I read that permanent magnetism is caused by electrons orbiting in a way which produces a net magnetic field. but how can a particle with no dimensions have a magnetic dipole? also I read that photons are the fundamental carriers of electromagnetism. where does that come in here?
121848
Experiments used to observe particle spin properties (such as ) rely on a varied magnetic field and a dipole-like reaction in the particle, deflecting it in one direction or another.  In the case of a point-particle such as an electron, how is it explained that a dipole with spatially separated poles exists in a single point?
77972
So I learned that the bar magnet makes a magnetic field and the field lines have a certain shape which kinda resembles 2 semi-ovals.  And if we pass current through a conductor then it makes a  magnetic fields and the field lines are in the form of concentric circles around the conductor.  My question is- why? Why do magnetic field lines take the shape that they take? There must be a reason behind it. Why aren't the field lines of a normal bar magnet shaped like the field lines of a conductor carrying electricity? And vice-versa?   Please explain it to me..i know that it delves very deep into the fundamentals of the working of science but still...please tell me the reason if anyone knows it
72320
I've been reading in Khan academy about the formula for calculating magnetic field lines (Ampere's law). Some materials have the ability to concentrate magnetic fields, which is described by those materials having higher permeability. The permeability of vacuum is $4\pi *10^-7$ , which means it can concentrate magnetic field lines. But how can nothing—no matter, energy, or force—influence or manipulate the waves passing through it (practically  through nothingness). I mean why is the permeability not 0? (This just comes out of the rigid intuition that vacuum does nothing to anything because it is nothing—ignoring the particle-antiparticle pairs or similar very distant stuff.)  As can be seen from the fuzziness of the question, I'm learning physics (not an expert) and any theoretical answer is okay but answers involving complex math like calculus or differentials are unwelcome but if it needs to be used let me be informed.
16675
I heard that a magnetic field only creates a force on a moving charge, not a stationary one. However, what if the solenoid containing the magnetic field was moving through a stationary charge. Would that create a similar force? Thanks!
106968
Frequently when EM waves are taught, it is said that the change in electric field causes a change in the magnetic field, which then causes a change in the electric field, and so on and so forth.   But from my understanding of basic electromagnetics, it is not necessarily a changing electric field that creates a change in magnetic field, but instead an accelerating charge. and it is this charge that creates both the electric and magnetic field. Again, from my understanding, a changing magnetic field is not generated by a changing electric field, but instead just happens to always be present perpendicular to a changing electric field due to the laws of electromagnetism.   Am I wrong? Or is the "mutual generation" concept between the electric and magnetic components of an EM wave an actual thing?
71546
Electric monopoles do exist, but why cannot magnetic monopoles exist?    The question is closed because I need to clarify it, but I don't know how I could ask it another way. However, I've recieved many answers that were appropiate and added something to my knowledge, so I consider it answered.
73549
I understand how one associates the spin of a quantum particle, e.g. of a photon, with intrinsic angular momentum. And in electromagnetism I have always understood the polarization of an EM wave as the oscillations of the E and M field, not necessarily being  aligned with the direction of propagation of the wave.     Questions:   But when one talks about the polarization of a photon in Quantum Mechanics, how does it really differ from its spin?  Are they somehow related and what's the physical idea behind photon polarization in contrast to photon-spin? Feel free to use mathematical reasonings as well if you see it fit!
36426
I have a bunch of magnets (one of those game-board thingies) given to me when I was a school-going lad over 20 years ago, and the magnets feel just as strong as it was the day it was given.   As a corollary to this question , is there a way to determine how long a permanent magnet will remain a magnet?   Addendum: Would two magnets remain a magnet for a shorter duration if they were glued in close proximity with like poles facing each other?
60036
As everybody here knows, Maxwell's equation would look more beautiful if a magnetig charge were present. Beyond the aesthetics question, if a single magnetic Dirac monopole would be found, the consequence is that the electric charge would be quantized.  While, I guess, everybody should be happy with the news, my experimentalist background doesn't allow me to understand the experimental setting. I therefore ask for the following advice -- a rather theoretically oriented question.  A recent paper,     Observation of Dirac monopoles in a synthetic magnetic field. M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen and D. S. Hall. .   claims to have observed such monopoles.   Can somebody shed light on the theoretical implicacions of this work? Concretely, should we now take care before saying that it's been proven that the electric charge is quantized?
7799
I learned that the force from a magnetic field does no work. However I was wondering how magnets can be used to pick up pieces of metal like small paperclips and stuff. I also was wondering how magnets can stick to pieces of metal like a refrigerator?
118255
A charged particle moving with an acceleration produces electromagnetic waves. Why doesn't a charged particle moving with a constant velocity produce electromagnetic waves? As far I understand, the electric and magnetic fields in space will still be time-dependent, if a charged particle is moving with constant velocity, so they could have given rise to electromagnetic waves, but they don't.  Also, why do accelerating charged particles produce electromagnetic waves? What is Nature's intention behind this phenomena?
102911
Maxwell's equations  very nicely if we add in magnetic monopoles: we get $$\begin{align*} \partial_\mu F^{\mu \nu} &amp;= J^\nu \\ \partial_\mu \tilde{F}^{\mu \nu} &amp;= \tilde{J}^\nu, \end{align*}$$ where $F_{\mu \nu}$ is the electromagnetic tensor, $\tilde{F}_{\mu \nu} := \epsilon_{\mu \nu \rho \sigma} F^{\rho \sigma}$ is the dual tensor, and $J^\mu$ and $\tilde{J}^\mu$ are the electric and magnetic current densities respectively. We can use differential forms to make these equations even more compact: $$\begin{align*} \mathrm{d} (\star F) &amp;= J,\\ \mathrm{d} F &amp;= \tilde{J}. \end{align*}\tag1$$  It is known that QED can be generalized to incorporate point magnetic monopoles, which leads to all kind of interesting phenomena like the . (This is done by essentially just excising the monopoles from spacetime completely and only defining the gauge field $A^\mu$ away from the monopoles, so that they act as topological defects that convert the trivial principle $\mathrm{U}(1)$ fiber bundle on which the gauge field lives to a nontrivial bundle. This is straightforward when the monopole trajectories are specified by hand, but I think there are also reasonably tractable ways to give the monopoles their own dynamics.)  But what about the case of continuous distributions of magnetic monopoles? (To avoid cheating, let's say that the $\tilde{J}$ field's support is the entire region of spacetime whose dynamics we are considering, although still bounded so that the fields are well-defined.) In this case the equation $\mathrm{d}F \neq 0$ prevents us from introducing a gauge field $A$ such that $F = \mathrm{d}A$ in the first place. It's probably not at all clear how to quantize such a theory, so let's just stick to the classical case. To make things even simpler, we can ignore the Lorentz force law and treat the source currents as background rather than dynamical fields, so that we only need to worry about the EM field's time evolution. In this case the equations (1) form a perfectly well-posed system of coupled differential equations. Is there any known/possible Lagrangian or Hamiltonian whose equations of motion are given by (1)?
96780
Magnetic forces vary from gravity and electromagnetic radiation (such as light) in that gravity and radiation diminish by the square of the distance from the source, but magnetism diminishes by the cube of the distance.  Why is this?
28112
Say I wrapped a piece of wire around an iron bar in a closed circuit connected to a DC power supply, the electrons starts flowing and moving charge produce magnetic field. Yet a bar magnet can produce a magnetic field without any charged particles moving around, are the magnetic fields produced by the two objects different types?
68843
If I was to take a bunch of magnets and arrange them in a sphere (And keep them there with glue or plastic or something) so that the north pole faces the outside of the sphere and the south pole faces the inside, would the magnet have the same pole no matter what way I turn it or would the magnet be neutralized or something.  I'm envisioning a sphere made of magnets so that no matter what way I turn it it is always the same pole and that a bunch of these repelling each other would be really cool, is this possible?  In effect would this create a monopole magnet?
102265
The electromagnet spectrum looks to be continuous, yet energy is discrete. Shouldn't one see evidence of the electron's "quantum leap"? What's going on?     Moderator, please size my photo. Thank you.
11743
I'm tutoring high school students. I've always taught them that:     A charged particle moving without acceleration produces an electric as well as a magnetic field.   It produces an electric field because it's a charge particle. But when it is at rest, it doesn't produce a magnetic field. All of a sudden when it starts moving, it starts producing a magnetic field. Why? What happens to it when it starts moving? What makes it produce a magnetic field when it starts moving?