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Homework Help: Explain why the electric field strength is zero

  1. Nov 13, 2004 #1
    I am stuck on my physics homework. I can manage the latter question, how ever when i work out the first one my working out has no rhyme nor reason (nor a right answer). Any help will be great.

    A small metal sphere on an insulating rod is placed on a top pan balance, as shown in the diagram below. An identical metal sphere carrying a charge of 2Q on an insulating rod is brought into contact with th sphere on the balance so each sphere aquires the same charge.

    When the two spheres are a separation of 68 mm, the top of the pan balance reading increases by 2.1 x 10 ^-3 N. Calculate the charge, Q, on each sphere.

    Explain why the electric field strength is zero at the midpoint between the two charge. ​
  2. jcsd
  3. Nov 13, 2004 #2
    sorry just read the sticky after posting
    This is how i attempted to do it, but then realised how completly stupid and random it was
    F/Q = E = V/d -> v/o = (2.1 x 10 ^ -3) / 2 -> 1.05 x 10 ^-3

    (1.05 x 10 ^ -3) / 0.068 = (2.1 x 10 ^ -3) / Q
    Q= (2.1 x 10 ^-3) / (1.544 x 10 ^ -3)
    = 0.136 C which is stupid
  4. Nov 13, 2004 #3


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    Staff Emeritus

    Coulomb's law states:

    F = (k*q1*q2)/r^2

    q1 = Q
    q2 = 2Q
    r = 68mm
    k= boltzmann constant (See your book for this value)
    F = 2.1mN

    Solve for Q

    Once you have Q it should be easy to figure out the electric field due to each charge.
  5. Nov 13, 2004 #4
    thank you!
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