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Homework Help: Conceptual Question 27.13

  1. Feb 11, 2009 #1
    1. The problem statement, all variables and given/known data
    A parallel-plate capacitor consists of two square plates, size L x L, separated by distance d. The plates are given charge +-Q . Each part changes only one quantity; the other quantities have their initial values.

    Part A

    What is the ratio E(final)/E(initial) of the final to initial electric field strengths if Q is doubled?

    2. Relevant equations

    I know that:E = kQ/r^2 ,where k= 8.99*10^9

    3. The attempt at a solution

    I am not sure where to start here for this question.
    Since it is a ratio of E final / E initial would Q cause E to be something like this?
    E = k(2Q)/r^2 => 1/2E for E final?
  2. jcsd
  3. Feb 11, 2009 #2
    I tried 1/2 but it was incorrect. Help please!
  4. Feb 12, 2009 #3
    You are using the formula for a point charge in the case of a parallel plate capacitor.
    The electric field inside the capacitor is constant and is given by E=V/d where V is teh voltage and d is the distance between the plates.
  5. Feb 12, 2009 #4
    i figured it out. it wasnt 1/2, for Efinal/Einitial = 2
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