Heat and Heat Transfer

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Heat and Heat Transfer
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Temperature

• Measure Of Molecule Energy
• Celsius scale depends on water
• 0o - freezing point
• 100o - boiling point
• Fahrenheit is common system
• Kelvin scale used for gases
• Starts at -273oC freezing point of He
• Absolute zero - no kinetic energy for any
  substance

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Temperature

• Temperature Is Not Heat
• Temperature measures average energy of one
  molecule
• Heat measures total energy of all molecules
  present

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Heat

• Kinetic Energy
• Can be transferred between objects due to change
  in temperature
• Always flows from hot to cold objects
• Once transferred, heat becomes internal energy
• Flow depends on thermal contact between objects

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Thermal Equilibrium

• Two objects in thermal contact are at thermal
  equilibrium when they have the same temperature
• Thermometer works on this principle
• Thermometer should not be too large compared to
  what it is measuring

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Internal Energy

• Translational, vibrational, rotational energies
  in substances
• Potential chemical energy is also involved

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Measuring Heat

• Heat is an extensive property
• Depends upon how much matter is present
• Cup of water _at_ 100 o C vs bucket of water _at_ 50 o
  C
• Units of heat
• Calorie--heat needed to raise 1 g water by 1 C
• Kilocalorie--1000 calories 1 food calorie
• SI unit is Joule 1 cal 4.187 J

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Specific heat capacity

• Gauges how quickly a material gains and loses
  heat
• Inversely proportional to heat flow
• Food has high heat capacity usually

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Thermal Expansion

• As object gains heat, molecules move farther
  apart
• Expansion vs. contraction
• Must be accounted for in these cases
• Dentistry
• Car engines vs piston
• Bridges and roadways

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Thermal Expansion

• Rates different for different materials
• Bimetallic strips--two metals connected which
  move when heated
• Motion toward side with greater expansion or
  contraction
• Thermostats use these
• Pyrex glass made with limited expansion
• Liquids expand more than solids
• Gases expand/contract more than either solid or
  liquid

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Special Properties of Water

• Specific heat capacity
• Takes long to heat or to cool
• Causes moderation of climate of areas near large
  bodies of water

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Special Properties of Water

• Expansion of water
• Like other liquids when heated
• Also expands as it freezes
• Freezes in an open structure with less density
• Freezing in a container will crack the container
• Ice floats on lake, keeps it from freezing solid

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Homework!!

• Chpt 21 RQ 2, 4, 6, 8, 10, 12, 13, 17 TE 2,
  3, 5, 6

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Heat Transfer --- Conduction

• Happens between materials in direct contact
• Due to collisions of atoms or molecules and
  electrons
• Conduction accounts for cooler feel of metals vs.
  nonmetals
• Good conduction feels cool
• Insulators, poor conductors delay heat transfer
• Liquids and gases usually poor conductors/good
  insulators
• Cold is not transferred

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Convection

• Heat transferred as hotter molecules move
• In a fluid, transfer by currents
• Rising fluid currents cool due to expansion and
  slowing down

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Radiation

• Heat due to energy waves passing thru space
• From electromagnetic waves of long wavelength
• Light also from objects emitting heat

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Absorption and Emission of Radiation

• Absorbers of radiation appear black
• Good absorbers also are good emitters--thermal
  equilibrium requires it

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Newtons Law of Cooling

• Rate of cooling is proportional to difference in
  temperature between an object and its
  surroundings
• Examples

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Greenhouse Effect

• Incoming radiation warms earth which then emits
  terrestrial radiation to warm the air
• Temperature of atmosphere depends on amount of
  both
• Atmosphere trapping terrestrial radiation is the
  greenhouse effect

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Homework

• Chpt 22 RQ 2, 6, 11, 13, 15, 16 TE 2, 4, 5,
  7, 10