1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Surface charge density problem

  1. Mar 7, 2010 #1
    1. A spark occurs at the tip of a metal needle if the electric field strength exceeds 4.00×106 N/C, the field strength at which air breaks down. What is the minimum surface charge density for producing a spark?

    2. surface charge density = Q/A

    3. The answer is 3.54*10-5 C/m2. How was this obtained? I couldn't figure out by playing with dimensions...
  2. jcsd
  3. Mar 7, 2010 #2


    User Avatar
    Homework Helper

    Gauss' Law ...
    Starting with Coulomb's kQ/r^2 = E , move the r^2 over with the E .
    ... x 4 pi , and it looks like the E-field is spread over a surface surrounding 4 pi kQ :
    => E.A = 4 pi k Q ... E has to pierce the Area A , which has to enclose charge Q
  4. Mar 7, 2010 #3
    I don't understand. E*A = 4pikQ but you don't know A or Q...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook