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Right hand thumb rule

  1. Sep 13, 2014 #1
    according to RHR , the curled fingers show the direction of magnetic field or flux & the thumb gives the direction of current.. but I am confused about the curling of fingers.. I want to make my doubt clear by the pictures I attached..
    In bothe first & second picture , the magnetic field is in downward direction irrespective of direction of current..
    but if we keep the hand in another way as in third , the direction of current is towards left but magnetic field is in upward direction..so which are correct way to find out the direction this thing is making me confused..!! Please help.
     

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  2. jcsd
  3. Sep 13, 2014 #2

    Doug Huffman

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  4. Sep 13, 2014 #3
    I'have already gone through that article but that's not answer to my query..!! Please read the question once again.. In both the the 2nd & 3rd picture , current is towards left but by changing the hand posture , magnetic field direction is being changed.. then which is the correct one?
     
  5. Sep 13, 2014 #4

    dlgoff

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    Look in the direction of the current vector and you'll see that the magnetic field direction is "circulating" clockwise in both cases. See your annotated attachments. Note that I only flipped the first image to get the second image.
     

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  6. Sep 13, 2014 #5
    so what should I infer from this..?? My doubt is still not clear... in 2nd & 3rd both pictures the direction of current is same but the direction of magnetic field is different so which one is wrong among 2nd & 3rd..how should we curl the fingers then..?? Are both right..??
     
  7. Sep 14, 2014 #6

    dlgoff

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    No. They are both in the clockwise direction, relative to the direction of current.
     
  8. Sep 14, 2014 #7
    ohkk so magnetic field is clockwise relative to current but how do we know whether it is in upward or downward..??
     
  9. Sep 14, 2014 #8

    dlgoff

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  10. Sep 14, 2014 #9

    sophiecentaur

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    The guy's using the same hand in all three pictures so it just has to be consistent. You may be having a problem with the 'spatial relationship' thing. It can't work differently, depending how he positions his hand, can it? Same fingers and same thumb must work for all orientations. Do it with your own hand and see how there's no change.
    The direction of the field is not 'up or down'. The magnetic field lines follow a circle, which has up. down. left and right at some point.

    BTW, I would call this rule the Corkscrew rule (Cork screws are all right hand thread). Fleming's Right Hand Rule tells you the direction of induced current when a conductor is moved through a magnetic field. It has a partner rule (the Left Hand rule) - for the Motor effect.
     
    Last edited: Sep 15, 2014
  11. Sep 16, 2014 #10
    see why I am being doubtful is in the attached fig.. according to book , the flux is upwards ..but how is so?? Can you please explain??
     

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  12. Sep 16, 2014 #11

    sophiecentaur

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    What flux? The field lines are closed curves. They point in all directions in a plane as you follow them round.
    That figure doesn't mean anything to me - there is no current carrying wire with field lines round it.
     
  13. Sep 17, 2014 #12
    the current is driven into top of the left coil , the corresponding flux is upward..!! This is what given in the book regarding this circuit..
     
  14. Sep 17, 2014 #13
    The dots indicate the coupling between the two inductors ( look up mutual inductance)- not that the flux is "up" - the diagram does not show physically how the coil is wound - it can be wound two ways around the core.
     
  15. Sep 17, 2014 #14

    dlgoff

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    Oh. Now I think I see what the problem is. Maybe this old thread might help.

    https://www.physicsforums.com/showthread.php?t=656294
     
  16. Sep 17, 2014 #15

    sophiecentaur

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    You are trying to read too much into that sketch. The way those coils are drawn, there is no what of knowing which is the back of a turn or the front.

    But I think you are mistaken from the start about this "right hand rule" and how to apply it (ref. my introduction of the name Fleming)
    The 'Corkscrew Rule' or "Right Hand Grip" tells you the direction of the lines of force around a straight wire, so it doesn't relate directly to a coil, in any case. This link has a simple diagram to tell you the polarity of a solenoid - but the solenoid needs to be drawn so that you know which is back and front.
     
  17. Sep 17, 2014 #16
    In this url.. many things are clear to me which I did'nt discussed about.. thanks for that.. but I am doubtful regarding the polarity ..in bothe loops of attached fig. current is approaching the dot so mutual inductance must be positive.. but in the equations given by gneil.. he took M as negative..!!
    why..??
     
  18. Sep 17, 2014 #17
    sorry , in the first equ. He took +M only.. so is'nt it right that mutual inductance will be positive for the 2nd loop too..??
     
  19. Sep 18, 2014 #18

    dlgoff

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    Here are two images from a "www.faculty.umassd.edu/xtras/catls/resources/binarydoc/4598.ppt" [Broken] that nicely shows how the dot convention is used with the mesh-current method.

    attachment.php?attachmentid=73280&stc=1&d=1411053747.jpg

    attachment.php?attachmentid=73281&stc=1&d=1411053747.jpg
     

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    Last edited by a moderator: May 6, 2017
  20. Sep 18, 2014 #19

    sophiecentaur

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    Going back to this post. What do you mean by "upward" and "downward"? There is no 'up or down' when a wheel is spinning on a horizontal axis - except when you look at just one particular part of the wheel.
    And what has the next part of the thread got to do with this particular problem? Why not sort out one thing at a time and then use another thread to discuss induction between coupled coils?
     
  21. Sep 18, 2014 #20
    According to this ppt.. when current enters the dotted terminal , the voltage induced is positive at dotted terminal..but in the attached fig.. it is negative at the dotted terminal.!!
     
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