Heat Transfer

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# Heat Transfer - PowerPoint PPT Presentation

Heat Transfer. How does the energy move from a hotter to a colder object? Three mechanisms Conduction Convection Radiation. Conduction. Stir your hot soup with a metal spoon Pretty soon you need a pot holder because the end of the spoon you are holding gets hot

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## PowerPoint Slideshow about ' Heat Transfer' - steven-edwards

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Presentation Transcript
Heat Transfer

How does the energy move from a hotter to a colder object?

Three mechanisms

Conduction

Convection

Conduction

Stir your hot soup with a metal spoon

Pretty soon you need a pot holder because the end of the spoon you are holding gets hot

This is heat transfer by conduction

Energy travels up the spoon from the end in the hot soup to the end in your hand

Conduction

We sense the movement of energy by the increasing temperature

This means the atoms and molecules have higher average kinetic energy

Primarily occurs by the movement of electrons in the material

The more easily the electrons can move, the better the conduction

Conduction

Metals have some electrons that are very loosely bound to the atoms in the material

These electrons can move easily and can rapidly pick up additional kinetic energy

Metals are good conductors

Wood and plastic don’t have loosely bound electrons, so they are poor conductors

Conduction

Air is a poor thermal conductor

If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warm

Snow is a poor conductor, while water is better

Makes igloos a useful as a house

Convection

A phenomenon in fluids

Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a current

These moving fluid currents are convection

Convection

When the radiator heats the air, it becomes less dense and rises

Cool air moves in to replace the air that rose

This generates the air flow

So radiators don’t need a fan to stir the air and to distribute heat throughout a room

The rising air cools until its density matches that of the surrounding air

Convection

We take advantage of the cooling that occurs during an expansion

We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space

Convection

Explains why breezes come from the ocean in the day and from the land at night

Energy carried by electromagnetic waves

Study waves later in detail

Wavelength is related to vibration frequency

Every object is emitting electromagnetic waves regardless of temperature

Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum

Most things emit primarily in the infrared

Night vision goggles, etc.

If they didn’t, they would run out of energy to emit

Good emitters are also good absorbers

Equilibrium established between emission and absorption

When something can’t equilibrate, it gets hotter or colder

Interior of a car on a sunny day

Sunlight comes in as visible light

Seats and interior are much cooler so they radiate in the infrared instead of visible

Glass in the windows blocks infrared so energy can’t get out

Car interior heats up!

A good absorber reflects very little energy

A poor absorber reflects a lot of energy

Think about snow that doesn’t melt in sunshine even though 1400 watts/meter2 are hitting it

At night, objects receive no input energy from the sun

But, they are warmer than outer space, so they continue to radiate energy

Thus, they cool off

Can we make ice in the desert without a refrigerator?

Newton’s Law of Cooling

Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings

Works both ways, cooling and heating

Rate of heating also depends on the temperature difference