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B Why magnetic force is perpendicular to the field?

  1. Apr 3, 2016 #1

    Kyx

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    Why is the force on a particle perpendicular to the magnetic field? In electric and gravitational field, the force is in the direction of the field. Is it due to the fact that electric and magnetic waves in electromagnetic waves oscillate at right angles to each other, or something else?
     
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  3. Apr 3, 2016 #2

    mfb

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    There is an electromagnetic field. For historical and practical reasons, this is sometimes split up into an electrical field (leading to a force along the field) and a magnetic field (leading to a force perpendicular to the field), but those two are really just components of the electromagnetic field.

    There is also gravitomagnetism, with similar effects as a magnetic field but for gravity. It is completely negligible in everyday life as macroscopic objects around us do not move at relativistic speeds.
     
  4. Apr 3, 2016 #3

    Kyx

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    Thanks. That makes more sense than them being separate :)
     
  5. Apr 3, 2016 #4

    Kyx

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    Does this mean that the electromagnetic force is (B*Q*q*v)/(4*π*r20) ?
     
  6. Apr 3, 2016 #5

    mfb

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    No. You can express it with 4-vectors as$${ d p_{\alpha} \over { d t } } = q \, F_{\alpha \beta} \, \frac{d x^\beta}{d t} \,$$
    Here F represents the electromagnetic field, x refers to position and p is momentum. See Wikipedia for an introduction. It is unavoidable to include special relativity if you want to have a fully unified theory of electromagnetism.
     
  7. Apr 3, 2016 #6

    Kyx

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    Thanks :)

    And don't worry about relativity ;)
    I have recently written a 7 page essay proving it (special relativity)
     
  8. Apr 3, 2016 #7

    mfb

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    You cannot prove physical theories.
     
  9. Apr 3, 2016 #8

    Kyx

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    I explained it :p

    That would be a better way of saying it ;)
     
  10. Apr 4, 2016 #9
    Doesn't electromagnetic theory speak about propagating fields, like RF radiation? It seems clear you can have separate electric fields and magnetic fields, at least static fields, like a magnet which only has a magnetic field or a static electric field which has no magnetic field, or am I totally off track here?
     
  11. Apr 4, 2016 #10

    mfb

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    Electromagnetism can deal both with the general dynamic case and with static cases. If your whole setup is static, splitting the electromagnetic field into two components can be very useful.

    Note that a magnet has an electric field in reference frames that move relative to the magnet.
     
  12. Apr 4, 2016 #11

    Kyx

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    That is also why the photon carries the electromagnetic force?
     
  13. Apr 4, 2016 #12

    mfb

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    In quantum field theory, photons appear as useful descriptions for quantized electromagnetic fields, but that has nothing to do with the previous discussion here.
     
  14. Apr 4, 2016 #13
    One can construct roads that only go east-west, or only north-south, too. In general, though, it is better for a civil engineer to be able to specify the construction of roads in any direction.

    In reality, if one measures the electromagnetic field at an arbitrary point in spacetime, the components will essentially never be identically zero.
     
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