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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
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