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Electric/Magnetic vs. Electromagnetic Fields?

  1. Aug 24, 2008 #1

    Former physics student here a bit rusty on his electromagnetic and quantum theory. I recall that an electric field in one frame of reference has a magnetic field component in another frame of reference, so E and B field are ultimately products of the same phenomenon (photon exchanges between matter). I also recall that an electromagnetic wave, like visible light, for example, possesses both E and B-field components oscillating orthogonally to one another. And yet EM waves don't interact with, say, a charged particle the same way an electric field does (shining light on a charged sphere doesn't exert a force on it the same way that exposing it to an electric field would). With this in mind, here are my questions:

    1) Is there a real distinction, either classically or quantum mechanically, between a magnetic field, as elicited from a moving current, for example, and an electromagnetic field, emanating from some radiating object, if all such fields apparently arise from photons according to the Standard Model?

    2) If EM radiation (light, for example) always has both E and B-field components, why don't these components interact with charged matter in the same way the electric field from another charged object does if all three phenomena describe are essentially the same? I know in some frame of reference, one observer's magnetic field is an electromagnetic field for an observer in another frame of reference, and yet I'm still confused as to the apparent discrepancy in how light, X-rays and radio waves (invariant regardless of their frame of reference according to STOR from what I remember), for example, are apparently distinct from what might appear to be a "pure" electric or magnetic field in my frame of reference?

    I'd appreciate any insight from those of you who are a bit more up on their physics than I am, and please feel free to point out any invalid assumptions I've made in the questions themselves.

  2. jcsd
  3. Aug 26, 2008 #2
    Any takers on this?

  4. Aug 26, 2008 #3


    Staff: Mentor

    Hi frogthoven, Welcome to PF!

    I think your confusion is due to a faulty premise. Light does in fact exert force on charges, this is the basis of the photoelectric effect that operates all photovoltaic devices and other related phenomena. Light carries momentum and a force is a change in momentum, so light exerts force on charges.
  5. Aug 26, 2008 #4
    light moves electrons in a conductor. thats why its reflected.
  6. Aug 26, 2008 #5
    Hi guys,

    Even though his first assumption may not be completely correct, his questions are still valid.

    If you're talking about the physical concept of magnetic field, then it is the very same magnetic field in both cases. The only difference in nature is that the one from the 'moving current' (by which I assume you mean to say 'moving charges or 'constant or direct current') produces a static magnetic field, while EM waves involve changing magnetic fields.

    Modelling the interaction of light with matter has always been a touchy issue. Sure, the reason light interacts with charges such as electrons is because of its magnetic and electric fields (Lorentz force). However, these fields are complex and changing as opposed to the easily calculated forces due to static 'pure' fields. Furthermore, it is impossible to actually model the photoelectric effect using the theory of electromagnetic waves, as an important part of history describes (Einstein's photon).

    Hope that helps.
  7. Sep 1, 2008 #6
    Thank you to all of those who replied. I look forward to participating in the forum in the future.
  8. Jan 18, 2011 #7
    What do I need to do to move a 3 lb object created by an electromagnetic field. I would like to move a small car generated by a electromagnetic field within a 15 second. please help.
  9. Jan 19, 2011 #8


    Staff: Mentor

    I would recommend an electric motor. That is a device designed for the express purpose of moving things using an electromagnetic field.
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