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: More Gauss' Law

  1. Feb 2, 2006 #1
    Consider a solid insulating sphere of radius b with nonuniform charge density [tex] \sigma = ar [/tex] where a is a constant. Find the charge Q_r contained within the radius r, when r < b. Note: The volume element dV for a spherical shell of radius r and thickness dr is equal to [tex] 4\pi r^2 [/tex].
    I got this part, the answer was Q_r = [tex] \pi ar^4 [/tex].
    The second part says if a = 2 x 10^-6 C/m^4 and b= 1 m, find E at r= 0.6 m. Answer in units of N/C.
    I used the equation [tex] E= Qr/4\pi E_0 R^3 [/tex]
    Plugging in the previous answer for Q gave me [tex] E= (\pi ar^4)r/ 4\pi E_0 R^3 [/tex]
    so [tex] \pi ar^5/4\pi E_0 B^3 [/tex] since R=B in this problem.
    then [tex] \pi (2 x 10^-6)(.6)^5/ 4\pi (8.85 x 10^-12) [/tex]
    This gave me 4378 N/C... which is wrong.
    Help please?
  2. jcsd
  3. Feb 2, 2006 #2
    [tex] E= Qr/4\pi E_0 R^3 [/tex]

    Did you mean Q_r?
  4. Feb 2, 2006 #3
    Yes, sorry I meant Q_r * r.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook