1" x 2" x .5" Neo weirdness?

[elt]

I bought a set of 1x2x.5 neos with the 3/16" hole in them with plans to use the holes to locate and hold the magnets on the rotor. Since these are the first and only neo's I own, I have no basis for what is "normal"... I don't know whether there's a question here or not; it just seems weird:

When holding a screwdriver or other long, thin object loosely so that the end can be attracted to the magnet; I find it nearly impossible to get the end to rest on the center of magnet. It doesn't take much at all to "force" the tip of the screw driver to the center but if you let it move on its own, it will jump to one side or the other. I made the following picture in an attempt to show where the "neutral" points are  where the tip of the screwdriver will stick (the areas in green)-

I can imagine that this magnet is topologically similar to a ring magnet; I did not expect the effect to be so profound.

I put a 1/2" long steel screw into the center wondering if somehow it wouldn't plug the magnetic hole there. On the side with the screw head, the screwdriver would attract to the screw head (where it sticks up over the surface of the magnet.) When moved slightly off the screw head, the tip of the screwdriver jumps to one of the regions shown in the picture. On the other side where the screw is flush with the magnet, there doesn't seem to be an difference whether the screw is in the hole or not. (The screwdriver tip won't go to the center, it goes to one of the regions shown in the drawing.)

As a ham radio operator (aa2mz) one thing I'd say when I saw a whacky radio antenna that appeared to work well was "well, radio waves have to go somewhere..." and I suppose that's true of flux, it has to go somewhere too. I wonder if a large amount of flux is going through the center hole and these magnets will be weaker for use in alternators or if the majority of the flux is just moved away from center hole and they will work about as good...

- Ed.

[Flux]

This is probably normal.

Attraction and pull are of little value in determining magnet strength.

Obviously there is no active flux in the hole and it will cause strange leakage flux paths to any steel object in the region. In many ways it will behave as a ring magnet.

The total flux lost will be marginally more than the area missing due to the hole.

You are unlikely to see any difference between those and magnets with no hole.

Interesting observation though,a bit like the method I suggested to find the pole areas on a square section magnet a few days ago.

Flux

[Ungrounded Lightning Rod]

Right.

What you're measuring with the screwdriver and indicating with the green spots is the local energy minimums, corresponding to a set of places where the sideways attraction from the magnetic material in all directions is balanced.

If you did this over a round magnet without a hole you'd get a dot in the center, and over a rectangular magnet without a hole you'd get an oval, like one of the two you get to the sides of your magnet.

You've found a new form of "cogging".  B-)

But it's not a big deal.

(Try dragging a wire - or the edge of a large coil - across the magnet surface at a constant speed and observe the waveform produced.  You should see a near-rectangle - rounded off near the sides by field spreading, and with a dip in the middle from the missing magnetic material in the hole.  The dip's deviation from the rounded rectangle will be the equivalent of removing the magnet and substituting an upside-down chunk of the same material that's the shape of the hole.)

[finnsawyer]

Not weird at all.  This is to be expected.  The iron in the screwdriver is attracted by the magnet's poles.  There is no pole in the center hole, so the attraction will be to the side of the hole where the pole actually is.  When the screwdriver or a rod is centered in the hole the attraction from all points around it is balanced out, but this an unstable situation like a ball sitting dead center on top of a nice rounded hill.  Any small disturbance will cause it to fly to one side or another.  There are also energy aspects involved here.  When the ball comes to rest at the bottom of the hill the total system (ball plus earth) is at minimum energy.  The mechanical energy the ball got in rolling down the hill is lost as heat.  You would need to replace that energy to put the ball back on top of the hill, which means increasing the total system energy.  In a similar fashion the magnet - screwdriver system is at lowest energy when the screwdriver is in contact with the pole of the magnet.  You need to exert a force and do work or provide energy to put it back at the center.  Without realizing it you were actually verifying various physical aspects of the magnetic system by moving the screwdriver around.  Your experiment was crude but still valid.