I've been watching several videos on DC electric motors and they all mention how current through a wire creates a perpendicular rotating electromagnetic field around the wire in a direction that can be found using the hand rule.

If electromagnetism is nothing more than the attraction of electrons due to relativity as they move, why is this field rotating, what causes it to rotate?

  • $\begingroup$ "If electromagnetism is nothing more than the attraction of electrons due to relativity as they move,[...]"...what? Also, can you find a non-video-source stating the thing with the "rotating field" and link it? $\endgroup$ – ACuriousMind Dec 29 '14 at 20:40
  • $\begingroup$ All my sources have been videos, they mention that the field around the single straight wire rotates. Since you don't understand my explanation of electromagnetism, can you provide an explanation of how electromagnetism works? $\endgroup$ – reddead Dec 29 '14 at 20:46
  • $\begingroup$ Well, not in such a nutshell (if you don't wanna hear "It's a $\mathrm{U}(1)$-gauge theory minimally coupled to matter", which I suppose you don't). But electromagnetism is more than "the interaction/attraction of electrons" - electromagnetic waves need to such charge carrier to propagate infinitely through space, for example. $\endgroup$ – ACuriousMind Dec 29 '14 at 20:53
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    $\begingroup$ The correct description of classical electromagnetic fields are Maxwell's equations en.wikipedia.org/wiki/Maxwell%27s_equations, however, what you want is a phenomenological description. Electric machines work because a wire that has an electric current flowing trough it will experience a force in a magnetic field. That's it. You can talk about it as a consequence of the Lorentz force en.wikipedia.org/wiki/Lorentz_force, but just knowing that a current in a magnetic field leads to a proportional force is enough to understand electrical machines. $\endgroup$ – CuriousOne Dec 29 '14 at 20:55
  • $\begingroup$ In a polyphase ac (not dc) motor there is something else rotating besides the shaft: the instantaneous peaks of the magnetic field is cycling through the poles of the stator. See this article and drawing: en.wikipedia.org/wiki/AC_motor . $\endgroup$ – hyportnex Dec 29 '14 at 23:11

You're probably talking about the physics summarized in this photo:

enter image description here

The arrow next to the letter I is the direction that current is moving, which refers to the actual direction that positive charges would move. I stress actual there because the arrows representing the magnetic field B do not represent a movement of any kind. The magnetic field is not rotating in the sense you may be imagining; it's not some substance that's moving around the wire in a swirling pattern.

Historically, magnetic field vectors were chosen to point "around" the wire because that is how tiny compasses would align if near the magnet:

enter image description here

That's really all the circular lines mean: The direction at any point along the circle tells us how a compass will point (in the absence of Earth's magnetic field). Experimentally, I would say this is "why" magnetic field lines wrap around current-carrying wires.

It turns out (turn, get it?!) that this direction also helps determine the direction in which a moving charged particle would experience a force due to the wire. (If you're interested, the force is found using a mathematical operation known as the cross product.)

The typical view that student in introductory physics classes have of the magnetic field is that there's nothing physically there pointing in that direction, nor is there anything actually rotating. The direction (and strength) just help us make sense of other observations.

  • $\begingroup$ that is exactly the examples that were presented in the videos although they spoke about it as if the magnetic field rotated. Thanks for clarification, also, why is it that the "force" or the way the needle would point to the magnetic field changes depending on the way the current is moving, isn't the magnetic field created by the current just the same? $\endgroup$ – reddead Dec 29 '14 at 21:09
  • $\begingroup$ True, the circular magnetic field vectors would switch directions if the current changed directions. Ultimately this is experimental fact, so I don't know how deep one can really go there. But remember that current has a direction; it is the direction that positive charges flow. So if you change direction, you might expect other things to also change direction. $\endgroup$ – BMS Dec 29 '14 at 21:41
  • $\begingroup$ ^ though I could imagine searching/asking this question about why the $B$-field switches when current switches separately here on Stack Exchange. $\endgroup$ – BMS Dec 29 '14 at 21:48
  • $\begingroup$ Ultimately, this is related to relativity - In its simplest form, a single, moving charge (say an electron) creates a magnetic field. But, let's say you are moving alongside the electron, at the same speed. Is the electron moving relative to you? No. Therefore, do you see any magnetic field due to this electron? No. en.wikipedia.org/wiki/Relativistic_electromagnetism $\endgroup$ – neonzeon Dec 29 '14 at 22:04
  • $\begingroup$ Excellent point. To continue with this reasoning, one would have to explain why electric fields tend to point away from from positive charges and toward negative charges. $\endgroup$ – BMS Dec 29 '14 at 22:05

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