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Right Hand Rule doesn't work for this problem?

  1. Mar 11, 2013 #1
    Right Hand Rule doesn't work for this problem???

    Consider an electron near the Earth's equator. In which direction does it tend to deflect if its velocity is directed in each of the following directions?


    I DON'T get this!

    I know they're basically saying this:

    Velocity of electron (v) x Directuib if earth's magnetic field (B) = direction ------> electron goes OPPOSITE this direction.

    But my question is, HOW do I go about determining what South x North equal????? or West x North equal??? It makes NO sense!

    I've tried using Fleming's Right Hand rule but I can only move my middle finger (which represents velocity v) in just a few directions -__-
  2. jcsd
  3. Mar 11, 2013 #2


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    Consider question (a). You are given that the electron is moving vertically downward toward the ground.

    Can you explain a little more why you are having trouble using the right hand rule? There are different conventions as to which finger should go with which vector, so it will help if you describe which finger you use for the B field and which you use for the force.

    [Note that the "x" in v x B represents a "cross-product" of vectors and not just ordinary multiplication. So, "west x north" indicates a cross-product of a vector pointing west and a vector pointing north].
    Last edited: Mar 12, 2013
  4. Mar 11, 2013 #3


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    Is earth's geographic north pole a magnetic north pole, or is it a magnetic south pole ?
  5. Mar 12, 2013 #4
    OMG, i swear im so stupid sometimes -.-

    Ok ok so I DO using Fleming's right hand rule where the index finger is ALWAYS pointing NORTH (horizontally), and the directions they give me for the velocity of the electron is where the middle finger should be pointing. And the thumb basically indirections the direction of the force that deflects it.
  6. Mar 12, 2013 #5


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    If the index finger points in the direction of the field and the middle finger in the direction of the velocity, then the thumb will point in the direction of the force on a negative charge (electron). So, you will not take the opposite direction of the thumb (unless you wanted the force on a positive charge!)

    You should probably double check the rule as explained in your text or in class. Usually the rule is designed to give the force on a positive charge rather than a negative charge.
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