sci.physics.electromag

On Feb 28, 3:13 pm, Mike D wrote:
> On Feb 28, 5:47 pm, 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)
>
> > 2. How does the magnetic field fall off with distance on the
> > centerline of the rod as you move away from the end surface.
>
> > 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. )
>
> > 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 ?
>
> > 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?
>
> > Thanks.
>
> The quickest way to get an answer to 1 is to use one of the online
> calculators. For example:http://www.dextermag.com/CalculationList.aspx
> Neo45 will have a Br of approximately 1.37 Tesla (13,700 Gauss)
>
> For 2: The field generally drops as a function of the square of the
> distance

Make that the cube of the distance (at distances large compared to the
size of the magnet); magnets are dipoles.

--
Jim E. Black