Heat Transfer

What is a heat?

Heat always flows away from a hot material to a cold material

Remember, heat moves in predictable ways, from a high temperature to low temperature.

Heat Transfer

• Hot objects in a cooler room will cool to room temperature.

• Cold objects in a warmer room will heat up to room temperature.

• So that everything is in a state of equilibrium, meaning equal.

Question• If a cup of coffee and a red popsickle were

left on the table in this room what would happen to them based on how heat moves? Why?

Question• If a cup of coffee and a red popsickle were

left on the table in this room what would happen to them based on how heat moves? Why?

• The cup of coffee will cool until it reaches room temperature. The popsicle will melt and then the liquid will warm to room temperature.

Heat Transfer Methods

• Heat transfers in three ways:–Conduction–Convection–Radiation

Conduction Conduction is heat energy moving from a high-

temperature object to a low-temperature object.

Occurs in solids only.

ConductionWhen you heat a metal strip at one end, the heat travels to the other end.Conduction is the direct transfer of heat or energy from one molecule to another molecule by collision.

In solids, every atom is bonded to another atom.

If we apply heat to one part of a solid the atoms begin to vibrate.

Conduction• The heated atoms vibrating causes atoms to

collide.• This passes the heat from one atom to the

next.• The energy spreads throughout all the

molecules of the solid, and the temperature increases.

ConductionThis is why when a metal pot or pan is placed

on a hot stove, the whole pan becomes hot, rather than only the bottom.

Conduction examples

Conduction LabIf you have a metal skewer with blobs of butter on it and heat the rod, why will the blobs fall off one by one? How is this an example of conduction?

ConvectionConvection takes place when heated molecules

move from one place to another, taking the heat with them.

Convection is the primary way that heat moves through gases and liquids.

Convection

Convection

What happens to the particles in a liquid or a gas when you heat them?

The particles spread out, rise and become less dense.

Fluid movement

Warmer liquids and gases r___ up.

Cooler liquids and gases s___.

ise

ink

Water movement

Hot water rises

Cooler water sinks

Convection current

Convection CurrentThe water or air at the bottom heats up first causing it to expand. Since the warmed air or water has a lower density (it’s lighter) than the air or water around it, it rises up.

At the top, the air or water cools, increasing its density, which causes it to sink back down to the bottom. This up and down movement eventually heats everything. The continual cycling is called a convection current.

Explaining Convection

• Convection currents cause the movement of magma within the

earth.

Why is it windy at the seaside?

Cold air sinks

Where is the freezer

compartment put in a fridge?

Freezer compartme

nt

It is put at the top, because cool air sinks, so it cools the food on the way down.

It is warmer at the

bottom, so this warmer air rises and a convection current is set

up.

Radiation

Radiation May Come From Other Sources • Have you ever sat too close to a campfire while

cooking marshmallows? You’re enjoying the warmth ….. only to notice that your skin is really warm?

http://video.nationalgeographic.com/video/environment/energy-environment/solar-power/

RADIATION

• Radiation is the transfer of energy by electromagnetic waves

• Radiation does NOT require matter to transfer thermal energy

• Radiation = heat escaping the sun• So now we are going to cook with it!