15-may-15 physics 1 (garcia) sjsu chapter 16 heat transfer
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Apr 18, 2023 Physics 1 (Garcia) SJSU
Chapter 16Heat Transfer
Apr 18, 2023 Physics 1 (Garcia) SJSU
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.
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Heat flows fromchild and air into the ice cream
Heat flows fromchild into air
Apr 18, 2023 Physics 1 (Garcia) SJSU
Conduction
Conduction is heat flow by direct contact.
Some materials are good thermal conductors, others are insulators.
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Wood is aninsulator
Tile is aconductor
Tile floor feels colder than wood floor
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Torch the Money
Wrap a dollar bill tightly around a copper pipe. Put it into a flame.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Air is a Poor Conductor
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).
Because air is such a poor conductor, some pizza ovens don’t have a door.
Apr 18, 2023 Physics 1 (Garcia) SJSU
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
Apr 18, 2023 Physics 1 (Garcia) SJSU
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.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: CandleVery HOT
Warm
Shadows revealrising air currents of hot air.
Rising hot air above a candle carries most of the heat generated by the burning flame.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Convection & BuoyancyCandle goes out while in free fall because in weightless state hot air
does not rise so fresh air cannot reach the candle’s wick.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Convection OvenConvection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Candle in a Tube
Candle stays light until the partition in the tube is removed. Why?
Apr 18, 2023 Physics 1 (Garcia) SJSU
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.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Radiation
Light has many different wavelengths, most of which are not visible to the eye.
All light carries energy, thus transfers heat.
Heat Lamp
Apr 18, 2023 Physics 1 (Garcia) SJSU
Emission of Radiant Energy
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.
Attics in this house were kept warm for growing marijuana.
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Apr 18, 2023 Physics 1 (Garcia) SJSU
Reflection of Radiant Energy
White objects reflect light, black objects don’t.
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.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Controlling Heat Transfer
Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum.
Silvered interior walls minimize heat transfer by radiation.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Greenhouse EffectGlass is transparent to sunlight (short-wavelength).
Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Earth’s Greenhouse Effect
Earth’s atmosphere acts as a greenhouse, trapping solar energy.
Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.”
Apr 18, 2023 Physics 1 (Garcia) SJSU
Check Yourself
What does it mean to say that the greenhouse effect is like a one-way valve?
Can this build up energy forever?
Apr 18, 2023 Physics 1 (Garcia) SJSU
Global Temperature Variations
Temperatures increased from 1910 to 1940.
Temperatures then cooled for 40 years until they started rising again in the 1980’s.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Greenhouse Carbon Dioxide
Over past 1000 years temperatures nearly constant until CO2 emissions increased starting with the industrial revolution.
Industrial revolution begins
Apr 18, 2023 Physics 1 (Garcia) SJSU
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, that’s 0.2 lb of carbon per mile, or about four to five charcoal briquettes per mile.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Anthropogenic* Global Warming
Rising temperatures are due to human production of greenhouse gases.
*Caused by humans
Tem
pera
ture
Apr 18, 2023 Physics 1 (Garcia) SJSU
Consequences of Global Warming
Weather modifications
Species extinctions
Melting of Polar Ice Caps
Apr 18, 2023 Physics 1 (Garcia) SJSU
Solutions to Global Warming
Many simple, small changes together could make a significant difference.
Efficient Appliances
Building Insulation
Efficient Lighting