thermal energy - gillam ap physics · 2019. 8. 12. · conduction: the transfer of thermal energy...

Thermal Energy

The atoms of all objects are in a state of constant motion, constantly "bumping" into nearby atoms.

When atoms bump (perfectly elastic collisions) the energy of one atom transfers to another .

This transfer of kinetic energy between atoms occurs when ever atoms collide.

In some materials the migrations of KE from atom to atoms is very fast, and in others it is much slower.

The reason for this can be complicated, but it partially depend on the ability of electrons to move between atoms.

In metals the electrons are free to roam from atom to atom and because of this KE can quickly pass from place in the material to another.

Metals are good "CONDUCTORS" of thermal energy.

Conduction:

Conduction: the transfer of thermal energy by

the contact of particles.

Conduction always causes thermal energy to flows from high temperature to low temperature.Until a state of equilibrium is reached (same temperature everywhere)

Heat flows out.

Cold does not flow.

Conductors: materials that

allow heat to flow easily

Insulators: materials that do

not allow heat to flow easily

Silver is the best

gases are the worst

Dense solids with atoms close together conduct the best. (especially if they have loose electrons)

Gases are poor conductors since there are fewer atoms to carry the energy, and fewer transferring collisions.

Heat cannot be conducted across a vacuum.

How would the Heat get out of this bottle?

Most types of insulation have trapped pockets of air to slow the conduction of thermal energy.

Down

Why do you need a pad when sleeping in a sleeping bag?

The sleeping bag will not insulate you from the ground because the weight of your body will compress the insulation (no trapped air spaces).

FYI: the most insulating material would be a tube of foam, however moisture would not escape, and water would condense inside the bag and you will wake up soaking wet.

If your bag does not "breath" you will get wet inside.

Jackets and sleeping bags are always made out of "breathable" materials.

Gore-Tex is famous for its ability to breath (let gasses pass), while blocking liquid water.

gore-tex

Convection: the transfer of thermal energy by

moving the particles that have the energy.

Occurs in fluids (liquids and gasses) due to the flow of particles from one place to another.

The Law of Thermal Expansion, tell us that materials become less dense as they get warmer.

The material gets bigger because the faster moving atoms move farther away from each other.

In fluids denser materials sink and "lighter" materials float (rise). So hot air, and hot water rise when surrounded by "heavier" cold fluids.

Convection occurs naturally in locations where a difference in the temperature of a fluid exists.

Golf stream convection current

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