conceptual understanding of electromagnetism

Discussion in 'General Electronics Chat' started by PG1995, Oct 30, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011

    I understand that this is a very complex topic but we will try to keep it simple and uncomplicated! Thanks.

    It is said that electrons in themselves are the reason for magnetism. As is explained in the linked page that if an electron is moving CW if viewed from the top, then electron's top is North and bottom is South.

    Linked page:

    I have a couple of questions to ask but asking them in steps would be a wise choice. In the first place I don't understand how we get circular magnetic field lines around a conducting wire. I understand that when DC current is flowing through a conductor electrons are continuous moving in one direction. But how do their collective movement create circular field lines?

    This is how electron's magnetic field looks:

    Could you please refer me to some diagram etc. which shows how electrons' magnetic field sums up to give circular lines, or explain it yourself?

    Magnetic field lines around a wire:

    Thank you for your help.

  2. iONic

    AAC Fanatic!

    Nov 16, 2007
  3. steveb

    Senior Member

    Jul 3, 2008
    I think using the magnetic field of the electron in your figure is misleading when talking about macroscopic fields, which is typical in electrical engineering. Magnetic moments of fundamental particles is important in physics but generally is not important for basic conduction of electrons and magnetic fields in wires.

    However, if we replace the electron in your figure with a small length element of current, we get a fundamental magnetic field pattern. This field pattern becomes the basis for using the Biot Savart law which is the analog of Coulombs law when we do basic magnetic field calculations.

    So just as we can calculate the total electric field by adding up all the fields of the point charges in a system, we can calculate the total magnetic field by adding up all the fields of the current elements in a system.

    The following lecture talks about this in a detailed way, but note that the lecturer makes a mistake when she says that Faraday's law can be used in symmetrical cases. It is actually Amperes law that is used in those cases.

    The above lecture is showing some complex stuff that you may not be ready for yet, but watching should give you a feel for the process. After watching these, try to apply the method to the simple case of an infinitely long wire. If you can't figure it out, do a google search and you can find the solution on line. However, this problem can be solved much more easily using Amperes law.

    The following lecture shows how to use Amperes law to calculate the field for the symmetrical case of a long wire.

    The above two lectures mention Gauss' law for electric fields, Faraday's Law and Ampere's Law. There is another version of Gauss' law which applies to the magnetic field. These 4 laws are the 4 so-called Maxwell Equations (once Ampere's law is modified according to Maxwell's great discovery) which you will be learning about more and more. When you master these equations you will no longer feel that this is a complex topic. The fundamental notions of electromagnetics are actually quite simple, but the subject seem complicated when the ideas are shrouded in the complexity of vector calculus.
    Last edited: Oct 30, 2011
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