1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Spherical balloon with conductive coating

  1. Jan 25, 2005 #1
    Hi, I have no idea of how to do the following problem and what formulas I should use. Please help! Thank you.

    A spherical balloon has a conductive coating and we propose to inflate the balloon to a diameter of 0.1 meters by connecting the surface to a high voltage source. Suppose that the maximum practical electric field at the surface is 2 x 10^6 volts/meter (in air, just before breakdown of air molecules). What is the largest voltage we can apply, and what is the outward pressure (N/meters^2)? How many atmospheres is this?
  2. jcsd
  3. Jan 25, 2005 #2


    User Avatar
    Homework Helper

    Find the total charge Q on the sphere in terms of the electric field - gauss' law works here.

    Then find the voltage on the surface of the sphere in terms of Q.... plug in Q from the first part, to get the voltage.
  4. Jan 25, 2005 #3
    Ok I understand the first part -- I can find the charge from Gauss's Law. For the second part, I'm still not sure what equation I should use. Should I use the equation W=QV or some other work equation? What about the outward pressure?
  5. Jan 25, 2005 #4
    Ok when I try it: using 1) E = kq/r^2 and 2) V = kq/r, I get V = Er = (2 x 10^6 volts/m)(0.1 m) = 2 x 10^-7 volts. This procedure looks too simple. Is it right?
  6. Jan 25, 2005 #5


    User Avatar
    Homework Helper

    Yes, those two formulas work because of the symmetry involved. But r=0.05m, so V=Er=2*10^6*0.05=100,000V.

    I'm not sure about the pressure part... if Force on a small area dA is F=(sigma*dA)E (where sigma is charge density).... And then you can find pressure by F/dA.

    But using F=(sigma*dA)E seems wrong to me as the charges are located inside the conductor where electric field is zero... Not sure here. Sorry!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?