2 basic magnetism questions

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  • #1
1. How is magnetism propagated? The descriptions I have read of magnetism explain about spinning electrons and the effects of magnetic fields, but I can find no explanation how magnetic force is propagated. Gravity is explained via GR and the effects of mass/energy on space-time, but I don't find any equivalent explanation for magnetism.

2. How is a north magnetic pole distinguished from a south magnetic pole?

Please note that I am an interested but mathematically illiterate layperson, so I will not be able to follow equations, though I assure you that I appreciate their elegance :smile:

Answers and Replies

  • #2
Jano L.
Gold Member
Magnetic effects are often supposed to exist in space, because there is some source of magnetic field (magnet bar, current in a coil, Earth). If nothing moves, the magnetic forces do not change.

But if the magnetic field is due to moving source (like rotating magnet in electromotor, or current in the emitting antenna), EM waves are generated and propagate out of the source. These exert oscillating electric and magnetic forces on everything they hit.

Concerning your second question, north and south poles are always present (they cannot be separated), so you will find out which is which simultaneously. You can do that with a help of reference magnet, for which you know its S and N ends; you find which end of the magnet studied is repulsed from the N pole of your reference magnet and this is defined to be the N pole too (like poles are repulsing each other). The other pole is then S.
  • #3
Thank you, but what are the EM waves made of? Aren't photons the carriers of the EM force, but I do not see references to them in discussions of magnetism.

I understand using a reference magnet to determine another magnet's north and south poles, but at some point one might not have such a convenient tool. How do north and south poles differ in characteristics so they attract opposites and repel likes? It appears there is something similar to the positive-negative opposition of electric charges, but I am a thoroughly unschooled layperson, so that may well be a simplistic and misguided assumption.
  • #4
Photons are the carriers of electromagnetic field but you have to consider the fact that when EM waves are observed on any macroscopic level, that wave is made of an incredibly large number of photons. What we observe is the averaging over of them in time and space. Particle-like carriers of the electromagnetic field were only first considered when quantum theory was first being developed, which is why any classical field theory or mainstream applications book either won't discuss them or they will be covered/considered in only a smaller portion of the discussion. So, if you were thinking on a particle-particle interaction level, EM waves are photons while if you were dealing with high energy waves, it's arguably a more intuitive convention to consider the EM waves as macroscopic fluctuations of the fields and not particles.

As for the magnets, the names were derived out of conventions mathematically and directionally. Consider the iron filings experiment with a bar magnet and a circular wire loop. If aligned properly, they will give the same pattern in the iron filings. Since the theory at the time this was originally considered said magnetic fields emerge from somewhere, loop away from the source, and return to close the loop and that the magnitude of the force was proportional to the current in the loop. The mathematics characterizing these effects were matched to the right-hand-rule convention in that the magnetic field circulates around a current in the counter-clockwise direction. Thus the magnetic field emerges from the center of the loop, curls around the loop, and back up through the middle. Simply, the 'north' pole of this arrangement was named for where the fields emerge and the 'south' pole is where they return. By the relationships between poles of magnetic sources at the time, the bars were thus given N and S pole names by how they interacted with the current loops.

The attraction principles can be noted simply by creating two identical magnets or current loops with definitive 'poles' and characterizing their interactions with each other. It's interesting to note that the Earth's field is a dipole that changes in time. For the last many thousands of years up to now, the magnetic N pole is located at the geographic south pole.