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Homework Help: Very fast right hand rule question

  1. Apr 27, 2005 #1
    The electrons in the beam of a television tube have a kinetic energy of 2.40×10–15 J. Initially, the electrons move horizontally from west to east. The vertical component of the earth’s magnetic field points down, toward the surface of the earth, and has a magnitude of 2.00×10–5 T.

    Just when i thought i had this right hand rule thing down, i come across this
    heres what i did
    used left hand, fingers down, thumb pointing right, force = out of page
    yet the answer is the force is directed south??

    what did i do wrong?
  2. jcsd
  3. Apr 27, 2005 #2


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    Homework Helper
    Gold Member

    The charge of an electron is negative. :)
    so f=qv*b. when q is negative, force is in the opposite direction.

    If you ask me, instead of thinking in terms of this rule, try think in terms of vectors and cross products
  4. Apr 27, 2005 #3
    i have no clue how to do cross products :/ and i used left hand since its an electron :)
  5. Apr 27, 2005 #4

    Doc Al

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    Staff: Mentor

    You are just interpreting the answer incorrectly. I assume your picture (mental or otherwise) shows East as going to the right of the page, and down as going down the page? The answer, given that, is out of the page--but out of the page = South! (Always translate the answer back into the language of the question.)

    Since you seem to be having quite a time with the right hand rule, check out the links I provide here: https://www.physicsforums.com/showpost.php?p=389039&postcount=8
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