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What causes induced magnetic fields?

  1. Nov 19, 2011 #1
    ok so you have a current flowing through a wire that induces a magnetic field, the direction of which is given by the right hand rule. my question is: WHY is there an induced magnetic field? what actually causes it? thanks.
    (fyi im in classical e&m right now)
  2. jcsd
  3. Nov 19, 2011 #2
  4. Nov 19, 2011 #3
    could you please explain it conceptually? (im in second semester physics, relativity is 3rd semester, "derive" sounds like youre certainly just speaking in mathematics---whereas im interested in the conceptual explination. Einstein said "If a physical theory cannot be explained to a child, it is probably worthless." so there should be a valid conceptual explination for something as basic as induced magnetism)
  5. Nov 19, 2011 #4
    There is.

    I figure by now you must be familiar with Galilean/Newtonian relativity. They both came to the realization that the way things look to an observer is dependent on whatever relative motion there might be between the person looking at the phenomenon and the phenomenon itself. Example:

    If a guy is standing on the moving platform of a flatbed train car and bouncing a ball, it looks to him like the ball is just moving up and down. To someone on the ground watching the train pass, though, the ball will seem to be describing a series of parabolas. Galileo and Newton asserted that neither one was "right" or "wrong". It's just a neutral fact that the observation of a phenomenon depends on the relative motion between the thing and the observer. "Rest" or "motion" aren't absolute things. We can only speak of something being at rest or in motion relative to something else. The general principle here is that the way something looks or behaves can be very different depending on how you and the thing are moving relative to each other.

    Einstein had a hunch this same thing applied to a magnetic field, that a "magnetic" field was really just a case of something being in motion relative to an electric field:


    In other words: there is no such thing as a magnetic field. Those things we call "magnetic" are, in fact, the result of something being in motion relative to an electric field. Or, equally valid, they are the result of an electric field being in motion relative to something else.

    In other words, if you and the field are at rest relative to each other, all its properties and behaviors are going to be those things we think of as "electric". If there is relative motion , though, the same electric field now exhibits "magnetic" behaviors.

    Now, having said "there's no such thing as a magnetic field," I have to contradict myself and point out that to the thing in motion relative to an electric field, all the magnetic effects it feels are quite real and definite. In the inertial frame of that thing, magnetic fields are quite real.

    So, what you see in a current-carrying wire is a very large herd of electric fields in motion along the wire having magnetic effects on anything which is at rest or moving differently relative to the herd. This is why a current-carrying conductor automatically has a magnetic field: because the electric fields traveling down the wire are pretty much guaranteed to be in relative motion to everything else in the vicinity.

    I hope that meets your challenge for a conceptual explanation a child could understand, least this physical theory be deemed worthless in your eyes.
  6. Nov 19, 2011 #5


    Staff: Mentor

    This quote seems to be misattributed:

    In any case, this is not a scientific criterion for physical theories.
    Last edited: Nov 19, 2011
  7. Nov 19, 2011 #6
    Not misattributed, but third hand (and therefore open to suspicion):

    Clark says that de Broglie said that Einstein said...
    True, of course.
  8. Nov 19, 2011 #7


    User Avatar

    Staff: Mentor

    An electric charge produces an electric field. Why does it do that? What actually causes the electric field?

    Newton's Third Law says that for every force there is an equal and opposite "reaction" force. What actually causes it?


    The point is that you can go only so far with "why" questions in physics. At some point you reach the limits of our current understanding. The fact that an electric current produces a magnetic field is the subject of one of Maxwell's equations which form the basis for classical electromagnetism. So asking why an electric current produces a magnetic field is equivalent to asking, "why are Maxwell's equations the way they are?"

    You might as well ask, "why are Newton's three laws of motion the way they are?"
  9. Nov 19, 2011 #8
    zooby - dude, gold star. lol i think i still need a nudge in the right direction in putting the pieces together though. ive been thinking about what you said (analogies REALLY help) and hopefully ill be able to post back tomorrow after work after ive thought it through some more. so thanks a bunch so far!!

    jtbell - first: "An electric charge produces an electric field." an electric charge producing an electric field is easier to accept than electric charges producing an electric field AND this new concept (for me) called a magnetic field. it really does leave a big gap to ask "where does it come from", which i believe is a question that should ALWAYS be asked---for every physical phenomena. i mean thats what science is all about and how progress is made within the field. (not that im hoping to make "progress within the field" with my current level of understanding, but hey gotta start somewhere)

    "At some point you reach the limits of our current understanding." im not sure why it seems difficult for a lot of physics enthusiasts to just say "we dont know". it really seems like many would rather say im asking a pointless question, rather than just admitting ignorance.
  10. Nov 19, 2011 #9
    To paraphrase Feynman, when you ask a "WHY" question, the answer is always in terms of some preexisting notion.

    Science provides a description of nature, not an explanation, certainly not a justification. There is no REASON why the law of nature are the way they are. They just are and we, who live in nature, try to write them down.

    So when you ask why, you have to also ask to what level are you willing to accept an answer?
    • That electric current create magnetic field is a observable phenomena for which we have a good description of?
    • That from SR, magnetism is just a different aspect of electricity, or more precisely, electromagnetism is one of the same thing. So the "reason" a current has a magnetic field is the same "reason" a charge has electric field.
    • That the laws of nature seems to have certain Gauge symmetry and electromagnetic field is the corresponding Gauge field: So the "reason" electromagnetism happens is due to certain symmetry of nature.
    At each level, you can keep asking why and it never ends. So yes, the WHY is ultimately pointless. The HOW, on the other hand, is useful because I can compare its prediction with observation. And science, unlike math, is only justified by observation.
    Last edited by a moderator: Sep 25, 2014
  11. Nov 21, 2011 #10
    Charged particles exert forces on each other and in motion those forces are reduced. As others here indicated, this doesn't need to be considered as necessarily due to a separate, magnetic field on top of an electric field; instead the magnetic field can be interpreted as a modification of the effective (electric) field strength due to motion. The electromagnetic equations are very similar to equations from fluid mechanics, and people such as Maxwell and contemporaries imagined all kinds of invisible fluid flows or even gear systems as models of what happens (and thus why it could happen that way).
    However, we can only see the effect and not the cause, and apparently there were too many possible models, so even Maxwell dropped those and just stuck with the equations which can be verified.
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