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Homework Help: Electrostatics -Electric Field and Potential

  1. Feb 15, 2012 #1
    1. The problem statement, all variables and given/known data

    A 30 cm diameter metal sphere hangs from a thread in the middle of a very large room. So its surroundings are essentially at infinity. If the electric field at tis surface is equal to the break down strength of air 3MV/m, what is the absolute potential at the center of the sphere?
    A) 450 kV
    B) 150 kV
    C) 300 kV
    D) 0

    The answer is from what i've searched online is A

    2. The attempt at a solution

    The Electric FIeld at the surface is 3,000,000 V/m so kQ/r2=3,000,000
    The only unknown is charge and solving we get Q=3*106*r2/k
    where (k=1/4pi epsilon nought)

    so potential at the centre =kQ/r=9* 105 V which is wrong so help please..
  2. jcsd
  3. Feb 15, 2012 #2


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    Homework Helper

    Your method is correct, V=kQ/r, so E=kQ/r^2, V=Er...but you have to use r=0.15 m, instead of 0.3m. It is the radius.

  4. Feb 15, 2012 #3
    Oh YES !! How silly of me Thanks !
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