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B field from Electron motion

  1. Mar 2, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.

    Hello PF friends.

    I know what the answer is, but I do not understand why the answer is.

    By the right hand rule I put my thumb west, but pointer up, and the field (middle) points North - BUT it is an electron so the field points South.
    I also tried pointing my hand west, curling my fingers up, and my thumb points North - again the electron means the field should be South.

    My problem is that the answer is North and I do not know why.
  2. jcsd
  3. Mar 2, 2017 #2


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    In using the right-hand rule, there are different conventions for what the fingers and thumb represent.

    Can you state what convention you are using?
  4. Mar 2, 2017 #3
    For the first one my thumb is the velocity, my pointer is the direction of the force, and my middle is the B field.
    For the second version I essentially point my entire hand towards velocity, and curl it in the direction of the force (curl up because the electron moves up), so the thumb is the B field.
  5. Mar 2, 2017 #4


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    I believe these directions are incorrect for a positive charge. Double check your textbook or notes.
  6. Mar 2, 2017 #5
    Ok I had my pointer and my middle fingers wrong for the first one, and for the second one my thumb points in the direction of the force and curls in the direction of the B field.

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