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Why is the magnetic field of a wire circular

  1. Apr 17, 2014 #1
    The title is enough
     
  2. jcsd
  3. Apr 17, 2014 #2

    jtbell

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    Look at the list of "Related Discussions" at the bottom of this thread, and you'll find two threads with the same question as yours. One of them has 24 responses, which looks promising!
     
  4. Apr 17, 2014 #3
    Could you please copy and paste these links in a reply because the mobile version doesn't allow make these links appear
    Thanks very much
     
  5. Apr 17, 2014 #4
    unfortunately, I couldn't get much from related discussions, could you provide a thorough explanation,
    thanks in advance
     
  6. Apr 18, 2014 #5

    UltrafastPED

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  7. Apr 18, 2014 #6

    WannabeNewton

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    It's simply due to the cylindrical symmetry of the system.
     
  8. Apr 18, 2014 #7

    I know the observation, but I can't figure out its reason. Bio-Savarts law will only guide me to the direction of the field mathematically not logically nor intuitively.
     
    Last edited: Apr 18, 2014
  9. Apr 18, 2014 #8

    How isn't it a physics question if I want a deeper answer??
     
  10. Apr 18, 2014 #9

    UltrafastPED

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    Mathematics _is_ logic; and if you have a good intuition for geometry, you will do better with physical intuition as well.
     
  11. Apr 18, 2014 #10
    Isn't there any other explanation, because bio-Savarts law is a way beyond my level b
     
  12. Apr 18, 2014 #11

    mfb

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    What else than circular could it be? Do you expect rectangles? Why would you expect one direction from a straight wire look any different from other directions?
     
  13. Apr 18, 2014 #12
    Why isn't there any poles
    Why are the lines rotating around the wire and not ending or beginning at certain poles ?
     
  14. Apr 18, 2014 #13
    "Why" questions in physics are always so ill-posed.

    Why not?
    Can you answer that?
     
  15. Apr 18, 2014 #14

    UltrafastPED

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    Because magnetic fields don't originate from "magnetic charges". Thus they always go in a loop.

    You see this if you have some iron filings (in a sealed plastic box) - as you bring any magnet close the filings will form an alignment which connects the poles.

    If you stick two magnets together, N-S or N-N, you will see changes in the alignment, but always in a closed loop.

    If you "break" a magnet it will always have two poles.

    One of Maxwell's equations describes this behavior, but you will need to study vector calculus to understand the math; at this point simply try to understand what you are seeing, and how it changes with the experiment.


    For example, what do you expect the magnetic field to look like if you bend the current carrying wire into a loop? Into a long coil?
     
  16. Apr 18, 2014 #15

    jtbell

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  17. Apr 18, 2014 #16
  18. Apr 18, 2014 #17
    Ampere Law states that

    2[itex]\pi[/itex]r[itex]\Huge[/itex] [itex]\propto[/itex] [itex]I[/itex]

    [itex]\beta[/itex] [itex]\propto[/itex] [itex]I[/itex]

    2[itex]\pi[/itex]r[itex]\LARGE[/itex] is the maximum circumference for the field??
     
    Last edited: Apr 18, 2014
  19. Apr 18, 2014 #18

    mfb

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    2πr is just the exact circumference of a circle with radius r.
    There is nothing "maximal". The (theoretical, ideal) field extends to infinity.

    That is a weird way to reduce Ampere's law.
     
  20. Apr 18, 2014 #19

    What does it mean that circumference is directly proportional to the intensity ? Since there are infinite concentric circles
     
  21. Apr 18, 2014 #20

    mfb

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    As I said, that is a weird way to reduce Ampere's law. You could read it "with larger current, you get the same field strength (which you omitted) at a larger radius".
    And constant factors like 2π are irrelevant anyway if you look at proportional quantities.
     
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