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Homework Help: Uniform Magnetic field

  1. May 22, 2009 #1
    1. The problem statement, all variables and given/known data

    In the figure, an electron accelerated from rest through potential difference 1.7 kV enters the gap between two parallel plates having separation 20.0 mm and potential difference 110 V. The lower plate is at the lower potential. Neglect fringing and assume that the electron's velocity vector is perpendicular to the electric field vector between the plates. What uniform magnetic field allows the electron to travel in a straight line in the gap?

    2. Relevant equations
    B=magnetic field

    3. The attempt at a solution

    I honestly don't know where to even begin, I know I've got to be missing a formula because distance has got to be used somewhere in there, I'm just interested in getting an idea how to start this problem.
  2. jcsd
  3. May 22, 2009 #2
    If the electron travels in a straight line it doesn't accelerate upwards/downwards, which essentially means that the sum of all forces acting in the direction of the y-axis is zero. The magnetic force has to be equal, but opposite to that of the electric force.
  4. May 22, 2009 #3
    Ok, so that means that F=-F, and i found another formula qvB=(mv^2)/r, but i'm sure that I need to find the velocity first and then use that and substitute and since i have two equations i would be able to solve for F which would allow me to find B, but i'm not sure how to find the velocity by only knowing the potential differences.
  5. May 22, 2009 #4
    Forget qvB=(mv^2)/r. This equation describes an electron (or any other particle with a charge q) moving in a circular path, this is not true in your assignment.

    If a particle with the charge q is accelerated with the voltage U it will gain kinetic energy according to


    From this equation you can solve the speed of the electron.
  6. May 22, 2009 #5
    Ok so, I found the velocity to be 24,449 m/s and then by combining the equations qvB=F and F=qE I got the equation vB=E so 24449 m/s x B = E and E=V(volts) / d

    so I plugged in numbers and 1.27e-10 but wrong... Is my math wrong?
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