# Magnetic field lines?

1. Feb 26, 2015

### hamburg21

Are magnetic field lines actually lines?

To me, they are just a way to pictorially represent the magnetic field, and there are no lines.

BUT, whenever you do the experiment with magnetic filings on a piece of paper, they align in actual lines.

HELP!

2. Feb 26, 2015

### Staff: Mentor

3. Feb 26, 2015

### hamburg21

There was something very useful on the article you sent:

"The use of iron filings to display a field presents something of an exception to this picture; the filings alter the magnetic field so that it is much larger along the "lines" of iron, due to the large permeability of iron relative to air."

I guess my question is a little deeper - why do the iron filing line up in lines if there is only a field? Shouldn't they be just distributed with a density that is proportional to the magnetic field strength at that position?

And if they affect the magnetic field themselves, has anyone modeled this feedback effect?

4. Feb 26, 2015

### nsaspook

5. Feb 26, 2015

### hamburg21

That thing is awesome - I have never seen it before!

Again, I realize what everyone is saying, I guess I am just looking for the physics behind the line "clumping". Has anyone modeled this effect? Meaning, has anyone written a numerical simulation that is seeded with these types of mini-particles, and verified that the lines form to "minimize" local energy. To me, if they are minimizing their energy, then where the lines form is highly dependent on the initial state of the particles, and the field strength itself is just the overall direction.

6. Feb 26, 2015

### Staff: Mentor

An analogy might be how lightning takes the path of least resistance to the ground.

7. Feb 26, 2015

### hamburg21

I like the analogy, but that is hand-waving the problem away.

8. Feb 26, 2015

### hamburg21

Just found this: http://van.physics.illinois.edu/QA/listing.php?id=27163&t=magnetic-field-lines-dont-really-exist

nice quote:

"It sure looks like field lines, right? Actually, this clumpiness has nothing to do with field lines; it's just a coincidence that it looks like lines (or perhaps it inspired the idea of field lines?). The reason for the creation of these pretty iron filing shapes is that each little iron filing, when subject to a magnetic field, becomes a little dipole itself. This dipole feels the force of the magnet, and aligns in thedirection of the field lines. In addition, each little dipole feels a small force from the other nearby dipoles, and they move to minimize their local energy. This causes the clumping into lines that you see, as the opposite ends of the dipoles move together.

The clumping is NOT a property of the magnetic field from the large magnet, it is a consequence of the magnetic fields of the small iron filings."

9. Feb 26, 2015

### davenn

yup, that about covers it :)

10. Feb 27, 2015

### tech99

Interestingly, Maxwell based his equations on a mechanical model of the medium which used actual lines.