospheric pressure p p BITSPilani, Pilani Campus
  9. 9. Scales of Temperature Gas thermometer – ideal gas scale T = 273.16(P/Ptp)• Celsius Ttp = 0.01º C, ice point = 0ºC, steam point =100.0 ºCC• Kelvin = ºC + 273.15 (Absolute), Coincides with the idealgas scale BITSPilani, Pilani Campus
  10. 10. Pure Substance Phase Behavior• Experiment with water at Constant Pressure: T-v behavior• Saturation Temperature – temperature at which liquid and vapor coexistat given P, ie., the boiling temperature• Saturation Pressure – pressure at which liquid and vapor coexist atgiven Y i th vapor pressure i Y, ie., the• The saturation T of water at 0.1 MPa is 99.6º C, and vice versa• At fixed pressure, the temperature does not change as long as the twophases coexist If heat is added the relative amount of vapor increases coexist. added, BITSPilani, Pilani Campus
  11. 11. Vapor Pressure• The vapor pressure of a pure liquid increases withincreasing temperature• The vapor pressure has a unique value at a giventemperature• The vapor pressure curve terminates at a critical pointbeyond which there is no distinction between liquid andvapor BITSPilani, Pilani Campus
  12. 12. T-v diagram for water BITSPilani, Pilani Campus