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Electromagnetism without any finger

  1. May 17, 2006 #1
    When I was a student, I remember that I disliked a lot these finger exercises.
    I soon realised that these were totally useless, and I solved everything without any finger rule.
    But since, I have forgotten how I did ! I only remember that I concentrated on currents, nothing else.

    Have some of you also avoided playing with fingers, and how ?

    Michel
     
  2. jcsd
  3. May 17, 2006 #2

    dav2008

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    Gold Member

    Are you referring to the right-hand rules?

    All those tell you is the direction of the cross product of two vectors in the conventional right-handed coordinate system.

    You can just understand where the respective vectors are pointing and which direction the appropriate cross product will point.

    Of course if you actually take the cross product mathematically, the resulting vector will obviously tell you its direction.
     
    Last edited: May 17, 2006
  4. May 17, 2006 #3
    I usually imagine myself facing the plane of interest, and clockwise rotation in that plane results in going forward through the plane, counterclocwise results in coming out, and vice versa ... sorta like how a clock would turn clockwise if you went forward in time and counterclockwise if you went backwards in time. I know it's sort of vague but its not hard to apply it to anything involving the right hand rule with E&M.
     
  5. May 17, 2006 #4
    Hmm....it isn't hard to visualize even without fingers.
     
  6. May 18, 2006 #5
    Greg825,

    I agree that some visualisation can make finger rules or corkscrew rules easier.
    But is that needed after all? Physically, fields are caused by currents. The finger rules are only needed because of conventions taken by physicists to allow them to think about fields and forget (temporarily) about the currents that caused them.

    Doesn't that mean that, going "back to the basics", it should be possible to avoid completely these rules? For the benefit of pedagogy, maybe.

    Michel
     
  7. May 18, 2006 #6
    Yes; the directions of various things in EM (such as propagation, given direction of E-field and B-field) will always be perpendicular. Whether or not we say parallel or anti-parallel to that direction is positive is a convention, which is often easily learnt using the "right-hand-rule".

    But we will always need some sort of convention, and using cross products/right-handed co-ordinate system is ideal.
     
  8. May 21, 2006 #7
    I think you're right, they should be avoidable, but are the physical reasons behind why a current creates a magnetic field at all really understood (let alone why it occurs in whichever direction)? I mean similarly to gravity, which is modeled very well, but not really explained.

    But maybe you meant avoidable for a reason other than that kind of understanding? I'm not sure of exactly what you mean.
     
    Last edited: May 21, 2006
  9. May 21, 2006 #8
    Greg825,

    My reasons have little to do with ultimate understanding.

    It is just that I remember how these conventions were far from real physics. For example, how emf induces a current may need to chain two 'finger' rules, while the end result is well known. I don't think there are examples where their use is really helpful, if one is willing to forget a little about it. In addition, now that differential geometry becomes popular, I have the feeling the 'old stuff' may be reconsidered.

    Am I right, and how to write that down?

    Michel
     
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