Heat and Thermodynamics Trefil

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Heat and ThermodynamicsTrefil Hazen, The
Sciences, Ch 4

• Great Idea
• Energy always goes
• from a more useful to a less useful form.

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Heat Temperature

• thermal energy that flows from object of higher
  temp. to object of lower temp.

• Measure of hotness, related to average kinetic
  energy per molecule in an object.

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30's hot 20's nice 10's cold zero's ice
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Temperaturemeasure of the average kinetic energy
of a substance

• Celsius
• H2O bp 100
• H20 fp 0

• Fahrenheit
• H20 bp 212
• H20 fp 32

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Absolute Zero (of temperature)

• No upper limit to temperature
• apply heat to matter and change phases solid
• liquid
• gas
• plasma

• Definite lower limit to temperature
• -273 C or 459 F
• absolute zero
• The point at which molecules lose all kinetic
  energy

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Weve never achieved absolute zero, but weve
come close!

• Temps of approx. 0.000001K have been reached,
  but 0 K has never been reached

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How do we know absolute zero exists?

• Gases expand when heated, contract when cooled.
• Gases at constant pressure, each 1oC drop in temp
  reduces volume by 1/273
• If a gas at 0oC were cooled to 273oC, it would
  contract down to a volume of zero

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Specific Heat Capacity

• A measure of the ability of a material to absorb
  heat energy
• The quantity of heat required to raise the temp
  of 1 gram of that material by 1oC.

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Specific Heats of Selected Substances

• Substance Sp Ht (J/g-oC)
• Water 4.18
• Aluminum 0.89
• Carbon 0.71
• Iron 0.45
• Mercury 0.14

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Thermal Expansion

• All phases of matter tend to expand when heated
  and contract when cooled.
• Think of examples of the above phenomena to share
  with your students.
• Discuss examples and engineering applications

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Thermal Expansion
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role-play phase changes

• Melting
• Solid to Liquid
• Heat input increases molecular motion to the
  point that molecular connections break

• Freezing
• Liquid to Solid
• Energy dissipates and motion decreases

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Evaporation vs Condensation

• Dry off outside shower
• Liquid water evaporates off of your body
• Water on your body is warmed and evaporates from
  liquid to vapor, gaining kinetic energy

• Mirror is steamed up
• Water vapor condenses on cooler glass
• Warm, moist air cools and condenses from vapor to
  water, losing kinetic energy

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Evaporation Sublimation
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Figure 4-4 Convection. Heat is transferred by
the bulk motion of the water.
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Boiling

• Evaporation that occurs beneath the surface of a
  liquid
• bubbles of vapor
• Depends on 2 factors
• Temperature
• Pressure

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Pressure and boiling

• At lower atmospheric pressure, boiling point is
  lowered
• Example At Denver, water boils at 95oC instead
  of 100oC.
• Why?

• At higher pressure, boiling point is increased
• Example common pressure cooker
• Why use one of these?

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Heat Transfer

• Conduction
• Convection
• Radiation

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conduction

• The movement of heat by collision of vibrating
  atoms

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convection

• Transfer of heat by the physical motion of fluid.
• (Dense cooler fluids descend and displace warmer
  rising fluids)

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radiation

• Movement of heat by electromagnetic energy.
• (The only mechanism that does not require atoms
  to facilitate the transfer.)

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Thermodynamics(Gr) movement of heat

• Conduction movement of heat through solids by
  atomic collisions
• Convection heat transfer in a gas or liquid via
  currents or bulk motion of the heated fluid
• Radiation transfer of heat through air or space
  as infrared waves

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Thermodynamics

• The study of the movement of heat
• Also refers to the science of heat, energy, and
  work

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Laws of Thermodynamics

• I Conservation of Energy
• In an isolated system, the total amount of
  energy, including heat, is conserved.
• II Entropy or disorder
• Energy always goes from a more useful to a less
  useful form.

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EnergyTransformation
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The tendency toward disorder or entropy
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Obstructions to the 2nd Law make life possible.

• 2nd Law of Thermodynamics and evolution

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In Trefil Hazens text

• Please read about trophic levels and the first
  law of thermodynamics
• Also see on the consequences of the second law of
  thermodynamics

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Review Questions from ch 4

• discussion questions 1, 2, 4, 6-9
• problems 1 6, p. 90.