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Homework Help: Mass of charged particle

  1. May 20, 2009 #1
    1. The problem statement, all variables and given/known data
    A positively charged particle (9.35pC) is suspended in equilibrium in an electric field between two horizontal parallel plates with opposite charges. Each one of the square plates has a charge of 5.5x10^-5C; and are separated by 6.25mm and their sides are 11cm long
    what is the mass of the particle?

    2. Relevant equations

    3. The attempt at a solution
    first i divided the charge of the particle by the elemental charge
    i got 58364544
    then i multiplied this number by a protons mass
    i got 9.764x10^-20

    the book says the answer is 4.9x10^-4
    soo confused!
  2. jcsd
  3. May 20, 2009 #2


    User Avatar
    Homework Helper

    Consider the E-Field between the plates.

    What does that come to?

    You know that F = q*E

    And you also know F = m*g

    So ...

    m = (q / g) * |E|
  4. May 21, 2009 #3
    wow, i feel like a moron for not seeing that! thanks!
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