Heat Energy Transfer Lesson 4

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Heat (Energy) TransferLesson 4

• Dan Hartung
• 2/22/08

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The Nature of Heat

• Definition Heat is the internal kinetic energy
  of the atoms and molecules that make up a
  substance
• Since it is a form of energy, it is measured in
  the standard unit of a Joule
• 1 Joule 1Nm 1 kg m2/s2
• More commonly, it is measured in the following
  units
• calorie heat energy needed to raise 1 gram of
  water by 1 degree Celsius. 1 calorie 4.186
  Joules
• Calorie commonly used to measure energy content
  of food. 1 Calorie 1000 calories
• British Thermal Unit (BTU) heat energy needed
  to raise one pound of water by 1 degree
  Fahrenheit. 1 BTU 252 calories 1,054 Joules

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Temperature

• Definition Temperature is the average speed or
  kinetic energy of the atoms and molecules in a
  substance
• Heat will not flow between two substances of the
  same temperature
• Heat is really energy in the process of being
  transferred from one object to another because of
  the temperature difference between them
• Temperature is measured
• by three different scales
• Celsius
• Fahrenheit
• Kelvin

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Brad Pitt
Heidi Klum
HOT
HOT

Contrary to rumor NO HEAT TRANSFER !!!!!
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Temperature

• Absolute Zero The lowest temperature possible
• Also signified as the temperature at which empty
  space is completely devoid of all motion and/or
  energy
• The coldest place in nature is the depths of
  outer space where temperatures are approximately
  3 degrees above absolute zero

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Heat Capacity

• Definition Heat capacity is the amount of heat
  required per unit increase in temperature
• It is a measure of how well the substance stores
  heat
• H. C. (heat added) / (change in temperature)
• Materials with large heat capacities hold heat
  well.
• Their temperatures will not rise much for a given
  amount of heat
• Example water, air
• Materials with small heat capacities do not hold
  heat well
• Their temperatures will rise quickly for a given
  amount of heat
• Example copper, soil

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Specific Heat Capacity

• Definition Specific heat capacity is the amount
  of energy required to raise the temperature of
  one kilogram (kg) of a substance by one degree
  Kelvin (K)
• It is a measure of heat capacity per unit mass
• Specific heat is measured in Joules per kilogram
  Kelvin
• J/(kgK)
• Substances with different specific heats require
  different amounts of energy
• Low specific heat Less input energy
• High specific heat High input energy

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Specific Heat

• Q Amount of heat added to the substance (in
  calories)
• c specific heat
• for water c 4.18 J/g K, for air c 1.0 J/g K
• m mass of the substance (in grams)
• ?T change in temperature by the substance

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• Example You add 12 calories of heat to 1 gram of
  air what is the temperature change?
• Q c x m x ?T
• ? ?T Q / (c x m)
• ?T 12 cal / (0.24 cal/g oC x 1 g)
• ?T 48oC
• So if the initial temperature of the air is 10oC,
  the final temperature is 58oC.

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Heat Transfer

• There are three ways that heat may be transferred
  between substances at different temperatures
• Conduction
• Convection
• Radiation

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Heat Transfer

• Conduction The flow of heat via collisions
  between atoms and molecules in a substance
• Collisions lead to faster molecules losing speed,
  and slower molecules gaining speed
• This results in the faster molecules transferring
  some of their kinetic energy to the slower
  molecules
• Thus, heat is always transferred from warmer to
  colder regions during conduction
• Solids, such as metals, are good conductors of
  heat
• Gases, such as air, are poor conductors of heat

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Heat Transfer by Conduction
Fast (hot) atoms
Slow (cold) atoms
Common Temperature
http//theory.uwinnipeg.ca/mod_tech/img115.gif
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Heat Transfer

• Convection The transfer of heat by the mass
  movement of a fluid (such as water and air)
• Takes place in fluids and gases
• Buoyant (warmer, less dense) blobs of air are
  called thermals
• Heat transported upward by rising thermals
• Cooler, heavier air flows to surface to replace
  rising air
• Horizontal heat transfer by fluid motions is
  called Advection

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Heat Transfer by Convection
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Heat Transfer

• Radiation Energy transfer by electromagnetic
  (EM) waves
• Every object emits EM waves
• Wavelength depend on temperature of object
• Stefan-Boltzmann Law E (const) T4
• Hotter objects radiate a lot more energy (heat)
  per unit surface area
• Objects with a temperature of 500 degrees C or
  greater radiate at wavelengths we can seecooler
  objects radiate at wavelengths that are too long
  to stimulate vision
• Example hot red coals and a room-temperature
  book

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Heat Transfer

• Radiation Shortwave vs. Longwave Radiation
• Hot objects emit radiation concentrated at
  shorter wavelengths
• Radiation from the sun is concentrated at much
  shorter wavelengths than radiation from the earth
• Sun vs. Earth
• Sun Shortwave radiation
• Earth Longwave radiation

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Heat Transfer

• Radiation What happens to incoming solar
  radiation?
• Scattering Incoming radiation is deflected in
  all directions
• Reflection Incoming radiation is deflected
  like scattering, although more is sent backward
  (towards space)
• Albedo Amount of reflected radiation compared
  to the amount of incoming radiation

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Typical Albedo of Various Surfaces
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Summary