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Homework Help: How fast is electron moving

  1. Dec 1, 2009 #1
    1. The problem statement, all variables and given/known data
    An electron is released from rest at the negative plate of a parallel plate capacitor. The charge per unit area on each plate is σ = 1.70E-7 C/m2, and the plates are separated by a distance of 1.59E-2 m. How fast is the electron moving just before it reaches the positive plate?

    2. Relevant equations
    E = σ/2εo
    F=q X E
    v2 - u2 = 2 a s = 2 (F/m) s

    3. The attempt at a solution
    E= 1.70E-7/2(8.85E-12)= 9604.52 N/m

    F= 1.6E-19 * 9604.52= 1.537E-15 N

    v^2= 2(1.537E-15/9.1E-31)*1.59E-2
    = 5.370E13...this is wrong...what am i doing wrong??
  2. jcsd
  3. Dec 1, 2009 #2


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    Homework Helper

    Did you remember to take the square root to get v instead of v²?
  4. Dec 4, 2009 #3
    yes, when i take the sq root, i get 7.328 E6 m/s...still not the answer!
  5. Dec 5, 2009 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    That formula is for a single plate. However, there are two plates here.
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