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: Capacitor with a conducting core

  1. Apr 22, 2008 #1
    A 0.380 m long cylindrical capacitor consists of a solid conducting core with a radius of 1.15 mm and an outer hollow conducting tube with an inner radius of 2.05 mm . The two conductors are separated by air and charged to a potential difference of 6.40 V .

    Calculate the charge per length for the capacitor.

    lambda =_______ C/m.

    Calculate the total charge on the capacitor.

    Q=_____ C.

    Calculate the capacitance.

    C=______ F.

    Calculate the energy stored in the capacitor when fully charged.

    U=_______ J.

    Well i figure out a couple of equations to use but i seem to get them wrong! all are wrong! Any Help!
    so the equation i used were :

    To Find C = 2 pi €o L ln(r2/r1) = 12.22 *10^-12 F = 12.22 pF.
    To Find U = (1/2) C V^2 = 250*10^-12 J = 250 pJ.
    To Find Q = C V = 78.2*10^-12 C = 78.2 pC
    To Find lambda = Q/L = 2.06*10^-10 C/m = 206 pC/m.
  2. jcsd
  3. Apr 23, 2008 #2


    User Avatar
    Homework Helper

    Hi sonrie,

    This is the wrong formula for the cylindrical capacitor. The natural log is in the denominator.
  4. Apr 23, 2008 #3
    so the formula should read C= 2 pi Eo L/Ln (r2/r1) Is that correct?
  5. Apr 23, 2008 #4


    User Avatar
    Homework Helper

    That looks right to me.
  6. Apr 23, 2008 #5
    Your the Best! Thank You!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook