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: Point Charge and Charged Sphere

  1. Sep 5, 2011 #1
    1. The problem statement, all variables and given/known data
    A point charge q1 = -7 μC is located at the center of a thick conducting shell of inner radius a = 2.6 cm and outer radius b = 4.9 cm, The conducting shell has a net charge of q2 = 1.1 μC.
    sxneB.png

    What is Ex(P), the value of the x-component of the electric field at point P, located a distance 8.9 cm along the x-axis from q1?
    2. Relevant equations
    (1.)For an electric field outside of a conducting sphere(where r>R), E=Q/ (4pi Epsilon naught r^2)
    (2.)Electric Field of Point Charge, E= kQ/r^2

    3. The attempt at a solution
    OK, so what I did was try to the sum of the electric fields first from the point charge in the center using equation (2.) and then I added that to the electric field outside the sphere to the point using equation (1.) and my Q as q2.
    Doing this I get -1774598 N/C when the answer should be -6703699 N/C. I am not very good at E&M so I would appreciate all help.
     
  2. jcsd
  3. Sep 5, 2011 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi ThirdEyeBlind! :smile:

    (try using the X2 and X2 icons just above the Reply box :wink:)

    Hint: what is the electric field outside a sphere of radius > b ? :wink:
     
  4. Sep 5, 2011 #3
    Alright so now I am getting the correct answer thanks to your hint but I am now stuck on another part.

    What is σb, the surface charge density at the outer edge of the shell?
    I was just thinking it would have been (1.1E-6 C) / (.049 m2 - .026 m2 * pi)
    which is the charge of the conducting shell divided by the area of the shell. However the answer is -0.00019554 C/m2 so I don't understand how they get that.
     
  5. Sep 6, 2011 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi ThirdEyeBlind! :smile:

    (just got up :zzz: …)
    Sorry, but this is completely wrong. :redface:

    For the surface charge density at the outer edge, you use only the area of the outer edge (isn't that obvious? :confused:).

    Also, it's 4πr2 for the area, not πr2.

    Also, you need to find the charge on the outer edge, not the net charge.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook