Change of Phase and Thermodynamics

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Change of Phase and Thermodynamics

• Physics 1010
• Dr. Don Franceschetti
• October 30, 2007

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States (Phases) of Matter

• Solid
• Liquid
• Gas (vapor)
• Plasma

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Evaporation

• Converts a liquid to gas phase.
• Only molecules with enough kinetic energy can
  leave. Lower energy molecules left behind.
  Evaporation cools.
• Perspiration is meant to take advantage of this!
  Dogs have to pant, pigs must wallow.
• Sublimation is the evaporation of a solid.

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Condensation and Humidity

• Gas changes to liquid.
• Is a warming process. Steam can burn.
• Humidity is mass of water per volume of air.
• Relative humidity is humidity divided by maximum
  humidity at that temperature.
• Saturation is 100 relative humidity
• Warm air rises, expands, cools, rain drops form.
  Discuss cloud seeding.

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Boiling

• Vapor pressure is the pressure of the liquid in a
  closed space above the liquid. It goes up with
  temperature. When it equals atmospheric
  pressure, the liquid boils. AT high altitudes,
  water boils below 100 degrees C.
• Pressure cooker, water vapor goes above
  atmospheric and boiling point goes up.

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Geysers

• Consist of a long, narrow, vertical hole into
  which underground streams seep. Heated by
  volcanic heat from below. Narrow shaft cuts off
  convection. Boiling begins near bottom, steam
  pushes out water above, lowering pressure,
  producing more boiling.

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Geysers
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Getting to the triple point
Boiling water freezes when the pressure is low
enough. Freeze dry coffee, et.c
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Freezing and Melting

• As atoms move faster they get unstuck from each
  other. Take away their random kinetic energy and
  they get restuck.
• Regelationmelting under pressure and then
  freezing again unique to water
• Phase transition temperatures are precise.
• Latent heat of fusion
• Latent heat of vaporization

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Background

• Thermodynamics means
• Motion of heat
• Motion from heat
• Begins with understanding of atmospheric
  pressure, needed to pump water out of mines.
• Steam Enginefirst effective conversion of heat
  to motion.
• Developed before atomic theory was accepted.

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Absolute Zero

• Gases seem to lose 1/273 rd of their volume for
  each degree below O C.
• Ideal gas just disappears at -273.15 C.
• Lord Kelvin (John Strutt) suggested an absolute
  temperature scale.
• Water freezes at 273.15 K
• Water triple point at 273.16 K
• Water boils at 373.15K

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First Law of Thermodynamics

• When heat flows to or from a system, the system
  gains or loses an amount of energy equal to the
  amount of heat transferred.
• Important to define what is meant by system
• Heat added to system increase in internal
  energy external work done by system
• Important to understand internal energy

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J. P. Joule and the First Law

• J. P. Joule determined the Mechanical Equivalent
  of Heat by doing various forms of work on water
  and measuring the temperature increase.

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Adiabatic Processes

• In an adiabatic process no heat enters or leaves
  the system. (Either because of insulation or no
  time for heat to flow).
• Heat added work done by system.
• So gases expanding adiabatically cool and gasses
  compressed adiabatically warm.
• Airplane cabin air is heated so much during
  compression, that it has to be cooled to be
  bearable.

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Meteorology and the First Law

• Air temperature rises as heat is added or as
  pressure in increased.
• The atmosphere can often be thought of as a
  collection of parcels, existing for minutes to
  hours and which are essentially adiabatic.
• The temperature of a parcel of dry air goes down
  10C for each kilometer it rises, due to the
  pressure difference.

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Second Law of Thermodynamics

• Heat of itself never flows from a cold object to
  a hot object.
• Equivalently, there is no process possible the
  sole effect of which is to convert a amount of
  heat into entirely into work.
• Impossible to build a perpetual motion machine of
  the second kind.

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Perpetual motion machines

• First kind Produce an output power greater than
  the input. Violates first law of thermodynamics.
• Second kind Convert 100 of heat energy into
  kinetic energy.

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Heat Engines

• A heat engine is any device that changes heat
  into work.
• Heat engine is thought of as working between two
  reservoirs, one hot, one cold.
• Max efficiency is difference in reservoir
  temperatures divided by absolute temperature of
  hot reservoir

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Fuel Efficiency of Automobile

• Energy in 36 cooling water losses
• 26 engine output 38 exhaust
• Engine output 3 acceleration work
• 6 rolling friction 3 accessories
• 4 coasting and idling 7 air drag
• 3 power transmission losses

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Order tends to disorder

• Second law in nature, high quality energy tends
  to transform into lower quality order order
  tends to disorder
• Entropy is a measure of the amount of
  disordersort of
• Change in entropy ?(heat)/T

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Entropy cautions

• Disorder needs to be defined very carefullyit
  takes quantum mechanics!
• Heat death of universea depressing idea
• The Last Question, good story by Isaac Asimov
• But black holes complicate matters
• Role of intelligence not fully understood