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Thermal Insulators and Conductors: Molecular
Name: Rosemary
Status: educator
Grade: 4-5
Location: MA
Country: USA
Date: March 2009
Question:
At a teacher workshop, we observed how the ice cube on the
aluminum block melted faster than the ice cube on the wood block.
While we understand that aluminum is a better conductor of heat, we
could not explain what was happening on the molecular level. What is
the difference between the molecules of wood and aluminum (or a
conducting or insulating material) that allows this? Movement? Space
between molecules?
Replies:
Rosemary,
Conduction is a particular way that heat may be transferred, requiring collisions
of atoms and molecules and resulting in the transfer of kinetic energy. A poor
conductor of heat would therefore either have (a) a low density - there are fewer
particles in a given volume to actually collide and transfer energy, and (b) low
mobility in the space between particles - if the material does not move around
well, it does not transfer heat. Comparing forms of the same substance, a shape
that offers a lot of contact surface between the heat source and the heat measuring
point means that more heat is transferred per unit time.
Keeping this in mind then, we can imagine that it may be okay to put our hands in
an oven (long enough to get out a batch of cookies, but it is not okay to touch the
cookie sheet with our bare hands. Both of these objects are at the same temperature,
but air is less dense than metal and therefore does not transfer heat well.
Likewise, we can touch a solid ice cube that is at 0 deg-C and not experience it to
be cold, but putting our hands in water that is at 0 deg-C will feel much colder
because the liquid water is more mobile.
Finally, we understand that a rough surface offers fewer contact points than a smooth
surface. Thus a rough surface would be less able to transfer heat through conduction
than a smoother surface.
Using this reasoning then, I am sure you can imagine why the atoms of aluminum and
the molecules of wood -at the same temperature- differ in their capacity to transfer
heat.
Greg (Roberto Gregorius)
Rosemary,
GREAT question!
Yes, the differences start at the molecular level and build up from
there. There are differences between the molecules, but there are also
differences between how the molecules are arranged. At the molecular
scale, the molecules may be configured in many ways (for example a
crystal such as metal or an amorphous solid such as some parts of the
wood). Or, you may have many different kinds of molecules and
arrangements (as is the case for wood). Looking a little larger in
scale, there may be macroscopic differences in the materials as well
(think of wood fibers or pits/cracks in the metal). In the case of
wood, it is composed of starches and proteins and lots of air voids.
These materials cannot store a lot of heat and they do not give up their
heat very easily. Metal, on the other hand, can transfer heat quite
readily. This is because of the electrical properties of metal atoms
-- they actually share electrons with neighboring atoms, which allows
them to conduct electrons and heat more readily. The surfaces also
matter -- if you have a rough surface with less contact, it will
transfer less heat than two surfaces with more intimate contact.
If you want more detail (or general rules) on different types of
materials and how their molecular structures lead to macroscopic
properties, then we get into a much more involved question. Let me
know if you are interested in this level of detail.
Hope this helps,
Burr Zimmerman
Hi Rosemary -
I have two answers which both contribute to aluminum being a better thermal
conductor, and the easier answer makes the bigger part of the difference.
Easier:
Aluminum is a metal, with lots of very free electrons, which make very good
electrical conductivity.
Wood of course is not an electrical conductor and has practically no free electrons.
It turns out these same free electrons also carry heat around faster than vibrational
bumping between neighboring molecules.
An electron amidst heated vibrating atoms tends to have a higher random velocity or
energy state of its own,
and it can diffuse quickly "across town" and give its own heat energy to the atoms in
a new place.
This situation with two distinct species (fixed atoms and flighty electrons) sharing
the same temperature, one more mobile than the other,
reminds me of steam permeating a pile of rocks.
A dry pile of rocks would be a poor conductor of heat, but with some steam and water
present, the heat would get around much faster.
Harder:
Even if the free electrons were all stuck to their homes, aluminum would probably
still be a few times more conductive than wood.
It's made of big crystals which are uniform (smooth) inside, so a wave-like vibration
of atoms can travel farther in any direction (like a sound wave)
before bouncing off an obstacle such as a heavy spot or a void (a bubble, a tiny hole
in the material).
We call these vibrations that make up heat "phonons".
The more broken-up the molecular structure is, the longer a bit of thermal motion
takes to spread out into its surroundings.
Wood has lots of soft cell-interiors, maybe partly dried out and air-filled, separated
by dense hard cell-walls.
And lots of empty capillary passages parallel to the fibers.
Even the parts that are filled up are often not repetitive like atoms in a crystal,
instead they're messy, randomly oriented, like atoms in liquid or glass or plastic.
So the photons will be bounced around and turned back very frequently by the structures
in wood.
They still migrate into cold places, just a lot slower.
Jim Swenson
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