# Magnetic force question

1. Nov 3, 2006

### allok

hiya

I assume the field lines represent the strength of magnetic field and show the direction of magnetic force. So if we put iron filings on paper with magnet near by, then fillings will arrange themselfs in such way to show the direction of the magnetic forces on these iron fillings.

And here is the confusing part: When learning about moving charged particles inside MF, suddenly magnetic force on this charge is perpendicular to magnetic field lines ( and these lines BTW represent magnetic force ).

Why if an object such as another magnet or steel enters MF, magnetic forces represented by magnetic field lines act on this object, but when charge enters MF, the magnetic force represented by magnetic field lines doesn't act on charge, but instead new force is created that acts on this charge and direction of this force is different than direction of magnetic forces represented by magnetic field lines ( this new force is only created if charge is not moving parallel with magnetic field lines )? In short, why does magnetic field behave differently depending on whether a charge or steel enters inside it?

thank you

2. Nov 3, 2006

### OlderDan

The nearest thing to a magnet that can be made with a single charge is a magentic dipole, which is a charge orbiting with a circular motion. Consider a charge moving in a magentic field where some force other than the one from the magnetic field is making the charge run around in circles, or it could be a group of charges running around on the same circle. If you like, let those charges be inside a little loop of wire. What force does the magnetic field apply to the charge(s) in different places along its circular path? How does this depend on the orientation of the circle relative to the magnetic field?

3. Nov 6, 2006

### allok

Uh, I don't know

4. Nov 6, 2006

### OlderDan

What you see when you look at the iron filings near a magnet is that the filings align themselves with the magnetic field lines. They are not forced in the direction of the field lines, they are twisted into alignment. The same thing happens with a loop of current in a magnetic field. One side of the loop is forced in one direction, and the other side is forced into the other direction. The loop is twisted into alignment so that the plane of the loop is perpendicular to the magnetic field. There is no inconsistency between what happens to a loop of current and what happens to a single charge moving in a magnetic field.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html