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Two spheres, solve for electric field strength

  1. Feb 4, 2009 #1
    1. The problem statement, all variables and given/known data
    Two 2.0-cm-diameter insulating spheres have a 6.50 cm space between them. One sphere is charged to + 89.0 nC, the other to - 95.0 nC.

    What is the electric field strength at the midpoint between the two spheres?


    2. Relevant equations
    Esphere = KQ/r^2
    k = 8.99 * 10^9
    3. The attempt at a solution

    Esphere1 + Esphere2 = Etotal

    (k*89*10^-9) / (3.25*10^-2)^2 + (k*95*10^-9) / (3.25*10^-2)^2
    = Wrong answer: 1.57*10^6 N/C
     
  2. jcsd
  3. Feb 4, 2009 #2

    Delphi51

    User Avatar
    Homework Helper

    Nasty tricky wording!
    Carefully sketch the spheres and mark all the distances clearly.
    Remember the wording on that force formula: "distance between the centers of the charges".
     
  4. Feb 5, 2009 #3
    Here was my 2nd attempt at a solution
    Esphere1 + Esphere2 = Etotal

    (k*89*10^-9) / (4.25*10^-2)^2 + (k*95*10^-9) / (4.25*10^-2)^2
    = Wrong answer: 4.58*10^6 N/C
     
  5. Feb 5, 2009 #4


    Actually, that solution is a calculator error.
    Here was my 3rd attempt at a solution (pressing the calculator correctly this time)
    Esphere1 + Esphere2 = Etotal

    (k*89*10^-9) / (4.25*10^-2)^2 + (k*95*10^-9) / (4.25*10^-2)^2
    Correct answer = 9.16 * 10^5 N/C

    Thanks for the help.
     
    Last edited: Feb 5, 2009
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