# Magnetic field lines and iron filings

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Something is really bothering me about magnetic field. So I know we use magnetic field lines to represent the intensity of the magnetic field. For example, the closer they are the stronger the field.

However when we look at iron filing aligning in a magnetic field, we see them grouping into distinct lines, almost imitating field lines, but this must mean that the field is weaker between the lines and stronger on the field lines, which makes no sense to me. Can someone explain why the filings form distinct lines and why there are gaps with no filings in them?

Edited by CasualKilla

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I would guess that each filing becomes a little magnet and thus one end of it is attracted to the end of another filing - so the filing tend to become lines. These lines will follow the field lines.

But an interesting question and, like you, I wait for some of the more knowledgeable answers.

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Good observation +1

As imatfaal says the iron particles line up so they are touching as continuous chains of tine induced magnets.

Why?

Well this is a good example of a system seeking to minimise its energy.

Without the filings the magnetic lines are 'evenly' distributed in space with no gaps.

Even is not exactly right since obviously the field strength diminishes with distance from the pole to pole centreline, but I think you know what I mean.

Just as with a transformer or motor core, the field lines (want to) congregate in a ferrous material ie they are drawn into it, leaving few lines in the free space not occupied by the ferrous material.

But these paths have to be continuous through the core.

Cores are not free to move, fine particles are.

So the system can rearrange itself when fine particles are introduced.

And yes, indeed there are few lines in the spaces and a concentration of lines in the chains of ferrous particles.

If you were to plot some lines at right angles to the chains you would find repulsive (N-N and S-S) forces in action, which is why you get spread out lines rather than a block in the middle.

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I would guess that each filing becomes a little magnet and thus one end of it is attracted to the end of another filing - so the filing tend to become lines. These lines will follow the field lines.

Yes I agree, the iron filing must become magnets, else they wouldn't align tangentially to the field lines in the first place. The question still remains, why are there gaps. I am still not sure why the iron filing becomes magnets in the presence of the field though.

Good observation +1

As imatfaal says the iron particles line up so they are touching as continuous chains of tine induced magnets.

Why?

Well this is a good example of a system seeking to minimise its energy.

Without the filings the magnetic lines are 'evenly' distributed in space with no gaps.

Even is not exactly right since obviously the field strength diminishes with distance from the pole to pole centreline, but I think you know what I mean.

Just as with a transformer or motor core, the field lines (want to) congregate in a ferrous material ie they are drawn into it, leaving few lines in the free space not occupied by the ferrous material.

But these paths have to be continuous through the core.

Cores are not free to move, fine particles are.

So the system can rearrange itself when fine particles are introduced.

And yes, indeed there are few lines in the spaces and a concentration of lines in the chains of ferrous particles.

If you were to plot some lines at right angles to the chains you would find repulsive (N-N and S-S) forces in action, which is why you get spread out lines rather than a block in the middle.

Yes, good point, i did not consider that the iron filing have much lower reluctance than the air, so the flux through them will be much higher. This will make the new iron filing much more attracted to the existing blogs/lines of iron than the air. So in the second diagram the field lines will actually change to look something more like the figure below.

If you were to plot some lines at right angles to the chains you would find repulsive (N-N and S-S) forces in action, which is why you get spread out lines rather than a block in the middle.

Still not 100% sure what you mean here, but I think that is the crucial final part of the puzzle since, with my new understanding I would expect all the filing to want to created one big blob around the magnet since this will provide the smallest possible reluctance.

Edited by CasualKilla

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Here is a very rough quick sketch, from which I hope you will catch the drift.

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Is the magnetic field, as expressed by the filings, still present in those dimensions and form in the absence of those filings?

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Not really, no, but we gloss over that for elementary demonstrations.

A flat steel sheet would be closer to the air/free space distribution, although more tightly 'pulled in' to the bar magnet.

The 'shape' remains the same, but the distribution of the lines changes.

Remember for free space or a flat sheet the lines can't interact since the tiny magnets are fixed in space.

But the use of filings allows the lines to interact, although it exhibits the same onion shape from the bar.

Edited by studiot

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Not really, no, but we gloss over that for elementary demonstrations.

A flat steel sheet would be closer to the air/free space distribution, although more tightly 'pulled in' to the bar magnet.

The 'shape' remains the same, but the distribution of the lines changes.

Remember for free space or a flat sheet the lines can't interact since the tiny magnets are fixed in space.

But the use of filings allows the lines to interact, although it exhibits the same onion shape from the bar.

Right. So, the onion configuration is still there but much more tightly packed in the absence of filings?

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This is the difficulty in comparing the field distribution in a medium that can be physically moved by the field and one that can't.

If you had a small enough compass you could use the following method to see that there is next to no field between the chains of filings.

Without the filings the field pervades all space and anywhere the girls starts her point you can trace a line.

But usually this is not discussed when filings are used to show the shape of the plot.

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Still not 100% sure what you mean here, but I think that is the crucial final part of the puzzle since, with my new understanding I would expect all the filing to want to created one big blob around the magnet since this will provide the smallest possible reluctance.

There is a limit to how much field a material can have running through it before it saturates; it depends on the permeability. You can get this effect (sometimes call flux return), and effectively shield the outside from magnet, but it takes a lot more material.

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There is a limit to how much field a material can have running through it before it saturates; it depends on the permeability. You can get this effect (sometimes call flux return), and effectively shield the outside from magnet, but it takes a lot more material.

Yes I know about saturation, but I don't think that would stop the iron forming one big blob. The saturated iron should still carry much more flux than the air and this should attract more iron. As you say, shielding would occur after the blob reaches some critical mass, which would mean the new filing outside the blob essentially experience no magnetic field. (The magnetic field continues infinitely outwards becoming weaker, but it will become negligible after some time)

I have added the most terrible drawing of 2016 to explain my point.

Here is a very rough quick sketch, from which I hope you will catch the drift.

Ah yes, I see you point now, but I am still confused. What stops the poles from lining up in an alternating fashion, or do magnets not attract each other like that? I have never tested it.

Edited by CasualKilla

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Because the nearest end of any particle to the bar magnet will be of the opposite pole!

This is as I tried to show in my sketch.

So the end of the first particle adjacent to the north pole of the bar will be a south pole and so on.

Here is an expanded sketch of the north end of the magnet showing what I mean.

Did you notice that the second video on the iron filings said that it is a three dimensional effect (which you can see if you look carefully).

We are only drawing two dimensional sections through this.

Edited by studiot

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Here is a very rough quick sketch, from which I hope you will catch the drift.

Good point but here's a question: if magnetic field is stronger closer to the magnet creating it, and becomes weaker further away then I would expect that repulsion between metal particles in adjacent lines to be the highest closer to the magnet and to weaken gradually as you move away.

So in my mind distances between lines would be big close to the magnet and small farther away but it's not the case as we can see in the original post. Why is that so?

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Because the nearest end of any particle to the bar magnet will be of the opposite pole!

This is as I tried to show in my sketch.

So the end of the first particle adjacent to the north pole of the bar will be a south pole and so on.

Here is an expanded sketch of the north end of the magnet showing what I mean.

Did you notice that the second video on the iron filings said that it is a three dimensional effect (which you can see if you look carefully).

We are only drawing two dimensional sections through this.

Ah yes, I get it now . Tnx Studiot.

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Yes I know about saturation, but I don't think that would stop the iron forming one big blob. The saturated iron should still carry much more flux than the air and this should attract more iron. As you say, shielding would occur after the blob reaches some critical mass, which would mean the new filing outside the blob essentially experience no magnetic field. (The magnetic field continues infinitely outwards becoming weaker, but it will become negligible after some time)

But you generally sprinkle the filings over the entire paper. Unless you have enough iron close to the magnet to contain the flux, there will be a field further away, and the filings will align with the field there.

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But you generally sprinkle the filings over the entire paper. Unless you have enough iron close to the magnet to contain the flux, there will be a field further away, and the filings will align with the field there.

Are you proposing that an interior line of filings will shield the area directly outside it (resulting in gaps with no filings), but the field will then becomes stronger again as you move outwards until it is strong enough to create another line of filing which will repeat the loop?

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Are you proposing that an interior line of filings will shield the area directly outside it (resulting in gaps with no filings), but the field will then becomes stronger again as you move outwards until it is strong enough to create another line of filing which will repeat the loop?

No, I'm not saying the field is created that way — the field already exists. Unless you have enough material to contain the entire flux without saturating, there will continue to be a field far from the magnet. Thus, there's no reason for it for this proposed blob to form. However, I do agree with the idea that the filings effectively reduce the field in the vicinity — the flux will preferentially be found in the filings rather than the air.

And even so, separate filing won't necessarily behave the same as a solid mass, where you have additional bonds holding the material together.

If my modeling software weren't so out of day this would be fun to look at. But if you looked at thin shells of material in a field, it's quite dramatic how strong the field is inside the high-permeability material vs outside of it.

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No, I'm not saying the field is created that way — the field already exists. Unless you have enough material to contain the entire flux without saturating, there will continue to be a field far from the magnet. Thus, there's no reason for it for this proposed blob to form. However, I do agree with the idea that the filings effectively reduce the field in the vicinity — the flux will preferentially be found in the filings rather than the air.

And even so, separate filing won't necessarily behave the same as a solid mass, where you have additional bonds holding the material together.

Ok yeah, the magnetic field in the air is definitely weaker than through the filings, but if we consider that just a single line of filings exists, I do not think we would see the field being weak just outside the 'ring' of filing then suddenly becoming stronger. MMF = integral(Hdl). I think you may be misunderstanding how shielding works.

So my reasoning is the field will always be stronger nearer to the blob and thus filing will want to be closer rather than further from it. This ignores the concept that Studiot explained, so I am my argument exist hypothetically in a world where those like pole repultions don't occur

Edited by CasualKilla

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Good point but here's a question: if magnetic field is stronger closer to the magnet creating it, and becomes weaker further away then I would expect that repulsion between metal particles in adjacent lines to be the highest closer to the magnet and to weaken gradually as you move away.

So in my mind distances between lines would be big close to the magnet and small farther away but it's not the case as we can see in the original post. Why is that so?

Fair question but think about a balance of competing forces.

Remember also that the magnetism in the filings is induced, not permanent.

I was trying to show that there is a net balance of forces on the particles from repulsion from those on either side in 2D and above/below in 3D.

I meant to add these into my sketch this morning but,

I am dealing with two domestic emergencies (a result of crappy electrical design) at this moment.

One in the fans of my fan ovens in the cooker and the other in the mains input filter to the washing machine.

Why can't modern manufacturers make things last for more than a year?

Edited by studiot

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So my reasoning is the field will always be stronger nearer to the blob and thus filing will want to be closer rather than further from it.

If the filings were free to move, sure. That's what happens when you put ferromagnetic material near a magnet — they attract. But the point of filings on paper is that there's friction and the filings won't necessarily move closer, regardless of "wanting" to. You just get alignment with the local field.

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CasualKilla, on 17 Mar 2015 - 11:18 AM, said:

So my reasoning is the field will always be stronger nearer to the blob and thus filing will want to be closer rather than further from it.

Swansont

If the filings were free to move, sure. That's what happens when you put ferromagnetic material near a magnet — they attract. But the point of filings on paper is that there's friction and the filings won't necessarily move closer, regardless of "wanting" to. You just get alignment with the local field.

It doesn't work like that, watch the movie carefully.

Again I say remember it is a complicated 3D effect, which is why many teachers avoid it.

Edited by studiot

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Ok yeah, the magnetic field in the air is definitely weaker than through the filings, but if we consider that just a single line of filings exists, I do not think we would see the field being weak just outside the 'ring' of filing then suddenly becoming stronger. MMF = integral(Hdl). I think you may be misunderstanding how shielding works.

I didn't say it suddenly became stronger. Not in the air, anyway. But since the field is amplified in a permeable material, it will be stronger in the material than in the air.

I think shielding works like this

http://www.kjmagnetics.com/blog.asp?p=shielding-materials

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If the filings were free to move, sure. That's what happens when you put ferromagnetic material near a magnet — they attract. But the point of filings on paper is that there's friction and the filings won't necessarily move closer, regardless of "wanting" to. You just get alignment with the local field.

So, if we were in a zero-g environment with a floating magnet in front of us and it drifted through a cloud of filings, would they all just stick to the magnet or would we see the field expressed via some of them?

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So, if we were in a zero-g environment with a floating magnet in front of us and it drifted through a cloud of filings, would they all just stick to the magnet or would we see the field expressed via some of them?

They'd stick to the magnet.

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They'd stick to the magnet.

I thought so. That makes it clear that friction holds them in position on paper.

If you stick two magnets together N-S end-to-end, and did the iron filings again, what would be the field configuration? Would it be like one magnetic field or two distinct fields, in some other pattern where they join?

Edited by StringJunky