Magnetic Units

From kirschen Thu Sep  5 21:19:15 1996
To: xxx@xxx.xxx.xxx
Subject: Re: Hello

There are four units commonly used to measure the strength of magnets or
magnetic fields.  Usage varies depending on whom you're talking to and
what situations you're referring to.

There are two unit systems in use in physics to measure things:
the CGS (centimeter-gram-second) system and the MKS (meter-kilogram-second)
system, also called SI (for Systeme Internationale, a french term).

In CGS one may measure magnetic fields in Gauss or Oersted (after the
physicists Gauss and Oersted).  The Earth's magnetic field is a few Gauss.

In MKS (which is becoming more preferred) one measures magnetic fields in
amps per meter (A/m) or Tesla.

10,000 Guass = 1 Tesla
10,000 Oersted = 1 Tesla

1 A/m = 4*pi Gauss.

1 Gauss = 1 Oersted in a vacuum or in non-magnetic substances (like air),
so people mix up Gauss and Oersteds.

How do you relate magnetic fields to other things?  It's tough -- and it's
why I really like A/m (or kA/m = 1000 A/m) or Tesla.

There is a law of physics which describes something called the Lorentz
force (after another physicist), which goes like this (in MKS):

F=q(vB + E)  the force in Newtons equals the charge of something (like an
electron) times the velocity times the magnetic field plus the charge times
the electric field.  If E=0:

F=qvB  for various reasons, the force is perpendicular to the direction the
particle is moving.  If q = 1 Coulomb (1.9 x 10^16, or 2 followed by 16 zeros
number of electrons), and v= 1 meter per second, and B = 1 Tesla, then
F = 1 Newton.  (gravity exerts a force of 9.8 Newtons on 1 kilogram of mass --
you weigh about 600 Newtons on Earth).

That's in units I can understand -- I have no understanding of Gauss, except
that they're 1/10,000 of a Tesla, and the Earth's field (moving compasses)
is a few Gauss.

Why are A/m easier than Gauss/Oersted?

If you have a wire, and you put 1 amp of current down it, at a distance of 1
meter, the magnetic field is 1/ (2*pi) A/m {The 2*pi comes in because if you
add up the entire magnetic field in a circle of radius 1 meter around the wire,
you have to multiply by 2*pi, so the sum of the field around a 1 meter circle
around this 1 amp carrying wire is 1 A/m).

Summary:
Tesla and A/m are used by scientists as a more rigorous magnetic unit.
Gauss (or Oersted) are used to estimate things because Gauss were used
more often historically, and are a more convenient size.

The Earth's magnetic field is a few Gauss.
The little bits on a computer disk or on a cassette tape are 20-50 Gauss
in magnitude _at_the_read_head_.
The fields used in hospital MRI machines are 2-7 Tesla (20,000-70,000 Gauss).