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Fields Confusion (Electro/Gravit etc)

  1. Jan 12, 2010 #1
    This is most likely a complete failure of comprehension on my part (as ussual), but Ive been thinking about this for the past while and have come to a unsatisfactory conclusion.

    In reference to an electro-magnetic field or a gravitational field, where do the particles get the energy to be attracted or pushed away?
    I know they have a POTENTIAL to do so, but where does that potential energy come from in the first place?

    Refering to gravitational fields, is it just because of a particular spacial configuration? Thats obvious in the sense that- we have 2 masses and they attract, you can give one potential energy in respect to the other by applying a force etc.
    However back to the original question where does that impeding attractive force come from to begin with?

    I reckon there is something fundamentally wrong with my understanding or this is edging on phillosophy. The concept of a potential field around say a static charged particle, a magnet or a large mass is a way of describing what is happening without saying where the energy came from.

    I will be very grateful if anyone can clear this up, or shead more light on it.

    Thanks.
    LTF.

    EDIT:
    We don't really know WHAT energy is in an absolute sence apart from little definitions. So I guess my question is edging philsophical ground. And the concept of potiential is indeed useful. I just don't like not knowing :P.
     
    Last edited: Jan 12, 2010
  2. jcsd
  3. Jan 12, 2010 #2

    Matterwave

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    The gravitational force, determined by Newton's law, and the EM force, determined by Coulomb's law.

    These are fundamental forces of the universe. Where they come from...is ambiguous. They are properties of our Universe.
     
  4. Jan 13, 2010 #3
    And the energy is just a definition, an ability to do work. So if it can do work it has energy (potential or otherwise).
     
  5. Jan 13, 2010 #4
    Yeah, I guess I'm thinking about it too much.

    Thanks for setting it straight.
     
  6. Jan 13, 2010 #5

    Dale

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    For EM I think a bit more can be said. An EM field itself has energy. Two oppositely-charged particles are attracted to each other because as they get closer more of their fields cancel, reducing the amount of energy in the field. Two like-charged particles are repelled for the same reason.
     
  7. Jan 13, 2010 #6
    I think something's wrong here.
    The energy density of the field is proportional to the square of the electric field, the electric field only gets stronger with the approach of oppositely charged particles.
    I can think of a proton and electron separated by a distance, an from the frame of the proton, the field line directed to the electron does not pass to the other side, nd the electron's field itself is pushing it in from there.
     
  8. Jan 13, 2010 #7

    Dale

    Staff: Mentor

    The fields exist over all space, not just over the region between the two charges. In that region they add, in other regions they cancel, the net effect overall is a decrease in the field energy.
     
  9. Jan 13, 2010 #8

    Born2bwire

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    While it is true that in the very near field the proximity of opposite charges enhances the field, in general the field suffers. For example, the monopole has a field that falls off as 1/r^2, the dipole 1/r^3, the quadrupole 1/r^4, etc. So when r << 1, the field will appear enhanced but otherwise the fields become more and more short-ranged, decreasing the total energy in the fields when you integrate across the entire volume of space. In addition, the strength of the multipole increases with distance between the charges. So charges that are closer together only diminish their moment.
     
  10. Jan 14, 2010 #9
    Good answer. The late Philip Morrison suggested many years ago that the only way this energy book keeping could also be accomplished in the case of gravitating (attracting, like-signed) masses would be if the gravitational field is mathematically imaginary. In that case the square of g would be NEGATIVE.
     
  11. Jan 14, 2010 #10
    Thanks again, alot of good responces.

    I was thinking about it too far, philsophically; which sometimes is bad.

    In short, the fundamental forces of nature in a sense are the innate ablity of particles/objects with particular properties (charge, movement, mass etc) to do work. This ablity we define as energy.

    Would this be correct? I know that definition could be alot more rigourous but just getting it straight in my head.
     
  12. Jan 15, 2010 #11
    You've got it straight. Don't listen to the field energy argument, although it is true, it doesn't help. In those situations work is still being done and that is the key.
     
  13. Jan 15, 2010 #12

    Dale

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    It helped me when I had the same thought.
     
  14. Jan 15, 2010 #13
    I'll apologize for that comment, it was self-centered. I really should have said something more like 'don't feel alone if thinking of it that way doesn't help, it never helped me'. Again, apologies.
     
  15. Jan 16, 2010 #14

    Dale

    Staff: Mentor

    No problem, I wasn't offended or anything. I think that is one reason that it is good to have multiple (correct) responses to a single post.
     
  16. Jan 16, 2010 #15
    I agree completely! But, again, that is why I apologized, I was a douche. :smile:
     
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