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Homework Help: Magnetic Field -- Some help with the right hand rule please

  1. Aug 18, 2015 #1
    1. The problem statement, all variables and given/known data

    A magnetic field of 0.0200 T (up) is created in a region.

    a) Find the initial magnetic force on an electron initially moving at 5.00 x 10^6 m/s (N) in the field.

    b) What is the radius of the circular path? Make a sketch showing the path of the electron.

    2. Relevant equations


    r=mv / qB

    3. The attempt at a solution
    I have already determined that a) is 1.16 x 10^-14 N, my question is how do I use the right hand rule to determine direction? Since the velocity is (N) I point my thumb north and since B is (up) I have been pointing my fingers upwards. Therefore my palm is pushing East. Is this correct?

    I found that r=1.4 x 10^-3 m so the magnetic force for also be (East)??

    Some help with the right hand rule, specifically to this question would be appreciated.
  2. jcsd
  3. Aug 19, 2015 #2


    User Avatar

    Staff: Mentor

    Are you familiar with the vector version of the Lorentz force? That helps me keep the Right-Hand Rule straight. So "X" represents the vector cross product:

    F = qV X B

    So first of all, remember that this question is about an electron, so qV is negative, or in the opposite direction from V. Does that make sense?

    Second, The way I use the cross product is to curl my fingers from the first term vector to the second term vector, and the resultant vector is in the direction of my thumb. So If I curl my fingers from the qV direction (which is which way?) to the B vector direction, which direction is my right thumb pointing in? :smile:
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