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Homework Help: Equipotential surfaces; finding the radius

  1. Jul 25, 2011 #1
    1. The problem statement, all variables and given/known data

    a metal sphere of radius 0.39 m carries a charge 0.55 μC. Equipotential surfaces are to be drawn for 100-V intervals outside of the sphere.

    Determine the radius of the first, tenth and 100th equipotential from the surface.

    2. Relevant equations

    V = kQ / r
    Volt = (Coulombs Constant * Charge) / radius

    3. The attempt at a solution

    r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (1 Nm/C)

    r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (10 Nm/C)

    r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (100 Nm/C)

    all units cancel out and I'm left with meters but I'm getting the incorrect values according to the homework system that I'm using
  2. jcsd
  3. Jul 25, 2011 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Read the problem more carefully. The equipotential surfaces are 100 V apart.

    What is the potential of the sphere itself? Start there.
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