Understanding Heat
Transfer:
Conduction, Convection
and Radiation
What is Heat?
• HEAT is the TRANSFER of thermal energy
• 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.
Question
• If a cup of coffee and a red popsickle were
left on the table in this room what would
happen to them? Why?
• The cup of coffee will cool until it reaches
room temperature. The popsickle will melt
and then the liquid will warm to room
temperature.
Heat Transfer Methods
• Heat transfers in three ways:
–Conduction
–Convection
–Radiation
Conduction
How it Happens: When you heat a material
at one end, the heat travels to the other end.
•As you heat the material, the particles vibrate.
•These vibrations make the adjacent particles
vibrate, and so on.
•The vibrations passes along the heat.
Conduction
Where it Happens: Conduction occurs in
solids and liquids.
Gases do not conduct heat well, because
the atoms/molecules are farther apart.
Why? Because the atoms/molecules in
solids and liquidsare close together, so they
can pass along the vibrations and heat.
Metals are good conductors
Remember metallic bonds?
Valence electrons in metals are free to move!
This movement lets
metals conduct heat
quickly (easier to
pass along
vibrations!)
Insulators = Poor conductors
Insulators (like wood and plastic) do not have free moving
electrons. So they do not conduct heat as well as metals.
When it is cold out, why does metal feel colder
than wood, if they are both at the same
temperature?
Metal is a conductor. Metal conducts the heat away from your
hands.
Examples of other good conductors: Stone, tile
Wood is an insulator (bad conductor). Wood does not conduct
the heat away from your hands as well as the metal, so the
wood feels warmer than the metal.
Example of an insulator: Styrofoam
Convection
•How it happens: As a fluid (liquid or gas) heats up,
the particles in it spread out.
•This makes it less dense. And it begins to rise.
•When it cools, it becomes more dense and sinks.
This creates a convection current.
Convection
What is a fluid?
Warmer
Where it happens: Convection occurs in
all fluids.
A liquid or a gas!
Colder
Water movement
Cools at the
surface
Cooler
water sinks
Convection
current
Hot water
rises
Why is it windy at the seaside?
Cold air sinks
Where is the
freezer
compartment
put in a fridge?
It is put at the
top, because
cool air sinks,
so it cools the
food on the
way down.
Freezer
compartment
It is warmer
at the
bottom, so
this warmer
air rises and
a convection
current is
set up.
Radiation
How it happens: Radiation is the transfer of heat
by electromagnetic waves (“EM”).
Radiation requires NO atoms/molecules. So it
can happen in outer space, where there are
almost no atoms.
All objects radiate energy. The hotter an object is,
the more energy it radiates.
Examples of “EM”:
•Light
•X-rays
•microwaves
•Ultraviolet (UV rays)
Radiation
Where it happens: It happens EVERYWHERE. All
objects radiate heat.
Radiation requires NO atoms/molecules. So it is
the only transfer that can happen in outer space,
where there are almost no atoms.
Remember: Convection and Conduction DO need
atoms/molecules to transfer heat.
Radiation
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 can travel through a vacuum (empty space)
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
Convection questions
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the
cool air ‘falls through’ the warm air.
Why are the heaters in a hot water tank placed at the
bottom of the tank?
Hot water rises.
So when the boiler heats the water, and the hot water
rises, the water tank is filled with hot water.
Conduction questions
Why does a metal bar placed in a fire get hot at the end?
The atoms and electrons in the bar begin to pass
along the heat as they vibrate.
How does a frying pan cook food?
The metal pan conducts heat from the stove into the
food
Radiation questions
How can you feel a fire even though you stand far
back from it?
The fire radiates the heat out into the room.
Why are shiny foil blankets wrapped around marathon
runners at the end of a race?
The shiny metal reflects the heat
radiation from the runner back in, this
stops the runner getting cold.
1.
Which of the following is not a
method of heat transfer?
A.
Radiation
B.
Insulation
C.
Conduction
D.
Convection
2. Which would be the BEST
conductor of heat?
A.
Solid
B.
Liquid
C.
Gas
D.
Fluid
3. How does heat energy reach
the Earth from the Sun?
A.
Radiation
B.
Conduction
C.
Convection
D.
Insulation