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Magnetic Field Theory Contradiction? - Repost

  1. Apr 7, 2014 #1
    I've been thinking about Magnetic Fields, and I think that I've found a contradiction in conventional physics theory.

    While comparing the Left-Hand-Rule for Motors (LHR) and the Right-Hand-Rule for Generators (RHR), I found this contradiction:

    The Left-Hand-Rule states that if the Magnetic Field (B) is on the y-axis, and the Velocity of a Current (I) is on the x-axis; Then the Force acts in the z-axis up. This uses a negative charge (an electron).

    The Right-Hand-Rule states that if the Magnetic Field (B) is on the y-axis, and the Velocity of a Test Charge (v) is on the x-axis; Then the Force acts in the z-axis up. This uses a positive charge.

    https://www.physicsforums.com/attachment.php?attachmentid=68407&d=1396915312

    Put another way, if both use a negative-charge as an example:
    The Left-Hand-Rule states the electron moving along the x-axis would have a Force-Up.
    The Right-Hand-Rule states the electron moving along the x-axis would have a Force-Down (Due to the negative-charge).

    https://www.physicsforums.com/attachment.php?attachmentid=68408&d=1396915312

    Can anyone tell me why equal charges, with equal directions, would have opposite forces?

    Is one of my sources flawed? Is this simply due to fact that one rule is for motors and one for generators? Is there some context I haven't considered?

    Sources:
    Wikipedia - 'Fleming's left-hand rule for motors'
    Cutnell & Johnson – Physics – Sixth Edition – ISBN 0-471-15183-1 (page 624)

    Thank you for your advice, and I'll be glad if anyone can explain this.
     
    Last edited: Apr 7, 2014
  2. jcsd
  3. Apr 7, 2014 #2

    jtbell

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    Staff: Mentor

    Where do you get that this uses a negative charge?

    On http://en.wikipedia.org/wiki/Fleming's_left-hand_rule_for_motors I see in the section "Conventions":

    That is, the charge-carriers are assumed to be positively charged.
     
  4. Apr 8, 2014 #3
    Thank you for the response. So, am I to understand that even thought 'I' delivers a negative charge, it is defined by the positive-potential that attracts that negative-charge? ...or am I incorrect in my concept of 'I'? Thanks again.
     
  5. Apr 8, 2014 #4

    jtbell

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    Staff: Mentor

    The magnetic force exerted on a positive charge moving in the +x direction is the same as on a negative charge moving in the -x direction. Likewise the magnetic field produced by a current of positive charges moving in the +x direction is the same as for a current of negative charges moving in the -x direction.

    For historical reasons, when we talk about the direction of electric current, we generally mean the direction of positive-charge flow that would produce that current, even though we know it's really a flow of negative charges in the opposite direction.
     
  6. Apr 8, 2014 #5
    Thanks again. That makes a lot more sense now. This graphic might sum it up well for others reading the post:

    500px-Current_notation.svg.png

    I appreciate the advice.
     
  7. Apr 8, 2014 #6
    Answer after Confusion

    Since an Electrical Current is a Positive-Flow,
    the two rules are equivalent:

    attachment.php?attachmentid=68445&stc=1&d=1396990991.jpg
     

    Attached Files:

  8. Apr 8, 2014 #7

    jtbell

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    Staff: Mentor

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