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Why is there magnetic force?

  1. Jul 28, 2012 #1
    This question has been bothering me and i haven't been able to find a satisfactory answer anywhere.

    A moving charged particle in a magnetic field experiences a force that is perpendicular to both the velocity of the particle and the direction of the magnetic field. My question is why is there such a force?

    I read somewhere that the moving charged particle creates a magnetic field, which interacts in the magnetic field the particle is placed in. The interaction of the two fields creates a situation where magnetic field lines are closer together (lets assume at the bottom of the particle), and further apart at the top. There is then a force that causes the particle to move up.

    Is this just a way to visualize the presence of magnetic force? Or does it explain magnetic force?

    If this is just a visualization (and not an explanation), then is there an explanation to why there is magnetic force, or is it just "There is a force"?
  2. jcsd
  3. Jul 28, 2012 #2


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    Staff: Mentor

    Physics cannot answer "why". It can explain how, and it can do so with several layers of abstraction, but if you keep asking "why" at each step, you will eventually see that it is just an experimental observation.

    You can derive the magnetic force in quantum electrodynamics, but the theory was just made to describe the electromagnetic interaction.

    "Moving charges generate a magnetic field, a magnetic field applys a force on moving charges" is just a description of the process, not an explanation.
  4. Jul 28, 2012 #3
    It's a relativistic effect. A magnetic field is really just an electric field that is lorentz tranformed.
    e.g. imagine two electrons moving parallel to each other somewhere in empty space. The electrons are repelling each other so their distance will start to increase more and more. But if an object is moving, time slows down. That's called time dilation and it is a form of lorentz transformation. That means the faster the electrons are moving relative to an observer the more slowly the electrons will move away from each other. The observer could now explain their observation with time dilation. But they could also claim that the moving electrons are creating a magnetic field that causes the electrons to attract each other, thereby partially cancelling out the repulsion.
    Both models - time dilation and magnetic fields - give a valid explanation for what is observed. So both models are correct. Theoretically you could get rid of magnetism and explain all of electromagnetism just with electric fields and lorentz transformations but that would be much more complicated than using the more simple model of magnetism.
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