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Homework Help: What is the energy of the electron just before it hits

  1. Jan 28, 2017 #1
    1. The problem statement, all variables and given/known data
    A 300 eV electron is aimed midway between two parallel metal plates with a potential difference of 400 V. The electron is deflected upwards and strikes the upper plate as shown.
    What would be the kinetic energy of the electron just before striking the metal plate?

    2. Relevant equations
    Magnitude of the resultant vector =
    = Sqrt [ Ex^2 + Ey^2 ] = Sqrt [ (300eV)^2 + (200eV)^2 ] = Sqrt [ (2e2)^2 + (3e2)^2 ] = 360 eV

    3. The attempt at a solution
    The electron energy in x direction is unaffected by the 400 V field. So it is present just before hitting the plate.
    And the electron goes thru half of the 400V = 200 V vertically picking up energy of 200 eV.
    So the resultant total energy is the vector sum of the x and y components of 200eV and 300eV.
    The answer sheet say the final energy before striking the plate is 500eV
    I do not see why is is not 360eV as calculated above.

    Thanks for your help
  2. jcsd
  3. Jan 28, 2017 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Energy is not a vector! (Just add them up.)
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