The document discusses key concepts in thermodynamics including: 1) Thermal equilibrium occurs when two systems in contact across a thermal conductor reach the same temperature. 2) A pure substance's phase behavior is characterized by its temperature-volume diagram, with liquid-vapor coexistence occurring along the saturation curve. 3) A substance's vapor pressure increases uniquely with temperature and defines its boiling point at given pressures, terminating at the critical point where liquid and vapor can no longer be distinguished.

1. BITS Pil i
Pilani
Pilani Campus
Lecture 2: P
L t 2 Processes; Th
Thermal equilibrium and
l ilib i d
Temperature; Phase behavior of pure substance

2. 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 effects
• Postulate – two intensive properties suffice to determine all
others (ie., determine the equilibrium state) of a single
phase, pure simple compressible substance. If i addition,
h i l ibl b t If, in dditi
the mass is known then so are all other extensive properties
• Also applies to a mixture of fixed composition such as air
in a single phase
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3. Equilibrium surface
• Properties are also called state functions
• Set of all equilibrium states constitutes a surface in
the space of independent intensive variables
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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 equilibrium
states – slow and controlled
• Isobaric isochoric, isothermal processes
Isobaric, isochoric
•If intervening states not equilibrium states, then shown
dashed
• Cycle – Initial and final states are the same
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5. Thermal Equilibrium
• Diathermal material – One which allows two systems in
contact across a rigid wall of such a material to influence
g
each 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
Systems separated by a di th
t db diathermal wall are i th
l ll in thermal
l
contact, and will reach thermal equilibrium
• Zeroth 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
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6. 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 can be used as one of the variables t characterize th
b d f th i bl to h t i the
state, ie., v= v(P,T)
• Thermometry: 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 gas
wire,
• T also determines as we all know the direction in which
heat transfer occurs, though we will introduce the concept of
g p
heat formally a little later in this course.
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7. Pressure
• Pressure: Normal force exerted by a fluid per unit area
P = δFn/δA
SI unit 1 Pascal (Pa) = 1 N/m2
1 bar = 105 Pa = 0.1MPa = 100kPa
1 bar = 105 Pa = 0 1MPa = 100kPa
1 atm = 101325 Pa = 101.325kPa
1 Torr = 1mm of Hg = 133.3224Pa
g
• Absolute Pressure and Gauge Pressure
• Hydrostatic Pressure – due to a column
of fluid of height h in gravitational field
∆P = ρgh is the pressure difference
ρg p
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8. Slide headings here
Barometer measuring Manometer measuring pressure
p
absolute pressure relative to atmospheric pressure
p p
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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 ideal
gas scale
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10. 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
given 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 two
phases coexist If heat is added the relative amount of vapor increases
coexist. added,
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11. Vapor Pressure
• The vapor pressure of a pure liquid increases with
increasing temperature
• The 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
vapor
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12. T-v diagram for water
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