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# Magnet confusion

## Magnet confusion

(OP)
Hi everybody,

I'll not lie: I joined this forum to see if I could find somebody so kind to help me with a simple question.

In the first picture I have represented the lines of force of a couple of magnets.
One is small, and represented in its entirety, the other one is much larger and only its North pole is represented.

So, question n. 1, are the line of force roughly right or not?

Based on the lines of force from the first picture, I have made another picture which shows what the vector power of the magnet should be.

The big N of the large magnet is pushing away the North of the small magnet while also pushing it in the South direction (right), while the South of the small magnet is being attracted to it vertically as well as being pulled to the right side.

The overall vector should result first of all in the rotation of the magnet along its axis, AND moving it to the South direction.

My question is: am I right or not?

Marco

### RE: Magnet confusion

(OP)
Damn I forgot to put a title to the thread, sorry :(

### RE: Magnet confusion

You also forgot to attach picture!

### RE: Magnet confusion

due to the effect of big magnet, the neutral section is not in the middle of the small magnet any more. the central line is now not perpendicular to the surface of the big magnet either. using Fe or Fe oxide soft magnetic powders one can see how the flux lines look like.

### RE: Magnet confusion

(OP)
Uhm... you mean that the small magnet becomes asymmetric in this particular case?

### RE: Magnet confusion

yes magnetically asymmetric.

### RE: Magnet confusion

(OP)
Uh ok, and what about the pushing/pulling thing?
Is my hypothesys correct?

### RE: Magnet confusion

(OP)
Oh and by the way... the large magnet is not so large.
I just simplified it because of posting the stuff here, but if this becomes a main parameter I will explain better.

It's a long magnet, whereas the North face is pointing to the small magnet.
In reality, it's as thick as the small magnet (3 mm), only longer, and the North pole is facing the small magnet.
So, not so big, and the South is right under the North

I can add that the power for square millimeter is practically the same for both magnets, N35 or N42, something like that.

### RE: Magnet confusion

the pulling direction is correct, if the small magnet can move, its south pole will move towards to north pole of the big magnet, the neutral section line will move from the center to south, vice verse, the neutral section of big magent will move towards upper direction.

b.t.w., N35 N42 are most likely the grades of NdFeB magnets, not square mm.

### RE: Magnet confusion

Went ahead and ran a 2D FEA sim with your updated specs. I made the larger magnet 20mm long; both magnets are N35 strength. Note the forces in the command bar--the first set belongs to the left half of the top magnet, the second belongs to the right half. I wouldn't pay too much attention to the absolute numbers, just their relative values and the angles.

### RE: Magnet confusion

(OP)
Cool, thank you everybody for your help

RyreInc can you tell me what's that software you used? Is it free or not?

I notice that the bottom magnet is not put in horizontal position, why? Is it possible to make it larger?

That's the essence of my question, as to also know if the small magnet would "travel" from North to South.

Thank you again, really grateful!

### RE: Magnet confusion

That software is not free, but there is a similar and free program called FEMM. The magnet is not in horizontal position because of unclear description and some assumptions I had to make. Look at the forces to see how the magnet will travel.

### RE: Magnet confusion

(OP)
Well I'm not a pro but I think from what I see that the bottom right corner of the small magnet will rotate to join the top right corner of the bottom magnet.
So I think the small magnet in this case will not travel anywhere, it will only rotate on its axis if I understand well.

The problem is, this is not the right configuration.
The bottom magnet must be large, much larger than the small one.

Thank you for the FEMM thing, I'm going to install it right now and I'll let you know

### RE: Magnet confusion

You use phrases and words than don't make any sense. If the small magnet were unconstrained, i.e., the entire magnet were placed in the designated spot by hand and released, it would move so that the S of the small magnet makes contact with the N ofthe large magnet. The typical electric toothbrush magnet will "travel" up to 10 times its length to join with a stationary magnet.

### RE: Magnet confusion

(OP)
OMG why is it so difficult to use it???
Move, rotate and scale are simply a hell!

Anyway, I managed to create some points and connect them with lines.
Now, how to tell which is North and which South?

I tried to use the properties menu but no way to assign anything to the points I selected.
I open the library and... can only watch the materials but not assign them?

### RE: Magnet confusion

(OP)
IRstuff, maybe I don't use the right words, because I have no university (and english is not my mother tongue eh eh), but I think it's clear what I say.

Assuming the bottom magnet is fixed, and the upper magnet is free, the upper magnet will move.
But it will not move in one direction or another, it will only rotate on its axis.
As you said, the final position will be contact South on North, not much of a science here.
But if you watch closely, the first point of contact are the two corners I mentioned.

This said, I understand that the problem I propose is not detailed enough to obtain the answer I need.
Let's see if I can make it a little bit more clear.

Imagine that the small magnet is free in movement on a horizonatl axis, and NOT free of rotating.
I suppose the magnet wouldn't move anywhere.
But still, I think that the simulation would be much different from this one if the bottom large magnet would be much larger than the upper one, like in my drawings.

### RE: Magnet confusion

I think we get into trouble thinking of magnets having a north and south pole. Really it's just an arrow pointing from south to north. There are no "north" regions or "south" regions, just a single region magnetized in a particular direction.

The way you have it drawn in the first picture, one could interpret it a few ways: The upper magnet could have a single arrow pointing right to left, indicating simple thru magnetization; or it could have an arrow pointing up on the left and an arrow pointing down on the right, indicating a two-pole (per face) configuration. These are very different! But using arrows instead of poles to begin with gets rid of this ambiguity. Plus, dividing the magnet into N and S regions implies that you could separate the two and be left with a pair of monopoles!

All of this is a roundabout way of saying that you can't specify North or South poles in FEMM, or any FEA software, only direction of magnetization. I've never actually used FEMM, so I don't have any specific help on manipulating the software.

(OP)
Thank you.

### RE: Magnet confusion

But it will not move in one direction or another, it will only rotate on its axis.

Unless it's actually pivoted, no. There will be rotation, yes, but there will also be translation. It's most like that the free magnet will wind up overhanging the right edge of the fixed magnet. Every termination of a line of force will be subject to attraction, so, even the portion to the right of center will be attracted and will move toward the fixed magnet.

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