Change of State and Thermodynamics

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Change of State and Thermodynamics
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Evaporation

• Change from liquid to vapor
• Requires energy and therefore causes cooling

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Condensation

• Opposite of evaporation
• Causes release of energy and therefore warming
• Cause of weather phenomenon
• Humidity
• Fog and clouds
• Rate of evaporation and condensation depends on
  saturation point

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Boiling

• Change from liquid to vapor at a pressure of 1
  atmosphere
• Happens when vapor pressure counteracts surface
  atmospheric pressure

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Freezing

• Forces of attraction between molecules overcome
  energy of the molecules
• Can be affected by dissolved solids in a liquid
• Freeze drying involves both boiling and freezing
  at same time

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Both At Once!!

• Boiling and freezing can occur at the same time
  under a reduced pressureas in a vacuum

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Regelation

• Since ice has an open structure, it can be
  crushed, but when this happens, it reforms the
  liquid
• In cases where the temperature is below the
  melting point, refreezing happens, which is
  called regelation

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Changes of State

• When solids, liquids or gases are heated, changes
  of state may occur if energy sufficient to break
  attractions is gained
• The opposite occurs when energy is lost

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Heating Curves

• The relationships between states and within
  states with energy can be seen best with a curve

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Refrigerators and Heat Pumps

• Refrigerators and heat pumps take advantage of
  the evaporation/condensation gain and loss of
  heat to do the jobs we require of them
• Heat pumps do it in reverse of below

Air conditioners work similar to refrigerators
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Homework

• Chpt 23 p342-3
• RQ 2, 4, 5, 7, 10, 19
• TE 1, 3, 6, 8

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Thermodyna-mics

• Basics
• The study of heat and its transformations to
  mechanical energy
• Must be a starting point of kinetic
  energy--Absolute Zero

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First Law of Thermodynamics

• Same as law of conservation of energy
• As applied to thermodynamics, becomes as follows
• Whenever heat is added to a system, it
  transforms to an equal amount of some other form
  of energy.
• As usually applied,
• Heat added increase in internal energy
• external work done by
  system

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First Law of Thermodynamics

• Combustion engines work this way

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Three Processes

• In thermodynamics, the first law allows work and
  heat energy to combine to make internal energy.
  This allows three possibilities
• Isothermal--where temperature of the system
  does not change
• Isochoric(isovolumetric)--where the volume does
  not change
• Isobaric--where the pressure does not change
• A fourth possibility, adiabatic, is not identical
  to the isothermal, since temperature changes, but
  heat does not

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Adiabatic Processes

• Changing gas volume with no flow of heat to or
  from system
• Diesel engine uses compression to make heat to
  ignite fuel
• Air masses cool as they rise by about 10o C for
  every 1 km rise
• Opposite happens when air falls--Chinook winds

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Second Law of Thermodynamics

• Heat will never of itself flow from a cold object
  to a hot object
• Energy always flows downhill except if an outside
  agency acts
• Heat engines cannot change all of the heat to
  work, since some heat is wasted

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Carnot Efficiency and Heat Engines

• Ideal efficiency (Thot -Tcold )
• Thot
• Where Thot is the temperature of the engine and
  Tcold is the temperature of the exhaust to the
  surroundings
• Highest efficiency can only be when absolute zero
  is exhausted to the surroundings
• Relatively high efficiency can be created if the
  ? T is large

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Order Tends to Disorder

• Natural systems tend to proceed toward a state of
  greater disorder
• This can be reversed within a system by work from
  the outside

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Order Tends to Disorder

• The measure of disorder in a system is entropy
• In the universe, entropy always increases in a
  system it can be reversed

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Homework

• Chpt 24 p 359-60
• RQ 7,10,16,17,18,23 TE 1, 4, 5, 7, 8, 12