[labview1958 (OP)]

I he 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. Why is that? can anyone help?

[Soul Surfer]

Sorry but the description of what you were doing is not detailed enough to comment. what was the magnetisation directions of your disk magnets and what were you measuring.

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[labview1958 (OP)]

The North Poles are facing each other and are perfectly aligned.

[labview1958 (OP)]

Ooops! I was measuring the repulsive force between identical magnets.

[Soul Surfer]

Presumably the magnets were magnetised along the axis of the disc ie short and fat.

How were you measuring this force and by how much did it vary? and when you say perfectly aligned I presume that you mean their edges mwere aligned all round.

There is no reason why the magnetisation of the discs will be totally uniform or along the disc axis as they were probably mgnetised in a pulse device and may not have been place very accurately in that.  Any small error in this will produce a small sinusoidal variation in the repusion that you observed

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[labview1958 (OP)]

The magnets are about 15 mm in diameter and 3 mm thick. I am using a weighing balance accurate to 0.0001g. I say the variations are about 5% from the maximum force. I had hoped to get a slightly wavy straight line. I thought there would many sine waves per turn. However there is only one sine wave per turn. Why?

[Soul Surfer]

Magnetised materials usually mutually stabilise their domains to produce a single peak pole at either end.  This would produce your single sine wave.  A variation of 5% sounds quite reasonable. you would not see any evidence of structure in the magnetism until you got to microscopic levels.

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[labview1958 (OP)]

Are you saying that the bottom of the upper disk magnet which is a north pole actually consists of two north poles 180 degrees apart? Similarly for the top of the lower disk magnet. That would produce two sine curves. I got only one.

[Soul Surfer]

No its just that the poles of the magnets are not perfectly aligned with the physical centre of the dic and/or the axis of the cylinder so the repulsive force varies a bit with angle as you rotate one magnet on top of the other

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[labview1958 (OP)]

Thanks. Let's say I move the upper magnet in the X-Y direction until somehow I get the poles aligned. Then there would not be a sine curve? Would there be just a random curve showing that the magnetic field on the disk is irregulat. I mean a curve with numerous tiny peaks and valleys?

[Soul Surfer]

Probably not. If you null out the first order effect it will probably be very smooth unles the magnets are made out of very non uniform material.

The fine structure in the magnetic field of a magnetised material is usually very fine and dependant on crystal or "magnetic domain" boundries.  You can resolve these details using a very fine magnetic powder on the surface of the magnet and a microscope  This is similar to the classic iron filings experiment.

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[labview1958 (OP)]

I suppose  there is only one peak on the surface of the disc. Only the peak is not in the exact centre of the disk. Is this reasoning correct. Where can I get published data to support this argument.

[daveshorts]

As it would be dependent on each individual magnet I think the only place you could get it from would be the manufacturer if you were lucky. otherwise you would have to measure it yourself somehow. possibly by scanning a small magnet across the big ones and mearing the repulsive/attractive force.

[labview1958 (OP)]

Is it possible to have the peak at the exact centre of the disc? Or are there disc magnets with the peak at the exact centre? Sometime ago I came across a diagram that showed two peaks on the pole surface with a small valley in the exact centre. Is that logical?

[Soul Surfer]

Why do you want to have a magnet that is specified in such great detail?

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[labview1958 (OP)]

I am interested to know how and where the magnetic field lines end or start on a pole surface. My hunch is that the magnetic field ends and start from the exact centre of a magnet and not on the pole surface.