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Chapter 22 Heat Transfer

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					Chapter 22 Heat Transfer

Conceptual Physics Chapter 22

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Heat Transfer
¤ Spontaneous heat flow is always from the warmer object to the cooler object. ¤ Heat transfer continues until thermal equilibrium is reached. ¤ Heat flow can occur in one of three ways:
¤ Conduction ¤ Convection ¤ Radiation
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Question
Why does a tile floor feel colder on your bare feet than a wood floor?

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Conduction
¤ Conduction results from atoms transferring thermal energy from one atom to the next during collisions. ¤ Examples:
¤ Food coloring in hot water ¤ Stirring a pot of soup ¤ Tile floor

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Thermal Conductors
¤ Good conductors are made of atoms with valence electrons that are only loosely bound to the nucleus. ¤ When a conductor is heated, the loosely bound electrons are caused to vibrate more rapidly and through larger amplitudes. ¤ This makes collisions between neighboring atoms more likely.
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Thermal Insulators
¤ Insulators have valence electrons which are tightly bound to a specific nucleus. ¤ The valence electrons in insulators do not have the same freedom of movement as those in conductors. ¤ Collisions between neighboring atoms occur far less frequently and heat transfer is localized.
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Conductors & Insulators
¤ There are no perfect conductors nor are there perfect insulators. ¤ Metals, as a group, are thought of as being good conductors. ¤ Wood, plastic, styrofoam, paper and air are thought of as good insulators. ¤ A good conductor is a poor insulator.
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Insulating Properties of Air
¤ Materials that are porous or have the ability to trap air pockets make good insulators ¤ Examples:
¤ Snow/Ice ¤ Goose Down Jacket ¤ Double Panel Glass Windows

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Convection
When a fluid is Convection occurs in heated, it fluids (liquids and expands, a result gases) as becomes of less dense, and a net movement of a rises above fluid mass of the the in more dense density response to fluid surrounding differences.it.

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Convection
Since at the a poor Any heat that reaches Waterwater istop of conductor, the ice the test tube at the ice stuck comes remains and remains bottom unmelted! to a boilof the test tube must do so by at the surface above conduction. the more dense water below it.

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Convection
The radiator forces When the water is hot air it near the heated,outexpands, floor where top and rises to the it rises toward a convection createsthe ceiling and sets up that causes current a natural circulation to occur the heatingto heat the room efficiently. more more efficiently.
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Convection
During the night, the During the day, the land cools more readilyland heats up more than the water. The readily than the warmer air above the water. The hot air water rises and is above the shore rises replaced by cooler air and is replaced by from above the shore cooler air from above resulting in an the ocean resulting in offshore breeze. an onshore breeze.
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Convection
¤ Convection occurs readily in gases and liquids, but does not occur at all in solid substances.

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Radiation
¤ Heat from the sun warms the earth after passing through millions of miles of empty space. ¤ Since conduction and convection can not occur in a vacuum, this heating must take place by a different mechanism – radiation.

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Radiation
¤ Radiant energy is transmitted via Radio waves, infrared waves, light waves, microwaves and electromagnetic waves.X-rays are all ¤ forms of radiation. These waves are capable of traveling through a vacuum – they do not rely in any way on a material medium in order to propagate.

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Radiation
¤ All objects emit radiant energy in a variety of wavelengths. ¤ The sun and other extremely hot objects emit short wavelength radiation, in the range of visible light. ¤ Objects at low temperatures emit radiation of longer wavelength that are invisible.
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Radiation
¤ The slower moving molecules associated with a cooler body will generally emit longer wavelength radiation.

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Absorption and Reflection
¤ Absorption and reflection are opposite processes. ¤ A good absorber of radiation is a poor reflector. ¤ Light-colored, highly polished surfaces make good reflectors of radiation. ¤ Dark-colored, dull or flat surfaces make good absorbers of radiation.
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Absorption and Reflection
¤ A white or light-colored t-shirt will keep you much cooler during a hot summer day than a dark-colored shirt. ¤ The silvered lining in a thermos reflects most of the radiant energy that reaches it.
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Absorption and Emission
¤ A good absorber of radiation is also a good emitter of radiation. ¤ If a good absorber of radiation were not a good emitter, it would experience a net gain in thermal energy and never reach thermal equilibrium with its surroundings.

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Absorption and Emission
Which of the two pitchers will keep hot tea colder icedcoffee hotter longer?

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Newton’s Law of Cooling
¤ The rate of cooling of an object (whether by conduction, convection or radiation) depends on how much hotter the object is in comparison to its surroundings. rate of cooling ~ ΔT ¤ Newton’s law of cooling also applies to objects which are being heated.
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Question
Since a hot cup of tea loses heat more rapidly than a lukewarm cup of tea, would it be correct to say that a hot cup of tea will cool to room temperature before a lukewarm cup of tea will?

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The Greenhouse Effect
¤ The greenhouse effect is a natural warming of the earth that takes place when solar radiation is trapped inside the earth’s atmosphere. ¤ Greenhouse gases – carbon dioxide, methane and water vapor – are transparent to the short wavelength visible radiation that reaches the earth from the sun, allowing this radiant energy to penetrate the atmosphere and be absorbed by the earth.
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The Greenhouse Effect
¤ The earth emits terrestrial radiation of a longer wavelength (because the earth is cooler than the sun). This radiation is primarily in the infrared region of the electromagnetic spectrum.
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The Greenhouse Effect
¤ Some of this terrestrial radiation passes through the atmosphere and escapes back into space. The remainder is blocked by the atmosphere and warms the earth. ¤ If the atmosphere were not opaque to this terrestrial radiation, the average global temperature would be approximately -18°C!

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The Greenhouse Effect
¤ This same effect occurs on a much smaller scale in an automobile left parked in the sun and in a florist’s greenhouse (although a florist’s greenhouse also restricts any heat transfer by convection).

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