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Homework Help: Magnitude of electric and magnetic fields question. Help!,

  1. Dec 11, 2013 #1
    1. The problem statement, all variables and given/known data

    An ink drop with charge q = 3 ✕ 10-9 C is moving in a region containing both an electric field and a magnetic field. The strength of the electric field is 8E5 N/C and the strength of the magnetic field is 1.3 T. At what speed must the particle be moving perpendicular to the magnetic field so that the magnitudes of the electric and magnetic forces are equal?

    2. Relevant equations
    F= q x v x B

    3. The attempt at a solution

    Right now I know the magnitude of the electric field is 8E5 so I can take that and set it equal to my other numbers... 8 x 10^5 = 3E-9 C x 1.3 T x ( ? M/s)
    Solving for m/s, I get 2.05E14 which turns out to be incorrect.
    Not sure what i am doing wrong any help would be nice!
  2. jcsd
  3. Dec 11, 2013 #2


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    ##q \vec{v}\times\vec{B}## gives you the magnetic force on the charge. ##\vec{E}## is the electric field, which is not a force, so you can't compare them directly. What's the force on the charge due to the electric field?
  4. Dec 11, 2013 #3
    Hint: What are the units on the left and right hand sides of your equation, and what should they be?
  5. Dec 11, 2013 #4
    N/C= C x T x m/s. those are my units.. Which I now realize I have done something wrong. Any more help?!
  6. Dec 12, 2013 #5
    What's the unit of force?
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