Chapter 16 Heat Transfer

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Chapter 16Heat Transfer
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Heat Transfer

• Heat always flows from high temperature objects
  to low temperature objects.
• Heat flow stops when temperatures equal.
• Various ways by which heat may flow.

Heat flows from child into air
75º
98º
32º
Heat flows from child and air into the ice cream
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Conduction

• Conduction is heat flow by direct contact.
• Some materials are good thermal conductors,
  others are insulators.

Tile floor feels colder than wood floor
98º
98º
75º
75º
Wood is an insulator
Tile is a conductor
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Demo Torch the Money

• Wrap a dollar bill tightly around a copper pipe.
  Put it into a flame.

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Air is a Poor Conductor
Because air is such a poor conductor, some pizza
ovens dont have a door.
Can safely put your hand in an oven. Metal is
good conductor so you need oven mitt to touch it
safely (cloth is a poor conductor).
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Demo Boiling Ice Water

• Water and glass are relatively poor conductors of
  heat.
• Can boil water at the top of a test tube with ice
  at the bottom of the tube.

Steel wool prevents ice from floating
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Convection

• Heat transfer in a fluid often occurs mostly by
  convection.
• Buoyancy causes warm air to rise, which carries
  thermal energy directly by its motion.

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Demo Candle
Very HOT
Rising hot air above a candle carries most of the
heat generated by the burning flame.
Warm
Shadows reveal rising air currents of hot air.
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Demo Convection Buoyancy

• Candle goes out while in free fall because in
  weightless state hot air does not rise so fresh
  air cannot reach the candles wick.

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Convection Oven
Convection oven has a fan to enhance the
circulation of the air, increasing the transfer
of heat.
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Demo Candle in a Tube

• Candle stays light until the partition in the
  tube is removed. Why?

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Fiberglass Insulation

• Air is a poor thermal conductor but easily
  transfers heat by convection.
• Fiberglass insulation is mostly air, with the
  fibers disrupting the convection flow.

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Radiation

• Light has many different wavelengths, most of
  which are not visible to the eye.
• All light carries energy, thus transfers heat.

Heat Lamp
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Emission of Radiant Energy
75º

• All objects radiate light higher the temperature
  the higher the frequency.
• At room temperature the radiated light is at
  frequencies too low for our eyes to see.
• Special cameras are sensitive to this infrared
  radiation.

98º
Attics in this house were kept warm for growing
marijuana.
70
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Reflection of Radiant Energy

• White objects reflect light, black objects dont.

Hole in a box with white interior looks black
because almost none of the light entering the
hole reflects back out.
White tubes look black inside.
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Controlling Heat Transfer

• Thermos bottle eliminates conduction and
  convection by having double-walled sides with
  vacuum.
• Silvered interior walls minimize heat transfer by
  radiation.

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Greenhouse Effect

• Glass is transparent to sunlight
  (short-wavelength).

Glass is opaque to infrared radiation
(long-wavelength) produced by objects inside
greenhouse, trapping the heat.
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Earths Greenhouse Effect

• Earths atmosphere acts as a greenhouse, trapping
  solar energy.
• Most of the trapping is due to carbon dioxide and
  water vapor, which is why theyre called
  greenhouse gasses.

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Check Yourself

• What does it mean to say that the greenhouse
  effect is like a one-way valve?
• Can this build up energy forever?

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Global Temperature Variations

• Temperatures increased from 1910 to 1940.
• Temperatures then cooled for 40 years until they
  started rising again in the 1980s.

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Greenhouse Carbon Dioxide

• Over past 1000 years temperatures nearly constant
  until CO2 emissions increased starting with the
  industrial revolution.

Industrial revolution begins
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Cars Carbon Dioxide

• One gallon of gasoline has about 5.2 lb of
  carbon. A 5-lb bag of charcoal (which is pure
  carbon) holds about 100 briquettes.
• At 26 miles per gallon, thats 0.2 lb of carbon
  per mile, or about four to five charcoal
  briquettes per mile.

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Anthropogenic Global Warming
Rising temperatures are due to human production
of greenhouse gases.
Temperature
Caused by humans
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Consequences of Global Warming
Melting of Polar Ice Caps
Species extinctions
Weather modifications
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Solutions to Global Warming

• Many simple, small changes together could make a
  significant difference.

Efficient Lighting
Efficient Appliances
Building Insulation