# An Introduction to Magnetic Attraction?

1. Mar 24, 2011

### JDoolin

Sorry to start yet another thread on the basics of magnetism, but I'm missing something...

Glancing through an introductory college physics textbook, the three chapters on magnetism are missing something ...and it is the one thing that every normal person thinks of when they here the word magnet...

In this text, there don't seem to be any equations or discussion involving the the attractive or repelling force between two magnets, or between magnet and metal. It seems like that should be a starting point for discussion in introductory physics, since those are the properties of magnets most people are accustomed to.

In the text, I see the effects magnetic fields have on currents, and the way current loops have a magnetic moment, and what kind of torque will be enacted on them, and the electromotive force resulting from a changing magnetic field, or changing current, magnetic field strength due to various configurations of wire, etc...

For one, how do you measure a magnetic field? The units are Tesla, and the direction can be determined by a compass, but how do you measure the strength?

Is the magnetic attraction force a side-effect of the two magnets aligning themselves, and the changing currents inside the materials, or is it a completely independent phenomenon?

2. Mar 24, 2011

### timthereaper

About measuring the magnetic field, you can measure it using the Hall effect.

As I understand it, the magnetic field of a magnet aligns the domains of a ferrous material, essentially inducing a magnetic field in the material. The pole of the magnet and the pole of the material closest to the magnet are opposite, so they attract. The force of the attraction is dependent on the ease with which a field can be induced in the material (I think it's called magnetic susceptibility).

If any experts out there know if I'm off, let me know.

3. Mar 24, 2011

### JDoolin

Okay, that is a helpful reminder about magnetic susceptibility and permeability. There's a discussion of paramagnetic, diamagnetic and ferromagnetic in my text but it doesn't break it down with the numbers. I found another text that did discuss susceptibility and permeability, but it still doesn't address my main question.

But I think I have a better way of asking my question.

The force of a magnetic field on any charge is always PERPENDICULAR to the magnetic field itself. However the force of a magnet on another magnet is predominantly PARALLEL to the magnetic field.

Yet there is no discussion in either book of that parallel force (except, to say that it happens.) I'm wondering about a quantitative description. Say you have the magnetic moment of two magnets and the distance between them... how do you calculate the attractive force between them?

4. Mar 24, 2011

Staff Emeritus
It's done this way for a reason.

If you would do this for elecrostatics, the first thing you would need to do is define "charge'. That's easy, because you could, at least in principle, do this by counting unbalanced electrons.

Doing this with magnets, the equivalent is the current loop. Thing is, once you start down that path, you might just as well explain the whole thing in terms of current loops. In for a penny, in for a pound.

5. Mar 24, 2011

### JDoolin

I looked in still another text book for the Hall effect. Apparently positive charges going up through a magnetic field are deflected the same direction as negative charges going down through a magnetic field. In the first case, postive charges accumulate, while in the second, negative charges accumulate. By checking which kind of charge accumulated, you can tell what your charge carriers were.

So with a more powerful magnetic field, you would have a greater accumulation of charge, and hence the voltage measured would be roughly proportional to the magnetic field, and hence the Hall effect can be used to measure magnetic field.

But it is still technically measuring a force perpendicular to the magnetic field--not the attractive force between magnets that ordinary non-physicists commonly think of when one says "magnetic force."

6. Mar 28, 2011

### JDoolin

7. Sep 27, 2011

### JDoolin

Based on the lack of replies to this, I guess this is somewhat of a mystery.

I happened to be thinking about it today, and I wonder whether magnetic attraction is a statistical thermodynamic effect?

Do the electrons in magnets gain more freedom of motion or "degrees of freedom" when they come come closer and align? Is this something that could be modeled by the canonical ensembles or grand canonical ensembles of statistical thermodynamics?