What ACTUALLY causes a magnetic field to be generated when there is a current?

  1. I have asked a few people this question and the most common answer is, "that just seems to be the way nature works." That definitely is not good enough. What is the physical REASON why a magnetic field is generated when a current is running through a wire?

    Thanks!
    ~Matt
     
  2. jcsd
  3. Pengwuino

    Pengwuino 7,118
    Gold Member

    That just seems to be the way nature works.

    Teehee. Really though, that's about as good as you'll get. Why does an accelerating charge produce light? Why does mass generate a gravitational field? The best physics can tell you is what mediates or mathematically describe what is going on. At some point you just have to accept something exists.
     
  4. Doc Al

    Staff: Mentor

    That approach limits you to only understanding things that can be explained in terms of things you already know. Doesn't hurt to ask, but often there's new physics to be found that cannot be understood using the old, familiar concepts.
     
  5. DaleSpam

    Staff: Mentor

    Out of idle curiosity, what sort of answer would be good enough?
     
  6. dx

    dx 2,004
    Homework Helper
    Gold Member

    Watch this video:
     
    Last edited by a moderator: Sep 25, 2014
  7. VERY interesting video! Man that is cool stuff. Thanks!
     
    Last edited by a moderator: Sep 25, 2014
  8. 1. Fantastic discussion, thank you for the question.

    2. Fantastic video. That guy... he's smart. And he brings up the natural question: if we don't yet understand why our hand doesn't go through the chair, then we're hopeless to know why there is a magnetic force.

    3. As an exercise for (super-) theoretical physicists, I would ask this question:

    How many arbitrary force fields could we assign to the universe which would obey the "conservation of energy" as we know it. What I'm saying is, since the electrical, strong/weak, magnetic, and gravitational forces are all "arbitrary" or "axiomatic" (as far as our understanding of them goes), I wonder how many different kinds of force fields there could actually be? Might a different universe have different force fields?

    I agree with you Matt. The equation F (magnetism) = qv x B is very curious. The equations for determining the magnetic field B are even more curious. Unfortunately, I don't see how we can ever get past the fact that our understanding of the universe is limited to our perception of it. We didn't come up with those equations on a chalkboard! We came up with them after years of rigorous, painstaking experimentation.
     

  9. To put in complicated terms..
    "consider the electron in the lab frame... when it moves you record a current and a mag. field..
    on move with the electron's reference... you can see ONLY a time dependent electric field..
    all electrons in the current will generate a superimposed field that will..show as a mag field.."
    ..this is a really confusing answer given by a certain physicist...

    the simplest explanation is...THE ELECTRON IS MOVING....since we know electric and magnetic fields are two facets of the same coin..from maxwell's laws..only seen completely in different frames.. it just depends on how you see it..if ur the experimenter..u'll see a force [qv B]...but while moving with the electron you will see a electric field..E(t)..
    that's my take on it..
     
  10. That is a very cool video and Feynman does a great job explaining dealing with the question "why" which science does not ask or answer. And Feynman show the problems in dealing with "why" questions. But you also need to note that Feynman is cheating just a bit because the original questions asked by the interviewer was really more along the lines of "how" does this happen and that is a different question from "why".

    If you ask "why is the sky blue?" or "why does a current generate magnetic forces?" An investigation into more and more details forces you further and further back in more "why" questions as Feynman explains in detail, until you are simply left with answers that the sky is blue because God made it that way! And then one has reduced a scientific question to a religious one.

    On the other hand if you ask HOW is it that the sky appears blue you have a different question. It is the one that Feynman is avoiding presumably because he doesn't want to say on TV that physics does not have "all the answers". And of course most people know that for light coming through the atmosphere we have a MODEL and by using that model we achieve an ANALOGY between that we do not know and that with which we are familiar. If we say the unknown acts like this other thing we know well, then we feel we have a certain handle on the unknown. And we do. For example, one can use our model to predict results in circumstances that we've never tried in an unknown phenomenon. That is downright practical! We still don't know "why" things act as they do but we do have a handle on "how" they act!

    Fine. So now when you ask the question what "actually" causes the magnetic field from a current, you really are asking "why does a current produce magnetic forces about itself?" Can't answer. "Why" questions are off-limits. Well, then, HOW does it do it? Well the usual approach is to flood you with mathematics. And that would be because the "model" that explains the phenomena is a mathematical model. But for you that isn't quite what you mean is it? Because a mathematical model is just an abstract creation with no anchor in reality. What you are asking is to understand the thinking of Maxwell who created the mathematics by using an analogy with the flow of fluids. And his model even invented a fluid to fit the bill they called "aether". And for a time people were satisfied with that as a "how".

    But as time went on and more things were measured, it was discovered that while the mathematics still predicts correct answers, the fluid model began to fail and soon it was totally rejected. Thus the problem from your point of view is that at present there simply is NO analogy that one can use for a model to "explain" how a magnetic field works. There are no "fluids" moving from here to there delivering forces or energy. There are no familiar mechanisms that we can recognize and instantly understand how a current delivers a magnetic force at a distance delayed by the speed of light. That model is today simply missing. For that matter we even know the mathematical model is not correct at all scales of measurement.

    So what is a "magnetic field"? Well, a field is a mathematical description of vectors over a region of space. In this case it is force vectors. So then a magnetic field is really just the "behavior" of those forces say about a current-carrying wire. But here's the fly in the ointment: Today there is no model to say HOW those forces arise. Therefore physicists make the illogical claim that these fields ARE "behavior"! They call them "behavior fields". Don't forget the forces are real and exist. So how does an object consist of only it's behavior? It's creating something out of nothing. It's illogical. It's as illogical as saying light waves are propagated without any medium. So the bottom line is that with no model, behavior alone is accepted as an "object".

    So what you've discovered is that scientists really don't like to stand up in front of the world and say "we really don't have any idea "how" that works. What they'll always say, is: "Here, let me tell all the things we DO know about that, whether relevant to your question or not!"

    But don't expect science to hand it all to you on a silver platter. If there's no model that's working for some phenomenon, then it's up to YOU to think about it and try to understand HOW it all could work. Maybe you'll be right. Maybe you'll be wrong. But that is what science is all about.
     
  11. I wouldn't say it's quite fair to characterize the aether concept of Maxwell and other prominent scientists as being a type of fluid. I think they all very much realized it could not be a fluid as we know them. Nor some type of super-gas which had been mentioned probably more often in those days. But the differential equations describing EM have pretty much the same form as for fluid behavior.

    Maxwell and subsequent physicists actually built mechanical models of the aether using cog wheels and belts and such. They obviously didn't confuse them with the actual behavior of the medium but used them as a crutch to form a more concrete idea of the stresses and movement of energy.

    Stress is probably the most powerful word to apply to magnetic behavior. Moving a charge causes flow of energy into a region of the medium and therefore causes a stress or build up of potential energy there. Nature urgently wants to remove stress whenever possible (just like us). So apparently she counteracts by venting the energy away from the built up region by forming a counterflow of magnetic energy.

    It wouldn't work to have a counterflow in the opposite direction as that would only increase or double the stress in the region that energy has moved from. So nature's only recourse is to create a neutralizing flow at 90 degrees.
     
    Last edited: Mar 31, 2010
  12. the first two answers gave you quite the best explanation.
    the magnetic field was and still is a miracle of nature because it does not exist so often in nature. and the only thing you could explain about it is why on some places there is a field and on others not. the best answer so far, after being considered by great minds as faraday maxwell and who else not is that when charge is not moving there is no mag field and if it does there is. and thats the most natural explanation there will ever be because movement is sort of a structural basis for everything we experience.
     
  13. By the way Matt, occasionally there is a post or two here encouraging a person to dig deeper and investigate things that have no clique answer. That's really how science works when it is functioning. I think you realize that you do not have to belong to the old gentleman's complacency club. I hope you never settle for less than your perceptive mind tells you it should.
     
  14. Thanks a lot for the video man !
     
  15. VERY good and informative answers everyone! This kind of thing is what fascinates me so much in Physics!!
     
  16. Certainly not an "ordinary" fluid, but interestingly enough, Maxwell's equations are based upon the model of an incompressible fluid. There are people even today who with some justification say that an electric current should be modeled as a compressible fluid. They point to the effects of ultra high currents on metal conductors as some justification for this view. I'm also guessing that the success of Maxwell's equations pretty much says that a gaseous compressible Aether was not a good choice.

    And let me add that Maxwell's mechanical model of an electrical transformer is quite a thing to behold! You'll find it on page 228 volume II of the Dover edition of Maxwell's Treatise on E&M. [Maxwell's section 584] It is an excellent "crutch" or what I'd call a "thinking tool" to help understand the rather strange behavior of transformers. But then "thinking tools" are really what all physical models really are!
     
  17. Well, in order to explain correctly what is magnetism, you need relativity. Math is a bit hard, but the concept is easy:

    Coulomb's law is only valid for electrostatics. When you move a charge, part of the electrical field is "transformed" to magnetic force. It's like the components of a vector when you rotate your referential, part of one component is transformed into another component.

    So, in essence, electricity and magnetism aren't as far apart as it sometimes seems to be when you look at the equations.
     
  18. I might be reading too much into this question, but it seems like the OP was trying to understand the fundamental cause of magnetism. And for the record, to the OP and to everyone else, "that's how nature works" is always a horrible answer because it doesn't tell you anything. You're just stating the obvious. "Magnetism has something to do with the alignment of spin" could have been a better starting point than the dismissive tone the OP encountered...
     
  19. Cleonis

    Cleonis 691
    Gold Member

    As Richard Feynman points out: taking explanation to the next level is possible only if the questioner has mastered the physics that is involved in that deeper level.

    It has been shown that special relativity unifies electricity and magnetism: it is possible to describe magnetic force as a relativistic side-effect of the Coulomb force.

    Daniel Schroeder has created a webpage with an exposition of how magnetism arises as a relativistic side-effect.

    On that page there is also a link to a 39-page text supplement
    Magnetism, radiation and relativity notes
     
  20. You can derive the Maxwell equations and the Lorentz force law from only Coulomb's law by demanding Lorentz invariance. So, like some others have suggested here, the answer is Special Relativity.
     
  21. Well, if you're willing to accept on faith the fact that stationary charges produce electric forces on other stationary, and the principle of relativity, then you can actually prove that moving charges must create another kind of force on other moving charges. We call this the magnetic force. (Yes, I know you may quibble with me about quantum effects like ferromagnetism, but I'm speaking classically.)
     
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