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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