sci.physics.electromag

Benj wrote:
>
> You guys know a lot. How about a quick and dirty answer for this one.
> I'm looking at some super strong magnets from United Nuclear.
> http://www.unitednuclear.com/magnets.htm
>
> The magnets are listed by size and lifting power. Say I'm going to buy
> a rod magnet(s) in some length. The questions I want to answer is
> 1. What is the magnetic field right at the surface of the magnet (with
> the magnet in free space)

Depends on bulk magentization and radius of curvature of the surface,
hence pole pieces. A field has amplitude, gradient, divergence, and
curl. Which of those are interesting to you?

> 2. How does the magnetic field fall off with distance on the
> centerline of the rod as you move away from the end surface.

A near-field dipole drops off as r^3. Don't bring two strong magnets'
opposite poles into proximity. Very, very, very bad idea.

> and
>
> 3, What is the field as above if I place two magnets along the same
> axis but spaced apart by some distance d. (only interested in the
> field in the gap on the centerline, not in the whole field
> distribution around the magnets. )

Depends. If you want a clean volume of uniform intense field, go for
a Helmholtz pair, cheaply with ferrites. Get a dead microwave oven
and recover the gyrotron.

If you want maximum sturm und drang, a linear or shaped Halbach
array. A Tesla in a one-inch bore will set you back $300,



Ignore the social bullshit

> Of course eventually I'll just buy some of these since they are pretty
> cheap and use a Hall probe to get the "real" answer, but it just
> seemed to me I could get a handle on this with just the magnet
> dimensions and the lifting force to help me decide what to order.
> Doesn't the lifting force equal something like BH/2 ?

Lifting force is field divergence. If you want big lift at a given
field you need tiny radii of curvature - a ribbed surface.

> > It almost seems to me that since the lifting force is per unit area
> > that the field at the centerline and surface is independent of the
> > diameter of the rods? Is that right?

Why a rod? A donut makes more sense. You can buy switchable ceramic
lifting magnet arrays from surplus places, or pay double at Edmund.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2