Download Conduction – Heat Transfer via Particle Collision

Conduction – Heat Transfer via Particle Collision by Paul Hewitt
Conduction is Heat Transfer through Touch
Heat conduction is the transfer of heat by the direct contact of particles
of matter. If you have ever held a warm mug of hot chocolate, you have
experienced conduction. Conduction only occurs
between two materials at different temperatures
and when they are touching each other. In
conduction, heat can also be transferred through
materials. If you stir the hot chocolate with a metal
spoon, heat is transferred from the hot chocolate,
through the spoon and to your hand.
Imagine placing a cold spoon into the hot
chocolate. Because the molecules in the cocoa have a higher average
kinetic energy than those of the spoon, the energy is transferred up the
spoon into the material with lower kinetic energy.
Another example of conduction might be found
when you hold an iron nail in a flame. When you
hold one end of an iron nail in a flame it quickly
becomes too hot to hold. Thermal energy at the
hot end travels along the nail’s entire length.
conduction
Atoms and Molecules
Heat conduction occurs when the atoms and
molecules in a material move easily. Every atom
has electrons that are orbiting the nucleus. Metal
atoms have loosely held electrons that are free to
easily move in the metal. Heat conduction occurs
when electrons collide inside the object being
heated.
Good Conductors are Metal
Solids whose atoms or molecules have loosely
held electrons are good conductors of heat.
Metals have the loosest electrons and are
excellent conductors of heat. Silver is the best,
copper next, and among common metals,
aluminum and then iron. If you want to transfer heat from the stove
burner to the pan, you will choose a metal pan because it is a good
conductor or heat.
Insulators are Poor Conductors
Wood, wool paper, cork, and plastic foam are
poor conductors of heat. Molecules in these
materials have electrons that are firmly
attached to the molecules. Poor conductors are
called insulators. If you want to put a hot liquid
into a cup to hold, you will choose a styrofoam
cup because it is not a good conductor of heat.
Wood is a good insulator and is used for cookware handles. Even when
a pot is hot, you can quickly grasp the wooden handle with your bare
hand without harm. An iron handle of the same
temperature would surely burn your hand. Wood is
a good insulator even when it is red hot. Since
wood is poor conductor, this explains how
firewalkers can walk barefoot across burning coals without burning
their feet.
Air is a Great Insulator!!
Air is a very poor conductor which makes it a
great insulator. The coat at the left is made of
down. The air spaces between the down
feathers help keep you warm in the winter. The
good insulating properties of such things as
wool, fur, and feathers are largely due to the air
spaces they contain. Porous substances are
also good insulators because
of their small air spaces.
The thermos to the right can keep coffee hot or iced
tea cold. The thermos uses a space of air between the
inside and outside wall to prevent heat transfer by
conduction. The air insulates the thermos to keep
your liquid from changing temperature.
Compare Conductors & Insulators
In the example to the right, the tile feels colder
than the wooden floor under the bare feet, even
though both floors are at the same
temperature. This is because tile is a better
conductor of heat than wood is, and so heat
is more readily conducted out of the foot
touching the tile.
Snow is also a poor conductor of heat. Snowflakes are
formed of crystals that trap air and provide insulation.
That’s why a blanket of snow keeps the ground warm
in winter. Animals in the forest find shelter from the
cold in snow banks and in holes in the snow. The
snow doesn’t provide them with thermal energy – it
simply slows down the loss of body heat generated by
the animals. Then there are the igloos in Artic dwellings
that are shielded from the cold by their snow covering.
Snow patterns on the roof of a
house show areas of conduction and insulation.
Bare parts show where heat from inside has
conducted through the roof and melted the
snow.
The walls and attics of homes built with wood are insulated with rock
wool, fiberglass, or cellulose. Interestingly, insulation doesn’t prevent
the flow of heat. Insulation simple slows down the rate at which heat
flows. Even a well-insulated warm home
gradually cools.
Insulation merely
delays the rate at which heat conducts
from a warmer region to a cooler one. In
winter, we wish to slow conduction from
inside to outside. But on hot summer
days, we wish to slow down conduction
in the other direction, from outside to
inside. Insulation slows conduction in
either direction.
Name ________________________________ Date ______
Name ________________________________ Date ______
Conduction Questions
Conduction Questions
1. What is a conductor? _________________________________
1. What is a conductor? _________________________________
___________________________________________________
___________________________________________________
2. What are two good conducting materials?
2. What are two good conducting materials?
_________________________________
_________________________________
_________________________________
_________________________________
3. What is an insulator? _________________________________
3. What is an insulator? _________________________________
___________________________________________________
___________________________________________________
4. What are two good insulating materials?
4. What are two good insulating materials?
_________________________________
_________________________________
_________________________________
_________________________________
5. A book and metal spoon are both at room temperature of 75* F
5. A book and metal spoon are both at room temperature of 75* F
BUT if you touch the book, it feels warmer and if you touch the
BUT if you touch the book, it feels warmer and if you touch the
spoon, it feels colder. How can you explain this?
spoon, it feels colder. How can you explain this?
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
6. If you hold one end of a metal nail against a piece of ice, the
end in your hand soon becomes cold. Does cold flow from the
ice and into you hand …or …does heat flow from your hand
and into the ice. ____________________________________
6. If you hold one end of a metal nail against a piece of ice, the
end in your hand soon becomes cold. Does cold flow from the
ice and into you hand …or …does heat flow from your hand
and into the ice. ____________________________________
Explain your thinking ________________________________
Explain your thinking ________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________