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Insulators and Density
Name: Tom
Status: educator
Grade: N/A
Location: CT
Country: N/A
Date: 6/21/2005
Question:
The big debate in our department is regarding insulators
and density. Is there a definite relationship? On one hand a higher
density material can have particles that have greater inertias and are
harder to move, meaning the can have some insulative properties. However,
higher density materials usually have closer packed particles which make
them more conducive to being conductors. What is the answer?
Replies:
It is not clear from the way you framed the question whether you referring
to the relationship between density and electrical conductivity or thermal
conductivity. If you mean electrical conductivity the relation is
convoluted and indirect. There are several different mechanisms for
electrical conductivity depending upon whether the material is a metal, a
semi-conductor, an ionic solution electrolyte, a gas (plasma), a
superconductor, a nominal insulator, or even a vacuum. Each type of
material has a different mechanism.
The electrical conductivity depends upon the mobility of electrons in the
material. This mobility is only indirectly related to the density of the
material, in the most simple approximation, the electron mobility is
independent of the density of the material. You can see that the density
varies from essentially zero (for a vacuum of low pressure plasma) up to
about 20 gm/cm^3 for a heavy metal. The web sites below goes into more detail
on the subject.
http://en.wikipedia.org/wiki/Electrical_conduction
http://en.wikipedia.org/wiki/Effective_mass
Vince Calder
Tom,
Structure of the material is more important than density. The only truly
perfect insulator is a vacuum: a vacuum has no molecules to transmit heat.
Liquids tend to be very poor insulators due to convection. Although they
might or might not transmit the heat from molecule to molecule very well,
the hot molecules can travel through the liquid quite easily. For a gas,
higher density tends to transmit heat better. Higher density results in
more heat carriers (molecules) to travel through the gas.
For solids, metals are the best conductors. The loose electrons can carry
the heat through the material quite easily. For other materials, what
matters most is how well adjacent molecules respond to each other. Many
crystals are poor conductors. The molecules have specific frequencies at
which they vibrate. The random patterns of heat do not inspire a resonance.
However, light of the proper frequency will enter and transmit through the
material quite easily.
As for plastics, I am not familiar enough with the structure and behavior to
comment reliably.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Tom-
Unfortunately the ancient Greek philosophers could not settle your
debate
without somebody first figuring out electrons and quantum-mechanics.
Or at least electrons-and-atoms. Two populations, together making up one
solid.
Only seen apart on special occasions which the Greeks never saw much,
namely: vacuum tubes. Batteries, generators, and galvanometers helped too.
Density is a composite parameter, the final result of a few
more-basic factors:
- the atomic weight of the elements,
- the crystal form in which the elements like to pack themselves
together, and
- the void volume: percentage of exceptions from that favored crystal
structure.
Density is history-dependent: diamond, graphite, glassy carbon, and
soot are all pure carbon 12.
Diamond, glassy carbon, and soot are insulating,
and the electrically conducting one, graphite,
has density in between diamond and glassy carbon.
So there must be some other picture dominating electrical conductivity.
That picture is made of about two things:
- wave-interference of free electrons trying to coast over the
wash-board ripples
of the atom lattice in the substance
(it can force electrons to require higher-than-thermal energies)
- the "sea-level" of electrons in that lattice, compared with said ripples.
These in turn depend on element valence, and structure details of the
particular crystal form.
So electrical conductivity has no simple relationship with density or
atomic number.
Void volume:
Any electrically conductive substance becomes gradually less conductive if
it has many voids
or other flaws which the electrons must swim around to proceed.
"Ideal" or "theoretical" density is slightly better for conductivity.
But it is also better for insulating strength, too.
Voids mean gas or vacuum space, and internal surfaces,
all of which help plasma discharges (sparks) to skip on through.
When I first read your question, I asked, "what kind of 'conductivity'?"
Electrical and thermal are possibilities.
Electrical conductivity varies over more than 15 orders of magnitude.
Things that pass 1 amp/cm2 given only 10^-5 volt/cm are metals, definitely
conductors.
Things that admit less than 10^-12 amp/cm2 with 1000 volt/cm, are
definitely insulators.
This degree of change must have a picture that includes "trapping" of free
carriers.
The particles all have dimples to be stuck in, or they do not, and they
drift around in any breeze.
Some insulators such as diamond, if they have very low density of traps,
begin to be slightly conductive
even though the free electrons are rare compared with the atoms.
Thermal conductivity has no such sharp distinction, no real trapping effects,
and it is more compatible with the kind of thinking you are doing.
The best and worst thermal conductors vary by only a factor of 1000 or so.
Everything is a thermal conductor, the question is one of degree.
"Particle inertia" can matter here.
Heavy elements conduct less heat per gram,
but similar amounts of heat per atom, or per unit volume.
Voids can locally reflect, globally diffuse, the random mechanical waves
that make up heat in a solid.
Jim Swenson
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