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I have noticed that when particles of metal are captured by a magnetic field. They separate into clearly visible patterns of lines. Why don't the particles form a more even distributed pattern?

Do these gaps/lines exist in the magnetic field itself?

Can someone explain why this happens?

As illustrated by this images.

Magnetic field example Magnetic field example Magnetic field example

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This is related to a recent question: Bend or concentrate magnetic field?, but I'm a bit shaky on the details and will hope that someone else can clearly explain the self-organization that is going on here. – dmckee Oct 17 '12 at 15:53

2 Answers 2

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Each particle of iron in a magnetic field becomes a tiny magnet, following the field lines. A small dipole. This means it will attract at its two poles and be neutral in the middle. This is a way of making a line.

The pictures you show are made for the purpose of displaying the field lines . If the whole area were filled with iron dust the effect would be much less visible, though there would be a direction, a type of flow of the dust, due to the gradual orientation of the dipoles along the field lines . The field lines themselves fill the space continuously.

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Is there anyway to visualize a magnetic field using a gas or liquid? – ThinkingMedia Oct 17 '12 at 15:59
People who study the sun dynamo and the corona work with both fluid equations and magnetic fields. I do not think though that this is what your are asking for – anna v Oct 17 '12 at 16:07

The presence of the magnet's field magnetizes each little strip so that, as Anna says, each strip becomes itself a tiny magnet. This has two consequences:

  • The energy of each strip is minimal when it is aligned with the field. This causes all filings to turn where they are and face (preferentially) the local direction of the field, which will already create a picture of the field.
  • The filings now interact with each other and are therefore attracted or repelled from each other depending on their relative alignment, as shown in the image below. This makes the head-to-tail configuration much more favourable than the side-to-side configuration (from the two that have $\mathbf{m}_1$ and $\mathbf{m}_2$ parallel; the others are discouraged by the original field from the magnet which is of course stronger) and the filings will therefore move around to achieve this as far as possible. This is what causes the lines you ask about.

enter image description here

(of course, to get an intuitive feel for how the dipole-dipole interaction really works, grabbing a pair of bar magnets and moving them about definitely beats staring at pictures!)

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One more bit...this self organization has an effect on the field. The answer Mathews subquery "Do these gaps/lines exist in the magnetic field itself?" depends on when you ask it. Before you introduced the filing, "No, the field is smoothly varying"; after you introduced them and they organize themselves, "Yes, the regions where there are no filings have depleted field relative nearby regions where the filing clump". – dmckee Oct 17 '12 at 16:48
very cool, thanks. – ThinkingMedia Oct 17 '12 at 16:59

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