Heat Transfer

Heat TransferHeat Transfer• Heat always moves from a warmer place to a

cooler place.

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

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

QuestionQuestionIf a cup of coffee and an ice cube were left on the table in this room what would happen to them? Why?

AnswerAnswerThe cup of coffee will cool until it reaches room temperature. The ice will melt and then the liquid will cool to room temperature.

HEAT TRANSFER METHODSHEAT TRANSFER METHODS

ConductionConductionConvectionConvectionRadiationRadiation

CONDUCTIONHeat travels along a substance from molecule to molecule (between two materials that touch each other)

Good conductors (silver, copper, gold)

Poor conductors ( glass, paper, Styrofoam)

Conduction

When you heat a metal strip at one end, the heat travels to the other end.

As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this?

Metals are different

The outer e_______ for metal atoms drift, and are free to move.

When the metal is heated, this ‘Sea of electrons’ gain k_____ energy and transfer it throughout the metal.

Insulators, such as w___ and p____, do not have this ‘Sea of electrons’ which is why they do not conduct heat as well as metals.

lectrons

inetic

ood lastic

Why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor, wood is an insulator. The metal conducts the heat away from your hands, the wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

CONDUCTION EXAMPLES

•water heating on an electric stove

•hot sand touching your feet

•touching a stove and being burned

•ice cooling down your hand

•boiling water by thrusting a red-hot piece of iron into it

CONVECTIONtransfer of energy as it is carried through a liquid or gas

heat transfer by a circulation of rising warm air (less dense) and sinking cooler air (denser).

“Hot air rises” the more dense air sinks forcing the less dense air upward

Heating fluids

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

The particles spread out and become less dense.

What is a fluid?A liquid or gas.

Fluid movement

Cooler, more d____, fluids sink through w_____, less dense fluids.

In effect warmer liquids and gases r___ up.

Cooler liquids and gases s___.

ensearmer

ise

ink

Water movement

Hot water rises

Cooler water sinks

Convection current

Cools at the surface

Why is it windy at the seaside?

Cold air sinks

Where is the freezer

compartment put in a fridge?

Freezer compartmen

t

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.

CONVECTION EXAMPLES

•macaroni rising and falling in a pot of heated water

•heat rising from a chimney

•an old-fashioned radiator (creates a convection cell in a room by emitting warm air at the top and drawing in cool air at the bottom)

RADIATIONCarries energy away from the hot object and causes it to cool down

The movement of heat in a wave-like motion through an empty space

The third method of heat transfer

How does heat energy get from the Sun to the Earth? There are no particles

between the Sun and the Earth so it CANNOT travel by conduction or by convection.

?RADIATION

Radiation

Radiation travels in straight lines

True/False

Radiation requires particles to travel

True/False

Radiation travels at the speed of light

True/False

RADIATION EXAMPLES•sunlight

•heat from toaster

•heat from a light bulb

•heat from a fire

•heat from anything else which is warmer than its surroundings

hot water rises and cold water sinks

CONVECTIONCONVECTION

stir frying vegetables

CONDUCTIONCONDUCTION

a spoon in a cup of hot soup becomes warmer

CONDUCTIONCONDUCTION

grilling hamburgers over a charcoal flame

RADIATIONRADIATION

hot air balloon

rises

CONVECTIONCONVECTION

you feel the heat from a campfire

RADIATIONRADIATION

a raw egg begins to fry as it hits a heated frying pan

CONDUCTIONCONDUCTION

Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the

cycle repeats.

CONVECTIONCONVECTION

boiling potatoes

CONDUCTIONCONDUCTION

microwave oven

RADIATIONRADIATION

Propane Heater

RADIATIONRADIATION

basement is cooler than the attic

CONVECTIONCONVECTION

radiator warms

your hands CONVECTIONCONVECTION

a person takes a warm bath

CONDUCTIONCONDUCTION

heat from a light bulb

RADIATIONRADIATION

hot pan is cooled by running it under cold water

CONDUCTIONCONDUCTION

you get sunburned

RADIATIONRADIATION

warm water at the surface of the swimming pool

CONVECTIONCONVECTION

ironing a shirt

CONDUCTIONCONDUCTION

wind currents CONVECTIONCONVECTION

ANSWER KEY• hot water rises and cold water sinks - Convection • stir frying vegetables - Conduction • a spoon in a cup of hot soup becomes warmer - Conduction • grilling hamburgers over a charcoal flame - Radiation• hot air balloon rises - Convection• you feel the heat from a campfire - Radiation • a raw egg begins to fry as it hits a heated frying pan - Conduction • Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle

repeats. - Convection • boiling potatoes in water –Conduction (water to potato)• microwave oven - Radiation• propane heater - Radiation• basement is cooler than attic - Convection • radiator warms your hands - Convection • a person takes a warm bath - Conduction • heat from a light bulb - Radiation • hot pan is cooled by running it under cold water - Conduction • you get sunburned - Radiation • warm water at the surface of the swimming pool - Convection • Ironing a shirt - Conduction • wind currents - Convection