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Relationship between electricity and magnetism

  1. Apr 14, 2009 #1
    Hey, first of all, I wanted to know that if you pass an electric current through an iron rod, for example, then what would be the relationship of it's electric field and magnetic field? Will they be parallel to each other or perpendicular?:confused:

    Second: suppose to fire a particle, let it be negatively charged, for argument's sake in a space where the electric field and the magnetic field are in the same direction, then describe the path of the particle? what if the electric field is perpendicular to the magnetic field? what if the electric field is in the opposite direction to the magnetic field?:confused:

    Third: If the photon is the fundamental electromagnetic force carrier, then why doesn't a magnetic field comprise of photons? Same for an electric field? If it has nothing to do with either, what then, are the magnetic fields and electric fields made of?:confused:
  2. jcsd
  3. Apr 14, 2009 #2
    These are interesting questions. Let me give them a try.
    If a dc current is along the rod, then the electric field of the charge in the current is radially outward from the rod, and the magnetic field from the current is azimuthal (circles) around the rod. This is the same for an iron or copper rod. If the current is an AC current, the current at higher frequencies (above about 100 kHz) is forced toward the outside of the rod, and flows in a layer called the skin depth.
    The force on a charged particle is along the direction of the electric field. Since the force can be parallel to the particle direction, the particle can gain or lose energy (energy = force times dx). On the other hand, the force on a particle in a magnetic field is perpendicular to both the magnetic field and the particle direction. Because the force is always perpendicular to the particle direction, the particle never gains or loses energy. If the electric and magnetic fields are perpendicular to each other and perpendicular to the particle direction, then the force on the particle will either add or subtract. For a given particle velocity, there is a ratio of the electric and magnetic fields where there will be no net force on the particle. This is sometimes called a velocity filter or crossed-field filter.
    Photons are comprised of both alternating magnetic and electric fields. These alternating H (magnetic) and E (electric) fields are perpendicular to each other and perpendicular to the photon direction. Radio waves are comprised of low energy photons. Visible light are shorter wavelength than radio waves, but longer than x-ray photons. So perhaps the fundamental electromagnetic quantities are the electric and magnetic fields, and not photons.
  4. Apr 14, 2009 #3


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    I think it's better to say that photons on the one hand, and alternating E and B fields on the other hand, are alternative models of an electromagnetic wave. The first is based on quantum electrodynamics, the second is based on classical electrodynamics. It's not safe to say that photons are simply tiny bundles of classical E and B fields, such that if we put a lot of them together, the fields add up to give a classical electromagnetic wave. The relationship is more complicated and subtle than that.

    Quantum electrodynamics is "more correct" because it makes correct predictions in situations where classical electrodynamics makes incorrect ones; but we still use classical electrodynamics a lot because it's much harder or even impossible (so far) to apply quantum electrodynamics in many situations that classical electrodynamics gives good enough results for (which is to say most of practical everyday electromagnetism).
  5. Apr 14, 2009 #4
    Then would a light (photon stream) be deflected in an electric field? how about a magnetic field?
  6. Apr 14, 2009 #5


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    No. Photons are indistingishable particles and do not interact with eachother.
  7. Apr 14, 2009 #6

    No, i was just asking about what would happen to light if it was introduced in an electric or a magnetic field...:rolleyes:
  8. Apr 15, 2009 #7


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    Photons are the energy/momentum packets of an electromagnetic wave. Electric and magnetic fields and photons are one of the same (though real photons only exist with the EM waves). As far as I can recall, fields and waves do not interact with each other either classically or quantum mechanically.
  9. Apr 15, 2009 #8
    okay that clears it. But now i wanna know WHY it doesn't interact!!!:bugeye:
  10. Apr 16, 2009 #9


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    Classically, they do not interact because for the most part fields and waves follow the rule of linear superposition. Barring the interaction of the fields/waves with sources, the total field/wave is the summation of all the individual sources. The addition of one source does not affect the fields/waves produced by any other source(again barrng the interaction of the external fields/waves on the source itself). Quantum mechanically, photons are indistinguishable bosons while particles like protons and electrons are indistinguishable fermions. Pauli's Exclusion Principle states that no two identical fermions can occupy the same state. This is why you have the binning of electrons in the various orbitals in an atom, as you add more electrons to an atom you have to put them into unique orbitals or states. But photons, being bosons, do not follow this exception, they can occupy the same states. If we confine two photons to a potential well, the ground state energy is the same as the case if we confined one photon. But if we have two electrons, the ground state energy has changed from the case if we have only one electron.

    Entanglement can occur due to the fact that photons are indistinguishable. We have to have a wave function that does not commit to identifying the photons to a particular state. You must have a wave function that is the summation of all the combinations of states possible.
  11. Apr 16, 2009 #10


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    Photons are not charged particles, they don't interact electro-magnetically. Virtual photons are the force carriers, not actual photons. They are different.
  12. Apr 16, 2009 #11
    Thanks a lot to all you guys! That cleared things up really well!:approve::smile:
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