Chapter 16 Heat Transfer

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Chapter 16 Heat Transfer - PowerPoint PPT Presentation

Chapter 16 Heat 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. Heat flows from child into air. 75º. 98º. 32º. Heat flows from child and air into

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Chapter 16Heat Transfer

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.

Heat flows from

child into air

75º

98º

32º

Heat flows from

child and air into

the ice cream

Physics 1 (Garcia) SJSU

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 don’t 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

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 candle’s 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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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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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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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.

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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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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.”

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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 1980’s.

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Greenhouse Carbon Dioxide

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

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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.

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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

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