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Re: magnetic fields
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, February 11, 2000 2:26 AM
Subject: magnetic fields
>Original Poster: CTCDW-at-aol-dot-com
>
>Hello, all,
>
> Just a curious question, or two...If we could actually SEE magnetic
fields,
>what would the field around the average, say 900va coil look like? how far
>out would it extend, etc...what shape would it take?
Interesting question.. A flat pancake coil, at a frequency where the
wavelength is much larger than the dimensions of the coil (which is the case
for a TC), has a "dipole" type of field (just like a very short bar magnet).
It extends forever (just like all fields), although the field strength would
fall off fairly rapidly. You'll see this discussed as the fields of a "small
loop" (where small means relative to a wavelength). In the far field (a
LONG way away), the field strength decreases as 1/r (for a single turn loop,
it actually H = pi*I*sin(theta)/r * A/(lambda^2), where theta is the angle
from the axis of symmetry, lambda is the wavelength, and A is the area of
the loop)
>
>Also, when a core saturates, what exactly happens...we know what its
effects
>are, heat, etc.., but what is actually happening to the iron and magnetic
>field?
Here is a grossly oversimplified explanation: ferromagnetic materials have a
bunch of microscopic "domains" each of which acts like a tiny spring loaded
magnet that can be aligned one way or another. When the material is
demagnetized, the domains point randomly, so it averages out to no field.
In the presence of an external field, the little magnets all start to line
up, and as a result the piece becomes magnetized. Take the external field
away, and the little magnets return to their original directions. However,
for "hard" materials, if you put enough field on it (exceeding the
"coercivity" as in coerce: to make go a particular direction) , the
"springs" permanently stretch (or break), and the domains remain pointed
even after the external field is removed, producing a permanent magnet.
As you increase the field, more and more of the domains point the way you
want them to go, until you get to the point where ALL the domains point the
same way, after which increasing the external field doesn't increase the
magnetization... a process akin to an amplifier limiting or "saturating".
Note that transformers are made from a material which has a "high saturation
point" (so you can put a BIG field on it without saturating), but which are
"soft" (i.e. they don't stay magnetized).
>
>thanks for any insights!
>
>Chris Walton
>
>
>