rotating a disk magnet above an identical disk mag

[labview1958]

I have slowly rotated a disk magnet above an identical disk magnet. The result: a sine wave. A graph of one peak and one vally per 360 degree rotation. The north poles are facing each other and are perfectly alligned. I am measuring repulsive force.Why is that? can anyone help?

[DV8 2XL]

Distribution of flux on both of the magnets isn't the same for the whole pole surface. when the high flux or low flux regions meet, the force changes.

[labview1958]

It is an extremly smooth sine curve with one peak and one valley for a 360 degree turn.

[m4rc]

Although i suspect that DV8 suggested has the right answer, another possibility is that the magnet is not rotating around its exact centre.

[Meir Achuz]

The magnetizaton in each magnet is probably in the plane of the magnet.

Then as you rotate the force between them would be a sine curve.

[labview1958]

The magnets are oriented in the vertcal direction. Thus north poles face each other. I also rotated a small disk magnet above another much larger (about twice the diameter) disk magnet. I got the same result.

[Meir Achuz]

"The magnets are oriented in the vertical direction. Thus north poles face each other. "

 

How do you know that?

[labview1958]

Well the bottom of the magnet is a north pole and the top is a south pole. Well the magnet must be oriented vertically.

[Meir Achuz]

Well the bottom of the magnet is a north pole and the top is a south pole. Well the magnet must be oriented vertically.

You have said that twice, BUT:

Your experiment demonstrates that the magnetization is directed along a diameter of the disk. Do you have any experiment to indicate that the magnetization is along the axis of the disk?

[labview1958]

Are you implying that half the bottom of the disk is a north pole and the other half a south pole? I have never come across such a magnet. It is a normal disk magnetwhich is available for sale on the net.

[Meir Achuz]

Are you implying that half the bottom of the disk is a north pole and the other half a south pole? I have never come across such a magnet. It is a normal disk magnetwhich is available for sale on the net.

I'm sorry. I thought you understood what the magnetization vector field in a permanent magnet was. I am afraid I will have to get a bit technical.

The magnetization vector, M, is what determines the magnetic properties of a ferromagnet. The concept of "magnetic charge" is a mathematical abstraction that behaves like a charge, but does not really exist as a physical entity. The "magnetic charge density" at the surface of a magnet equals the normal component of the magnetization at the surface. If the magnetization vector were perpendicular to the plane of the magnetized disk, then there would be positive (called "north") magnetic charge spread over one face and negative magnetic charge (called "south") spread over the other face. It would look like the top face were a north magnetic pole.

(Even then, the "pole" would not be at one point, but would be spread out over th etop face.)

In the case of your magnetized disks, the manetization vector is in the plane of the disk. Then positive (north) magnetic charge is spread out over one half of the circumference of the disk, and negative (south) magnetic charge is spread out over the other half. There is no single "pole", but a spread out distribution of magnetic charge. The density of the magnetic charge varies like sin\theta, which is why you see a sin\theta depndence of the force in your experiment of rotating one disk with respect to the other.

[labview1958]

Let me clarify. I am actually getting a sine wave which shows that the force is always repelling. I do not have attractive force, just more repelling or less repelling force. Put it another way: a "DC" sine wave. My guess is that the discs were magnetized perpendicularly. Only I could not get them aligned as the point with the strongest field is not the centre of the disc. for example the earth's geographical north pole is not where the magnetic north pole is.

[DV8 2XL]

That's the explanation I'm betting on too. You could do the iron fillings test with the magnet under a sheet of paper to see if this is what's going on.

[labview1958]

Is the magnetic field on the pole surface uniform? Or it has ONE peak pole? Or it has more then one peak per surface? Where can I can get info or formulas regarding the value of the magnetic field exactly on it's pole surface?

[DV8 2XL]

You would have to use a point magnamometer,or like I said above the - iron fillings test

[labview1958]

I suppose someone would already have done that. Where can I get hold of the data concerned.