Thermodynamics lecture 2

BITS Pil iBITS PilaniPilani Campus

L t 2 P Th l ilib i dLecture 2: Processes; Thermal equilibrium and Temperature; Phase behavior of pure substance

State Postulate• Phase – Spatially uniform (in chemical composition and physical 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 that of volume change, no magnetic, electrical, effects etc., Also we will normally ignore surface effectswe will normally ignore surface effects• Postulate – two intensive properties suffice to determine all others (ie., determine the equilibrium state) of a single h i l ibl b t If i dditiphase, pure simple compressible substance. If, in addition,

the mass is known then so are all other extensive properties• Also applies to a mixture of fixed composition such as airAlso applies to a mixture of fixed composition such as air in a single phase

BITSPilani, Pilani Campus

Equilibrium surface

• Properties are also called state functions

• Set of all equilibrium states constitutes a surface in

the space of independent intensive variablesthe space of independent intensive variables


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 equilibrium states – slow and controlled

• Isobaric isochoric isothermal processes• Isobaric, isochoric, isothermal processes•If intervening states not equilibrium states, then shown dashed• Cycle – Initial and final states are the same


Thermal Equilibrium • Diathermal material – One which allows two systems in contact 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 an interaction as above when in the form of a rigid wall separating two systems, ie., it is a thermal insulator

S t t d b di th l ll i th l• Systems separated by a diathermal wall are in thermal contact, and will reach thermal equilibrium• Zeroth Law of Thermodynamics – If A and B are separatelyZeroth Law of Thermodynamics If A and B are separately in thermal equilibrium with C, then A and B will be in thermal equilibrium with one another• An experiment with gases – equation of state


Temperature and Thermometry• States in thermal equilibrium with one another have the same temperature T• Equation of state: A relationship between P, v, and T, characteristic of a substance

T b d f th i bl t h t i th• T can be used as one of the variables to characterize the state, ie., v= v(P,T)• Thermometry: Such a relationship that enables one toThermometry: Such a relationship that enables one to determine the temperature from a measurement of a property for eg., height of mercury in capillary, resistance of a wire pressure of a fixed volume of a gasa wire, pressure of a fixed volume of a gas• T also determines as we all know the direction in which heat transfer occurs, though we will introduce the concept of g pheat formally a little later in this course.


Pressure

• Pressure: Normal force exerted by a fluid per unit areaP δF /δAP = δFn/δASI unit 1 Pascal (Pa) = 1 N/m2

1 bar = 105 Pa = 0 1MPa = 100kPa1 bar = 105 Pa = 0.1MPa = 100kPa1 atm = 101325 Pa = 101.325kPa1 Torr = 1mm of Hg = 133.3224Pag

• Absolute Pressure and Gauge Pressure• Hydrostatic Pressure – due to a column• Hydrostatic Pressure – due to a columnof fluid of height h in gravitational field∆P = ρgh is the pressure differenceρg p


Slide headings here

Barometer measuring absolute pressure

Manometer measuring pressure relative to atmospheric pressurep p p


Scales of Temperature

Gas thermometer – ideal gas scaleT = 273.16(P/Ptp)

• Celsius Ttp = 0.01º C, ice point = 0ºC, steam point = 100.0 ºC100.0 C• Kelvin = ºC + 273.15 (Absolute), Coincides with the ideal gas scale


Pure Substance Phase Behavior

• Experiment with water at Constant Pressure: T-v behavior• Saturation Temperature – temperature at which liquid and vapor coexist at given P, ie., the boiling temperature• Saturation Pressure – pressure at which liquid and vapor coexist at i Y i thgiven Y, ie., the vapor pressure

• 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 two phases coexist If heat is added the relative amount of vapor increases


phases coexist. If heat is added, the relative amount of vapor increases

Vapor Pressure

• The vapor pressure of a pure liquid increases with increasing temperature• The vapor pressure has a unique value at a givenThe vapor pressure has a unique value at a given temperature• The vapor pressure curve terminates at a critical point beyond which there is no distinction between liquid and


beyond which there is no distinction between liquid and vapor

T-v diagram for water


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Thermodynamics lecture 2