Heat Transfer How does the energy move from a hotter

to a colder object? Three mechanisms Conduction Convection Radiation

Conductors are substances that conduct thermal energy very well.

Insulators are substances that do not conduct thermal energy very well.

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. (increasing temperature.)

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

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

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

Radiation Energy carried by electromagnetic waves Study waves later in detail Light, microwaves, radio waves, x-rays Wavelength is related to vibration

frequency

Radiationaverage frequency absolute temperature

Radiation 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.

Radiation Things also absorb radiation 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

Radiation 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!

Radiation A good absorber reflects very little energy Think about dark pavement 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

Radiation 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