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Homework Help: Guassian infinite sheet and charge with kinetic energy

  1. Jun 9, 2008 #1
    1. The problem statement, all variables and given/known data

    two infinite sheets of charge (one positive, one negative) are arranged parallel to each other and separated by a distance of 2m. a charge of 3 microcoulombs is realeased at the positive plate. when it reaches the negative plate it has a kinetic energy of 0.6 joules. what is the electric field between the plates

    2. Relevant equations

    electric field of charge sheet E = sigma/2(epsilon_o) where sigma = charge density, epsilon_o = constant 8.85*10^-12

    energy U = 1/2 CV^2 where C is capacitance, v is electric potential

    C = Q/V where Q is charge, V is electric potential

    3. The attempt at a solution

    do i assume sigma as constant?
    do i assume both sheets have the same electric field, so electric field between the sheets is the sum of the two?

    how does the energy play a role in determining the electric field, the equation for electric field is already specified, and can be solved in terms of constants, assuming sigma is constant too.

    if i use the energy and capacitance equation i get:
    U = 1/2[(Q/V)(V^2)]
    0.6 = QV/2

    i am not sure how Q, V relate to electric field, i'm lost actually...

    help appreciated
  2. jcsd
  3. Jun 10, 2008 #2


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    Yes, yes. Constant sigma. Constant E field. Now start throwing out equations you don't need. Don't stop too soon. You only need one.
  4. Jun 10, 2008 #3
    still somewhat lost, i did find in actuality that using :

    U = 1/2[(Q/V)(V^2)]
    0.6 = QV/2

    V = 400000 volts when Q = +3*10^-6 coulombs

    but how does V relate to electric field E?
  5. Jun 10, 2008 #4


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    You are CONSTANTLY using WRONG equations. Take U = 1/2[(Q/V)(V^2)]. What kind of a situation does that apply to? What's U, is it kinetic energy? What's Q? What sort of charge is it? Can it be the 3*10^(-6) coulombs? If you have a formula involving U,Q and V, that doesn't mean you can use it in EVERY problem that has the letters U,Q and V in it. Don't only remember equations, remember what sort of physical setup they apply to.
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