sci.physics.electromag

On Feb 28, 5:47=A0pm, 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.un=
itednuclear.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, =A0What is the field as above if I place two magnets along the same
> axis but spaced apart by some distance d. =A0(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

For 3: The same webpage above will give you a reasonable approximation
of the field between two magnets (although admittedly on a steel
yoke).

It's a little difficult to figure the magnetic field output from
lifting force because it depends on how much magnet volume is
involved. Most times a lifting force rating is based on a single
magnet on a piece of steel and it is hard to determine what volume of
steel is involved.

Best regards,
Mike