Interaction Between Magnetic and Electric Field Lines

  1. Drakkith

    Staff: Mentor

    Hey all. I was wondering what the interaction between Electric and Magnetic field lines are. For example, when you hold up a magnet against a charged electrode. I tried searching in google, but everything just turned up one or the other or how a moving charge makes a magnetic field and etc.
  2. jcsd
  3. It's all about relative motion. If the electric field (or charged particles) and the magnetic field are not moving relative to one another then their interaction is governed by the rules of Electrostatics. If there is relative motion then their interaction is governed by what used to be known as Electrodynamics, now commonly referred to as Electro-Magnetism in acknowledgement of the intimate connection between electricity and magnetism.

    Einstein's original paper "Zur Elektrodynamik bewegter Körper" ("On the Electrodynamics of Moving Bodies") is what became known as the Special Theory of Relativity.
  4. Drakkith

    Staff: Mentor

    I don't really know what you mean by that. Are you saying that a static magnetic and electric field act just like two electric fields?
  5. I like Serena

    I like Serena 6,181
    Homework Helper

    In a static situation (like holding a magnet near a charge) there is no interaction.

    Electro-statics and Magneto-statics are completely independent from each other.
    The electric field is completely determined by the static distribution of charge (Gauss's law).
    And the magnetic field is completely determined by static current (Ampère's circuital law).

    However, when the electric field changes or the magnetic field changes, they start to interact (Maxwell's extension to Ampère's circuital law and Faraday's law).

    So if you wave your magnet near an electric charge, things become more complicated.
    (Like waving a magic wand! :wink:)
  6. Drakkith

    Staff: Mentor

    What exactly do you mean by no interaction? The magnet wouldn't repel or attract the electrode?
    Do you happen to know where a diagram of the field lines for both the electric field and magnetic field in the same picture is?
  7. I like Serena

    I like Serena 6,181
    Homework Helper

    Yes, that is what I mean.

    The only diagrams I'm familiar with that show both fields, is where it is explained how light works.
    Like this one: 122/light-electro-pictures/electromag-wavelength.jpg
    Last edited: Jun 29, 2011
  8. Drakkith

    Staff: Mentor

  9. I think what the OP is getting at is what if you have a solenoid, and place an infinite charged plane parallel to the solenoidal axis? Then isn't there a nonzero Poynting Flux (ExB)? But how can this be if the fields are static?
  10. Drakkith

    Staff: Mentor

    Yeah, that's a good example. I don't know about the infinite plane though. How would a realistic electrode, like about the size of the solenoid, react?
  11. I don't know the answer to this. Here's a pdf that has the paragraph:

    "At carefully examination of Maxwell’s equations and sequence of their conclusion (for example, [1]) it is visible, that these equations describe only variables electric and magnetic fields which cause each other. Nevertheless, many authors (even in textbooks on the physics for high schools) try to apply them and, in particular, Poynting’s vector to stationary fields. It leads to absurdity. For example, flowing of energy of a field from an environment to a conductor with a direct current, or existence of some stream of energy about located close each other a constant magnet and the charged condenser."

    and the last part about a stream of energy located close to a constant magnet and charged condenser is what I thought you were asking.

    I do know that it is true that the energy of a field flows into the conductor of direct current. Surrounding a wire are B-field circles, and inside the wire are E-fields, so ExB suggests that energy is flowing into the wire. That makes sense because the wire is guiding electromagnetic waves into the resistor, thereby heating it up. But having a constant magnet and charged condenser confuses me. If you orient them so that E and B are perpendicular, then there should be an energy flow? Are they interacting somehow to give an energy flow?
  12. Drakkith

    Staff: Mentor

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