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Electric/magnetic lines of force's property.

  1. Jun 10, 2009 #1
    There are certain properties of lines of force, I know they are not real, but following these 'properties', given position and intensity of magnetic/electric field sources we will be able to reconstruct these lines without any practicals.

    So I have a problem with lines of forces.

    Sources say -

    Assuming 2 sets of lines for forces emerging from any 2 plate sources (so as to make the lines parallel to each other), the lines which are parallel (i.e having the same direction of propagation) will repel each other, and those which propagate in the opposite direction will try and merge with each other.

    Diagrammatically, lines parallel to each other -


    Will repel, and those having different direction of propagation -

    -------------------> <-------------------
    -------------------> <-------------------
    -------------------> <-------------------
    -------------------> <-------------------

    Will attract and try to merge with each other.

    I do not find this true.

    Suppose, in this case, suppose both the plates are positively charged -

    |---> <---|
    |---> <---|
    |---> <---|
    |---> <---|

    Although the direction of propagation of these lines are against each other, they will repel, similarly, putting a negatively charged in front a positively charged one -

    |---> --->|
    |---> --->|
    |---> --->|
    |---> --->|

    Though these lines are parallel, they will try and merge with each other.

    This is against what all the sources say.

    So I say -

    Electrical lines of forces will repel each other if its origin is from the same polarity of charge, else they'll attract...which appears to be working fine...
  2. jcsd
  3. Jun 10, 2009 #2
    Although this does not exactly address the parallel plate situation posed by the OP, here is an applet for showing the field lines for two proximate equal or opposite charge point charges. You can use your mouse to drag the charges around, and vary the the separation.
    here is another
    http://qbx6.ltu.edu/s_schneider/physlets/main/efield.shtml [Broken]
    Last edited by a moderator: May 4, 2017
  4. Jun 10, 2009 #3


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    There is no such thing as lines of force. The contradictions that arise here should be obvious.
  5. Jun 11, 2009 #4
    You mean those properties are just rough?

    I cannot view the applet....
  6. Jun 11, 2009 #5


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    What sources?
  7. Jun 11, 2009 #6
    Last edited: Jun 11, 2009
  8. Jun 11, 2009 #7


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    No, draw me a diagram for the lines of force for a magnetic field. Or a system with two charges after a finite amount of time has passed. How about for a charged particle that is accelerating or moving at relativistic speeds?
  9. Jun 12, 2009 #8

    God damn...where do the sources go when you need them?? :mad:


    Page 14.

    And various e-books.

    Issue resolved...after installing the 'missing' plugins which firefox recommend and RESTARTING the browser (which I was not doing initially)...the issue resolved.

    BTW that link is Microsoft savvy...I'm on Linux.

    So...after looking at those applets, yeah, I understand lines of forces, and can recreate them with accuracy...like I did in OOo draw and inkscape.

    I cant do this one...cause my M.F concept is very much rocking......cause right now I don't know E.F itself.

    So what exactly is the conclusion?...are the sources wrong?
  10. Jun 12, 2009 #9


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    There is no such thing as general lines of force for electric/magnetic fields. The lines of force for an electric field are different for a positive charge than it is for a negative charge. The lines of force for bar magnets due to magnetic fields are different than the lines of force for a negative charge at rest which are different from a positive charge moving in +x which are different from a negative charge moving in -y, etc. The force on charged particles from electric and magnetic fields is defined by the Lorentz force, not just by the field vectors themselves.
  11. Jun 12, 2009 #10
    YES, I AM aware of that but they can be mapped so they should have a property, I clarified that in the beginning of the question...I guess.

    I think you did not get the question.

    Thanks for the help till now :)
  12. Jun 12, 2009 #11


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    The point is that they cannot be mapped, they are not dependent solely upon the fields.
  13. Jun 13, 2009 #12


    So what exactly determines them...in homogeneous space?

    But there are some properties you know.............I mean everyone lists them...
  14. Jun 13, 2009 #13
    I'll be offline for sometime...going somewhere + installing gentoo :(

    Not an easy job.
  15. Jun 13, 2009 #14
    If you calculate, measure, or visualize the equipotential lines between two equal and opposite charges, the imaginary "lines of force" are orthogonal to the equipotential lines everywhere.
  16. Jun 13, 2009 #15


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    But there isn't a way to show it for a general situation for electric or magnetic fields. The lines of force for a electric fields are dependent upon the charge of particles. If he draws the lines of force due to charged plates, the picture will be different if he introduces a negative charge in comparison with a positive charge into the system. And for magnetic fields it is worse because not only is the force dependent upon the sign of the charge but the direction of the charge's velocity (which will change over time from the acceleration). The only real situation where it makes sense is the force from a static magnetic field acting on a ferromagnetic metals. Thinking in terms of lines of force is wrong here because it unnecessarily complicates the problem and hides the underlying physics.

    "Lines of force attract/repel when anti-parallel/parallel." What does that even mean? Forces do not attract each other or repel.
    "Electrical lines of forces will repel each other if its origin is from the same polarity of charge, else they'll attract." It's like reading stereo instructions. He wants to know what is the force for a given problem he just needs to visualize the fields and apply the Lorentz force. The fields interact simply through linear superposition and using the Lorentz force means he doesn't need to come up with all these convoluted rules and different pictures for the slightly different situations.
  17. Jun 14, 2009 #16
    Ok...so this is one of their properties.

    Assuming we know the intensity and geometry of charges, we can recreate them right?

    Lets just talk E.F...since M.F are more complex.

    Yeah I myself do not prefer using lines of forces, it DOES complicate thing up, last time I quit electrostatics cause of this very problem...analysing things using lines of forces.

    But I just wanna know the properties you know, though I will not use it; nor prefer to.

    Its said that these lines are like stretched rubber bands, i.e they are not forces, but they are a medium to transfer the force, just a like a field (sorta).
  18. Jun 15, 2009 #17
    For the relativistic transformation of the electric field, see the last four lines of
    Note that a moving charge creates a transverse magnetic field, and that the transverse component of the electric field is multiplied by gamma.
  19. Jun 15, 2009 #18


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    I'm asking about the lines of force, not the fields. This is the point I have been trying to make about this. In the case of relativistic moving charges, there is a resulting magnetic field that will act on other charges but is dependent upon the velocity of the affected charges. This is something that lines of force are completely inadequate in showing.
  20. Jun 18, 2009 #19
    Lets consider statics only...no MF, no motion...what do we have to say then?
  21. Jun 24, 2009 #20
    I'm stalled with electrostatics cause of this...
  22. Jun 24, 2009 #21
    It is easy to visualize the equipotentials (lines, planes eic.) between two charges of opposite polarity. The gradient vector is orthogonal to the equipotential lines, and these gradient vectors provide an imaginary grid of the direction (lines of force). One post above by Born2bwire points out that for the Lorentz q(v x B) force, both the direction and magnitude of the force depend on the particle velocity, so it is difficult to map.
  23. Jun 26, 2009 #22
    aaa...I didn't understand this part actually. How can the lines be orthogonal to itself?

    Gradient vector? And vector of which thing? :confused:

    We're actually not considering motion here; even if you're trying to figure out the polarity of the source charge by the M.F that it generates on a frame which's at relative motion, I think it cannot be stated that the polarity of the charge cannot be determined simply cause it's a function of the direction of the relative motion...you have a few ways to determine the 'original' polarity of the source charge (which born2bwire says it's relative and so there's no 1 specific charge).

    Anyway, I'm trying to figure out the properties of these lines of forces in a static frame; though they do not exist and might cause major hindrance when seen with a relative velocity, they do have some specific properties in static conditions where they stand correct i.e if mapped correctly (following the properties) will predict the nature of M.F at any point if the source charges are known on geometry and intensity.
  24. Jul 3, 2009 #23
    Every source says there are a few properties.

    I wanna confirm this one.
  25. Jul 8, 2009 #24
    Oh no!
  26. Jul 12, 2009 #25
    Ok, since no one would answer, I would like to know WHERE can I get the answers.
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