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Homework Help: Radius of a circle with a magnetic field and electron velocity

  1. Feb 26, 2010 #1
    1. The problem statement, all variables and given/known data
    An electron moving perpendicular to a magnetic field of 2.2*10^-2 T moves in a circle of certain radius. If the electron is moving with a speed of 1.5*10^7 m/s, what is the radius of the circle?

    2. Relevant equations
    B field = u0*I / 2 pi r
    F = qvB

    3. The attempt at a solution
    I'm confused about how to find the current I in order to solve for the radius. I know that F = (-1.9*10^-19)*(1.5*10^7)*(2.2*10^-2) = -6.27*10^-10 (but positive because magnitude), but I can't figure out how to solve for r with two unknowns (r and I). Can someone help?

  2. jcsd
  3. Feb 26, 2010 #2


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    The electron moves in a circle, so that the magnetic force is providing (equals) the centripetal force.
  4. Feb 26, 2010 #3


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    Hello skibum143,

    Um, why are you solving for I? There are no wires carrying current involved in this problem (well, none that we know about anyway). You already know the magnetic field (it's given in the problem statement), so there's no need to calculate that.


    The charge of an electron is -1.602 x 10-19 C, btw. Be careful of your exponents too.

    Well, you have the capability of finding a force on the electron. The magnetic force (in this problem) is perpendicular to the electron's instantaneous velocity, such that it travels in a circle. The electron's speed is given in the problem statement. Can you think of any equation that relates force, velocity and radius of things traveling in a circle? :wink:

    [Edit] Hint. You can always look up the mass of an electron if you need to.
    Last edited: Feb 26, 2010
  5. Mar 1, 2010 #4
    I see, thank you!!
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