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Question about Fields.

  1. Aug 4, 2011 #1
    I was told that when I bring 2 bar magnets together and their field lines alter, that it was because the field of one affects the matter of the bar magnet and alters its own field. And that the fields themselves don't alter each other.
    If this is right how does this work with gravity. If I bring 2 massive planets next to each other how does the field of one affect the matter of the other one?
  2. jcsd
  3. Aug 4, 2011 #2
    It will affect the matter by imparting on it a force. Magnetism is different to gravity in many ways, so the interaction between the field of object A and object B will be different.
  4. Aug 4, 2011 #3
    ok thanks for your answer, but fields don't interact with other fields directly.
  5. Aug 4, 2011 #4


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    MikeyW that certainly makes sense but doesn't seem to answer the question in any way.

    cragar, I don't know the physics of magnetic materials so could be wrong but this SOUNDS completely wrong to me.

    Look at it this way (also COULD be wrong). Magnetic fields are generated when you run a current through a wire. Do that in two wires put close to each other. The fields change each other. I don't think this is because of any change in the matter inside the wires I think it's a direct interaction between the fields.

    Again, this is NOT something I'm expert in so I welcome comments by more knowledgeable people.
  6. Aug 4, 2011 #5


    Staff: Mentor

    Gravitational fields do. The EInstein field equations are non linear.
  7. Aug 4, 2011 #6
    So gravitational fields interact with each other. When you say they are not linear does that mean that they don't obey linear superposition.
    And if E fields interacted with each other then i could alter the E an B components of an EM wave, which im pretty sure we cant do.
  8. Aug 4, 2011 #7


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    hi cragar! :smile:
    i don't think that's correct …

    don't the individual field lines stay the same, but when we "add" them, we get new joint field lines?
  9. Aug 4, 2011 #8


    Staff: Mentor


    Maxwell's equations are linear, so that is a big difference between gravity and EM.
  10. Aug 4, 2011 #9
    You misread my response, it should be read as "the interaction between (the field of object A) and (object B)". The field of object A will influence object B and the field of object B will influence object A. The total field is the superposition of the two individual fields (at least in classical physics) and the field of an object does not affect the object itself.
  11. Aug 4, 2011 #10
    When you say it does not affect the object itself, are you taking about self interaction or other objects?
  12. Aug 4, 2011 #11

    What do you mean by this??

    electric and magnetic fields interact....say in a transformer....

    Here is an example of ferromagnetic interaction.....


    Another simple example would be placing a magnetic dipole in a magnetc field...it experiences a torque T = u x B....so the force (torque) is magnetic field (B) dependent.

    If such realignments don't take place via field forces it would seem to be "spooky action at a distance"...

    If there is an alternative quantum description I'd sure like to see it. I think gravity is the exchange of gravitons and electromagnetic interactions the exchange of photons....each are the quanta of the respective fields.
  13. Aug 4, 2011 #12
    (Cragar: This may be different than an expected answer, but sometimes is useful.)

    Let's see: the gravitational [potential] energy is lowered. [Nobody would notice anything different locally, but distant observations would be affected. Time slows a bit relative to distant observers, for example.] Entropy increases. Also the rotational kinetic energy of each planet is increased...they will accelerate and begin rotation about a new center of rotation....but because gravity is so weak relative to nuclear forces, the atoms pretty much stay in place as do electron orbitals....unless the gravitational forces are really massive and one planet rips the other apart.
  14. Aug 4, 2011 #13
    What do you mean when you say that E and B fields interact in a transformer? My time varying B field inside my solenoid creates an E field and that E field pushes the electrons in the secondary coil. What do you mean by interact. And if the fields themselves interact then why can't I alter a photons E and B component with an external E field.
  15. Aug 4, 2011 #14
    I meant the fields of the electron in the secondary and the field from the primary windings were interacting.

    I was neither opposing nor supporting your statement that:

    I am simply trying to understand your perspective. I would have thought "of course" but "interact directly" may have a different meaning to each of us.

    I was trying to think in concrete terms when I posted examples above (picked pretty much at random) magnetic dipole alignment, hysteresis and transformer....

    The electric field at a point is different if there are two electrons instead of one being considered, right? In general, it has different magnitude and different direction....so to me the field has changed....but [as in standing waves, say in a transmission line,] maybe you are wondering if the component waves remain the same?? I never thought about that...or have forgiotten if I did!


    Wikipedia says:
    so now I am wondering why it couldn't say instead...."....how fields of matter interact with each other.."

    Regarding Dalespams post, the non linear aspect of gravitational waves reflects their interaction, alteration, of one another....that's what requires the tensor formulation...As he posts, electromagnetic waves ARE linear...they obey simple superposition as noted above.
  16. Aug 4, 2011 #15
    I just think that E and B fields will only alter the paths of charged particles or particles with magnetic moments or matter. Not alter things that just have E and B fields like light.
  17. Aug 5, 2011 #16
    That seems ok in classical mechanics...Maxwell's equations, I'm just not positive; but don't the atoms of all materials have magnetic moments??

    yet we know photons have momentum for example...so it would seem to me whan an uncharged particle encounters the momentum of an electromagnetic wave, something must happen.

    I read this which may have some insights, but I'm just not sure:


    It would seem the underlying math described here should have an answer:

    Anyway, I've gone as far as I can..maybe an "expurt" will join....Interesting issues.
  18. Aug 7, 2011 #17
    If we could alter the E an B field of a photon with another field and create a disturbance in its field then could we make a photon emit another photon. I know that sounds crazy but isn't an EM wave a disturbance in an E or B field. Thats also why I don't think we can alter an EM wave with another field.
  19. Aug 7, 2011 #18


    Staff: Mentor

    Only if they interact. Which they don't.
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