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Homework Help: Deflection of electron beam in oscilloscope

  1. Aug 19, 2012 #1
    1. The problem statement, all variables and given/known data

    Calculate the deflection of an electron beam as it passes between the plates of a CRT tube.
    In the picture, the parallel plates create an electric field, with the positive plate on top and the negative plate on bottom, causing the electron's path to be deflected upwards as it travels to the right.

    2. Relevant equations

    Kinetic energy of electrons: K = 3.2 x 10^-16 J
    Electric field between plates: E = 1.2 x 10^4 N/C
    Distance along plates = 15mm

    3. The attempt at a solution

    I know the equation F=Eq is involved. I've been told that the force is centripetal, so that the equation for centripetal force is also involved, but im not entirely convinced. This is because centripetal force is always perpendicular to the velocity, but in this example that can't be the case.
    So im stuck and need help.

    [edit] and the answer is 0.34mm
    Last edited: Aug 19, 2012
  2. jcsd
  3. Aug 19, 2012 #2


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    Homework Helper

    A constant force F=qE acts on the electron in the vertical direction while its initial velocity is horizontal. The problem is analogous with projectile motion, the trajectory is parabolic. You can decompose the motion into horizontal and vertical direction.

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