# Direction of magnetic force for magnetic dipoles

1. Aug 24, 2013

### san203

A charged particle in a magnetic field experiences a magnetic force that is perpendicular to it.
But is that the case with magnetic dipoles?

There was this section in my text book describing Torque on a Bar magnet in a uniform magnetic field which states that the magnetic force m.B on one pole of bar magnet acts along the magnetic field. But that does not fit with the above statement that Magnetic force always acts perpendicular to particles.
Please Explain.
Thanks

Last edited: Aug 24, 2013
2. Aug 24, 2013

### Staff: Mentor

A magnetic pole is not a particle, and does not actually exist physically. It is a fiction that lets us pretend that a magnetic dipole is similar to an electric dipole made up of two opposite electric charges.

Particles with "magnetic charge," i.e. magnetic monopoles, do not exist, as far as we know. Maxwell's equations do not include magnetic charge, although they can be "extended" hypothetically to include it.

3. Aug 24, 2013

### san203

I understand that monopoles do not exist physically. But that was not my question. My question was why does a bar magnet in a uniform magnetic field experience magnetic force in the same direction as the magnetic field?

4. Aug 25, 2013

### Staff: Mentor

I think you're mis-reading your textbook. The torque on a magnetic dipole is $\vec m \times \vec B$ (vector cross product). The dot product $\vec m \cdot \vec B$ (actually the negative of it) gives you the potential energy of the dipole. In both cases it applies to the entire dipole, not a single pole.

Magnetic moment

5. Aug 25, 2013

### san203

@jtbell
I am sorry. The m.B was actually multiplication of m&B.

Link to the original webpage

As you see in this figure, the Dipole is subjected to a force mB which is along the magnetic field.This is what is confusing me. No prior explanation was given in my text book about magnetic forces acting in the direction of field(i thought they always act perpendicular). Does it happen only in case of dipoles? Because i am pretty sure that the force on an electron in Magnetic field would be perpendicular to it.

6. Aug 25, 2013

### Staff: Mentor

How does the book define m, exactly? It sure looks like to me like it's using m as the symbol for a fictitious magnetic monopole strength, analogous to electric charge.

7. Aug 27, 2013

### san203

Yes. Your right. The symbol m is described as magnetic pole strength in my text book.

Edit: I read Wikipedia and found out that their are 2 kinds of Magnetic field H and B? I also read their are two models to describe magnetic field and that the H model was used to describe forces on magnets. I am confused now.

Edit#2 : I read some more. Correct me if i am wrong. The physically real magnetic field is because of moving charges or currents. The pole model is hypothetical but is easier to use. The magnetic force described in my previous post is also a consequence of this model that states that a pole in a magnetic field would be subjected to forces along the magnetic field something similar to electric charges in electric field. Hence the force m.B acting on the Magnetic dipole causes rotation.

Last edited: Aug 27, 2013
8. Aug 27, 2013

### DrewD

don't worry about this yet.

That's sounding good. I think the most fundamental mistake you seem to have made was equating the force on an electrically charged particle with that of a magnetically charged particle. The latter don't seem to exist, but even if they do, the would not behave the same way.

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