here to go to our companion permittivity page
here to go to a page with a table of permeabilities of various
here to go to our main skin depth page
here to go to our physical constants page
This discussion about permeability
recently came in from Walt.... thanks!
"Permeability is a
magnetic property of space; space either filled with stuff or
empty. You've undoubtedly heard of ferromagnetism... the ability
of materials to be attracted to a magnet. Permeability is the
unit of measure for ferromagnetism. It is also extremely important
in the history of electricity and magnetism. Permeability along
with the speed of light and a property called permittivity are
the three measurable constants of Maxwell's
Materials with unpaired
electrons such as iron have microscopic domains that are themselves
little magnets. An externally imposed magnetic field, such as
produced by an electric current through a coil which is wound
around (or next to) the iron can cause the magnetic domains in
the iron to line up with each other and the material is said to
be magnetized. The alignment is such as to make the entire piece
of ferromagnetic material a magnet of the opposite polarity as
the driving field. That's why ferromagnetic materials are drawn
toward either end of a magnet. It is magnetized at least while
the magnetic field is imposed. (If it stays magnetized after striking
it, it is a permanent magnet.) The tiny domains of magnetic field
are extremely powerful; much more powerful than even the strongest
magnet. But in their natural state the domains randomly cancel
each other out. A driving magnetic field, although small can trick
some of these domains into alignment thus increase the local field
by a large factor. (One might think there's a violation of thermodynamics
here, but no, the amplification of the magnetic field happens
without any opportunity for perpetual motion. Sorry.) The degree
to which it increases the magnetic field is the permeability for
the material. Permeability is the measure of the amount of ferromagnetism.
(Ferromagnetism is a property not a measure.) Some materials have
domains that actually fight the external magnetic field. These
are called diamagnetic materials. A diamagnetic material is repelled
rather than drawn toward a magnet. Alas, diamagnetic materials
are no where near as dramatic as ferromagnetic. It takes a huge
superconducting magnet to feel the effects of diamagnetism for
even the strongest diamagnetic materials. You can see in our table
of magnetic materials how unsymmetrical the paramagnetic values
are for materials of this universe. Notice that there is no zero
value of permeability. Empty space does not fight or aid an applied
magnetic field. The important thing to remember about empty space
is that it conducts a magnetic field as if it was made of stuff."
Back to our original text...
Permeability is a property which is associated with how much a material
responds to a magnetic field. The definition is
where B is the flux density,
and H is the magnetic force.
The permeability of a vacuum
is denoted as 0.
The units of permeability are Henries/meter. The permeability of
vacuum is exactly 4x10-7,
which is "approximately" 1.25663706E-06. Most materials have
permeability very close to that of a vacuum; the common exceptions
are materials that some fraction of have iron, chrome, and nickel.
Permeability controls skin depth.
The higher the relative permeability, the less an electromagnetic
wave will penetrate into the material. Seems kind of backwards,
but that is how permeability works!