# B Magnetic field composition?

1. Aug 8, 2017

### Mediocrates

So are there photons "flowing" from one end of a permanent magnet to the other? If so what determines the frequency?

2. Aug 8, 2017

Staff Emeritus
No, and that makes the second part moot.

3. Aug 8, 2017

### Mediocrates

Of what then is the field composed?

4. Aug 8, 2017

### weirdoguy

Nothing. Field is a field, entity that lives on it's own. Photons are "excitations" (whatever that means) of quantum electromagnetic field, but that doesn't justify the claim that em field is composed of anything.

5. Aug 8, 2017

### rumborak

There was another thread just yesterday that veered off massively, so I think it's worth pointing out that "electromagnetic" isn't just a convenient moniker to refer to the magnetic and electric field together, but rather that they are really are one combined, inseparable entity. That is, the classic "electric field" and "magnetic field" are manifestations depending on the observer, and indeed different observers might entirely disagree on whether a certain action was caused by an electric field, or a magnetic field.

6. Aug 8, 2017

For the OP @Mediocrates The best simple explanation for the source of the magnetic field in a permanent magnet=[e.g. a cylindrical magnet with uniform magnetization vector $\vec{M}$,(that points from the "-" end to the "+" end), that essentially has a "plus" (north) magnetic pole on one endface and a "minus" (south) magnetic pole on the other endface], is the explanation of "bound surface currents" from the magnetization that "circulate" around the outer cylindrical surface with the same geometry as that of the currents of a solenoid. Since these currents are the resultant edge effect of what could be viewed in a simplistic model as currents from the magnetic moments of individual atoms, there is no actual electrical charge transport from these currents. In any case, the magnetic field $\vec{B}$ of the permanent magnet, both inside and outside the magnet can be computed from these hypothesized "surface currents" using Biot-Savart's law, and the results are consistent with the magnetic field that is observed.

7. Aug 8, 2017

### rumborak

@Charles Link , that is an engineering approximation that hardly elucidates the actual generation of the magnetic field in a permanent magnet.

8. Aug 8, 2017

The exchange effect is an additional effect necessary to explain ferromagnetism, but the magnetic surface currents can be quantified with what I believe are highly accurate results. For a simple calculation, see the following post: https://www.physicsforums.com/threads/magnetic-field-of-a-ferromagnetic-cylinder.863066/ Hopefully this doesn't start to lead too far off topic before the OP gets a chance to respond. $\\$ Also, Griffith's Electrodynamics book contains a very good derivation of the surface currents as he computes the vector potential $\vec{A}$ from an arbitrary distribution of magnetization $\vec{M}$. I believe this result is in section 6.2 of his book.

Last edited: Aug 8, 2017
9. Aug 9, 2017

### davenn

Charles .... you are making this far more complex than what a B level topic should be

10. Aug 10, 2017

### Mediocrates

How would one determine the existence of a field in the absence of particles?

11. Aug 10, 2017

### rumborak

That's probably more of a philosophical question since that scenario wouldn't be our universe.

12. Aug 10, 2017

### Mediocrates

Quess my question was, "can there be a field without a particle?" I thought particles give rise to fields?

13. Aug 11, 2017

### weirdoguy

No it's the other way around, particles are states of (quantum) fields.