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I What is actually happening to a charge in a magnetic field?

  1. Jan 14, 2017 #1
    Hi everyone, I was talking to a friend of mine today who's studying electrical engineering and he essentially asked me why a charged particle feels a force when moving through a magnetic field. I thought about it for a moment, and realized I didn't have a good answer for him. Would anyone be willing to help me understand this or know of any resources where I could read about what's actually physically happening to a charged particle as it moves through a magnetic field? I'm curious to know the answer as well.

  2. jcsd
  3. Jan 14, 2017 #2


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    HI Travis
    welcome to PF :smile:

    I don't think the why question can be answered, it's just one of those things in nature
    Physics instead describes the result of the charged particle moving in a magnetic field ... the is the Lorentz Force

    There masses of info on the www about the Lorentz Force, here is one of them


  4. Jan 14, 2017 #3
    Ah ok, that explains why I wasn't really able to find anything regarding the interaction beyond the resulting forces we observe. Thank you for the quick reply!
  5. Jan 15, 2017 #4
  6. Jan 15, 2017 #5


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    This question needs a more thorough background. For example, does this friend of yours have a same question on why a charged particle feels a force in an electric field? Since this was not asked, one may assume that he doesn't. So why is that? Why does he (and even you) have no problem with a charge having force in an electric field, but then have a problem understanding the force it has when moving in a magnetic field. After all, BOTH came out of the same Lorentz force equation!

  7. Jan 15, 2017 #6
    I suppose that the force being subject to a cross product is what sparked his curiosity revolving specifically around charged particles in magnetic fields. You're right though, that is another question that came to mind and made an appearance in our conversation. Perhaps I need to work on asking complete questions.
  8. Jan 15, 2017 #7


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    But why would the appearance of a cross product be an issue?

    You may think, for example, that F = qE is "easy". But it really isn't, because "E" can be nasty as well. And who is to say that this is where we should begin. After all, E can be written in a nastier form of Coulombs law where it may involve a 3D integral. So having a cross-product is the LEAST of our worries here.

    The problem in the question is the use of the word "why". At the most fundamental level, physics deals with "how", not "why". Other things at the higher level may have a "why" by employing mechanism more fundamental than them, but if you dig down deeper and deeper, there is no "why". There is only how.

    This is why I asked about the force in electric field and why you guys aren't having the same question. While they may be slightly different mathematically, physically there is no difference. A force is a force, of course, of course.

  9. Jan 15, 2017 #8
    So, correct me if I'm misinterpreting your response, this becomes a question of how a force is exerted on a particle in a generalized sense?
  10. Jan 15, 2017 #9


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    That is part of it. But the other part is a human nature question. What makes it so that you asked specifically about the magnetic field? How come you find that field strange but not the electric field? From a physics standpoint they are equally weird and inexplicable, but for some reason people seem to "grok" the E field easier.
  11. Jan 17, 2017 #10
    Perhaps we found attractive and repulsive forces a bit more intuitive due to it being something we were taught at a much younger age, as opposed to cross products not being introduced until freshman physics.
    At any rate, this certainly helped me redefine my question so that google may be of more use. I found the wiki page on "force carriers" which looks like it might be a good place to begin reading about this.
  12. Jan 17, 2017 #11
    I had my cousin ask me similar question once - what is EM wave? So I try and give him a usual explanation (which he was already aware of), but that wasn't enough.
    How can it propagate through vacuum? What is it that physically "wiggles" or "vibrates" that produces the wave? I could not give him a satisfying answer.
  13. Jan 17, 2017 #12
    I cannot speak for OP, but perhaps electric field feels more natural because of the similarity to the gravity? Since birth, we accepted that Earth is round and pulls us to its centre; Sun pulls the Earth, etc. Imagining a point and a vecor radially towards it becomes natural. Magnetic force on the other hand feels a bit more complicated (to me at least).
  14. Jan 18, 2017 #13
    This will help you for detailed information:

  15. Jan 18, 2017 #14


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    This is vague. More "complicated"?

    I still don't understand the issue here, but since you based your understanding (or at least consider things to be "less complicated") on FAMILIAR situations, let's try this.

    The magnetic force on a charged particle has two characteristics based on what we know from the Lorentz force law: (i) the force is always perpendicular to its direction of motion, and (ii) it depends on its velocity.

    But are these two characteristics THAT unfamiliar?

    The centripetal force for a uniform circular motion has a direction that is always perpendicular to the particle's direction of motion. I don't hear people saying that this is "more complicated".

    The drag force when an object falls through air has a direct speed dependence. Again, I don't hear people saying that this is "more complicated".

    These are all familiar forces that never appeared to be giving people problems in understanding or conceptualizing, and yet, together, they have similar characteristics to the magnetic force on a charged object.

    Here's the thing about physics. You simply can't point out an issue or a "problem" simply based on personal preference or a matter of taste. There has to be a more rational reason for something to be problematic. Otherwise, we will be arguing about a favorite color.

  16. Jan 18, 2017 #15
    I understand, this was poor choice of words on my behalf. I was trying to explain why I (as a student) find one concept less intuitive than the other.
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