15-May-15 Physics 1 (Garcia) SJSU Chapter 16 Heat Transfer.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 16Heat Transfer


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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32º

75º

Heat flows fromchild and air into the ice cream

Heat flows fromchild into air


Conduction

Conduction is heat flow by direct contact.

Some materials are good thermal conductors, others are insulators.

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75º

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Wood is aninsulator

Tile is aconductor

Tile floor feels colder than wood floor


Demo: Torch the Money

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


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.


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


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.


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.

http://set.lanl.gov/programs/DX2/explorations/CandleShadowSci.htm


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.


Convection OvenConvection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.


Demo: Candle in a Tube

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


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.


Radiation

Light has many different wavelengths, most of which are not visible to the eye.

All light carries energy, thus transfers heat.

Heat Lamp


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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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.


Controlling Heat Transfer

Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum.

Silvered interior walls minimize heat transfer by radiation.


Greenhouse EffectGlass is transparent to sunlight (short-wavelength).

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


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.”


Check Yourself

What does it mean to say that the greenhouse effect is like a one-way valve?

Can this build up energy forever?


Global Temperature Variations

Temperatures increased from 1910 to 1940.

Temperatures then cooled for 40 years until they started rising again in the 1980’s.


Greenhouse Carbon Dioxide

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

Industrial revolution begins


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.


Anthropogenic* Global Warming

Rising temperatures are due to human production of greenhouse gases.

*Caused by humans

Tem

pera

ture


Consequences of Global Warming

Weather modifications

Species extinctions

Melting of Polar Ice Caps

http://upload.wikimedia.org/wikipedia/en/a/a4/Hurricane_Katrina_August_28_2005_NASA.jpg


Solutions to Global Warming

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

Efficient Appliances

Building Insulation

Efficient Lighting

15-May-15 Physics 1 (Garcia) SJSU Chapter 16 Heat Transfer.

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