1

I have spent all day puzzling over this (when I should be revising...) I have just about got my head around how special relativity allows the magnetism to work, and how in different frames of reference this mysterious force acting on a moving charged particle is either electric, magnetic or elements of both, this being the electromagnetic field, unifying both the electric and magnetic elements. I have got to grips with this using the familiar analogy of the positive (lab reference) charged ion and current travelling through the wire, and as the ion moves with the electrons, the length contractions occur, applying an electric force to the ion. my understanding of a magnetic field

So that's all well and good. However I have two main questions, that with hours of scouring the internet I can't find the answer to.

  1. How does this idea of relativistic electromagnetism explain why a permanent magnet has a magnetic field? Does the electron "spin" act as a current loop, and how does this create a magnetic field?
  2. With both the idea of a magnetic field from a permanent magnet and a current carrying wire, how can this explain the motor/generator effect?

TLDR: How does relativistic electromagnetism explain permanent magnets, and the motor effect?

Many thanks!

EDIT: Thank you for all the replies thus far. I am just hoping to refine my question as it is causing a little confusion.

With the idea that magnetism is a force when a charged article is moving with a current, how does this apply to a permanent magnet? In a permanent magnet, If the electron creates a current loop that is instead moving in a circle, does an outside charge have to be moving at a certain velocity to for length contraction to occur in the atom and therefore the magnetic force exhibited my permanent magnets?

How does this logic of special relativity work with permanent magnets?

And then, later on, how this can all come to together with a current carrying wire and permanent magnets in the motor effect...

Matt Smallwood
  • 73
  • In order to apply the same logic for a current loop, you must consider how the electric and magnetic fields transform in a non-inertial (rotating) frame. This gets nasty very quickly, and leads to a situation where the "electric field" doesn't really act like an electric field anymore. So I wouldn't recommend trying to use that same process here. – probably_someone Apr 02 '18 at 22:50
  • That said, what precisely do you mean by "special relativity allows the magnetism to work"? – probably_someone Apr 02 '18 at 22:51
  • 1
    permanent magnets exhibit ferromagnetism, which is due to spin alignment. See http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/ferro.html See also https://en.wikipedia.org/wiki/Ferromagnetism#Origin_of_magnetism – ZeroTheHero Apr 03 '18 at 02:16
  • 1
    Relativity unites electricity and magnetism, and in the example you show, it happens that a force that is purely electric in one frame is purely magnetic in another. That doesn't mean that in all examples, a force that is purely magnetic in one frame can be made into a force that's purely electric in another frame. –  Apr 03 '18 at 02:40
  • 1
    Permanent magnetism isn’t explained by relativity. It can’t work without quantum mechanics, see the Bohr-van Leeuwen theorem. – knzhou Apr 03 '18 at 08:27
  • @probably_someone Great this was the sought of comment I was hoping for. So using the same logic for a non inertial current loop gets rather complicated, but essential is the same underlying logic present? Is there a Classical Electromagnetic theory that explains this somewhat? - Just about your second comment, by "allowing magnetism to work" I am trying to express that SR explains this mysterious force in terms of electric and magnetic fields together. – Matt Smallwood Apr 03 '18 at 08:31
  • @knzhou So is the Bohr-van Leeuwen theorem the classical method of explaining magnetism? In reality, will SR still be the underlying (theoretical) explanation for a permanent magnet, but it is simply to complicated to model (at least from my bedroom that is). I can understand this way of thinking, as with Electrical Engineering we treat electric and magnetic fields as though they are classical mechanics, which hugely simplifies the maths. – Matt Smallwood Apr 03 '18 at 08:38
  • Furthermore, this post claims that ferromagnetism is a quantum effect rather than a relativistic effect? Surely they are linked? https://physics.stackexchange.com/a/159025/191224 – Matt Smallwood Apr 03 '18 at 08:56
  • @MattSmallwood It's not that SR is too complicated. You can have permanent magnetism if you have QM, and you can't have it if you don't have QM, and SR has nothing to do with it. If you try to calculate the amount of permanent magnetism anything displays without QM, eventually everything will cancel out to zero, whether or not you use SR. There are classical explanations of magnetism, given in introductory physics classes, but they're all 'lies', i.e. they neglect some effects to get a nonzero result. – knzhou Apr 03 '18 at 09:33
  • @MattSmallwood For more info, see here! – knzhou Apr 03 '18 at 09:35
  • @knzhou Ok great! This is starting to make sense! In summary: Permanent magnets are the result of a Quantum Effect, and electromagnets are the result of Special Relativity. So in the Motor/Generator effect, this must be an interaction between the QM permanent magnet and SR current carrying wire. Can a permanent magnet's magnetic field still be seen as an electric field from a different reference point, or is it a purely Quantum Effect, and therefore electromagnetism and ferromagnetism are seperate? I will sit down and have a thorough read of your link! – Matt Smallwood Apr 03 '18 at 09:51
  • sorry, but absent magnetic monopoles, magnetic fields only arise from moving charge per special relativity - quantum spin gives "moving charge" so hat some materials can have "pemanent magnetism" without obvious flowing current – f5r5e5d Apr 03 '18 at 10:48
  • @f5r5e5d thank you for the comment. So you are saying that magnetism is a relativistic effect, but the quantum mechanics behind electron spin allow there to be a moving charge, and therefore magnetism due to special relativity? This makes logical sense in my head, that magnetism is caused by the same effect in both cases. Do you have any further details about what actually is the electron "spin" and how it affects the charge? – Matt Smallwood Apr 03 '18 at 16:08
  • the Wikipedia Magnetism article looks OK, refs Feynman Lectures feynmanlectures II_34 – f5r5e5d Apr 03 '18 at 18:13

0 Answers0