Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why are magnetic feilds formed from an electric current?

  1. Mar 23, 2005 #1
    Why are magnetic feilds formed from an electric current? and why does the feild occur in an anticlockwise direction?
  2. jcsd
  3. Mar 23, 2005 #2
    -A neutral current carrying wire is usually more or less electricaly neutral
    -Relativity says objects contract in their direction of motion (lorentz contraction)
    -If you move wrt the wire their is a diffence in contraction between the moving electrons in the wire and the positively charged cores standing still!
    -This difference in contraction means a difference in charge density
    -This net charge density constitutes an electric field we call a magnetic field

    This accounts for the magnitude (the faster you move the larger the contraction) and direction (the contraction is only in the direction of motion) of the magnetic field.
  4. Mar 23, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Just out of curiosity, how come you are not puzzled by the reverse process? Why not ask why electric currents are formed by changing magnetic fields? This is very commonly used in power stations to generate electric currents.

  5. Mar 23, 2005 #4
    Even better (MHO):

    Consider a coil of 10 "primary turns" of AC current on a highly permeable iron (permalloy is better) ring with relatively large diameter to minor diameter thru the iron. Almost all of the magnet field remains inside the iron of the ring.

    Now imagine a single turn copper ring of wire a little thinner than the iron cross section, but with the same major diameter as the iron ring. Furthermore imagine that the plane of copper ring is perpendicular to that of the iron ring and one part of the copper ring passes thru the center of the iron ring. That is the two circular rings are topographically joined.

    There is essentially zero magnet field where there is any copper, yet the secondary loop (the copper ring) has current induced in it approximately 10 times greater than the primary current in the 10 turns!

    Don't tell me any thing about "magnetic lines of force" cutting thru the copper ring. Even Faraday, who invented the concept of "line of force/ field," knew they were not real, just a convenience to aid thinking. Remember, no part of the copper ever has any significant magnetic field near it!

    Who said classical physics did not have mysteries just as good as a single photon going two separate slits!
    Last edited: Mar 23, 2005
  6. Mar 24, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm trying to figure out what you're saying here, but I can't visualise the placement of the two rings...
  7. Mar 24, 2005 #6
    They are linked, like two links of a chain. Both are circles of the same size. The planes of the ring are perpendicular and each passes thru the center point of the other's circle. Hope that helps - I can't stated it any more clearly.
  8. Mar 24, 2005 #7
    If all that is true then why is it that magnetic forces and electric forces dont interact with eachother directly ?
    Last edited: Mar 24, 2005
  9. Mar 24, 2005 #8
    I think hes asking the big question of, "WHY"
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Why are magnetic feilds formed from an electric current?