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Homework Help: Electric potential

  1. Apr 25, 2013 #1
    capacitance cylinder

    1. The problem statement, all variables and given/known data

    Hey guys! :)
    I have an exam tomorrow i am a stuck with this question while reviewing :S..
    Can anyone help me plz?

    consider a cylinder capacitor of lenght 1m , inner radius 0.1cm and outter radius 0.2cm. You connect the capacitor to a 10V battery. What is the electric field strength at the inner radius?
    a) 0v/m
    c)7.7 kv/m

    2. Relevant equations

    C= k*L/ln(b/a)

    3. The attempt at a solution

    first capacitance of a cylindrical capacitor is known to be
    C=K*L/ln(b/a) (b=0.5/a=0.1)
    then i thought to calculte the charge q=C.V
    but i can't see the relation to find the electric field E :S
    and also the thing about the inner radium made me confused..
    Last edited: Apr 25, 2013
  2. jcsd
  3. Apr 25, 2013 #2


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    Homework Helper

    You see a cylindrical capacitor in the figure. Try to use Gauss' Law to get the electric field at the inner radius r=0.1 cm.


    Attached Files:

  4. Apr 25, 2013 #3
    will it be =0?? cause in the inner radius no charges?

    this is gauss law : E=q/epsilom0*A :S
    i tried the calculation no answer fits mine.. so i thought it might be 0 because it is in the inner radius?
    Sorry if it seems that i am so narrow minded.. i have been studying physics since this morning,got look of chapters and exams coming this week..
    Thanks again! i really appreciated your help
  5. Apr 25, 2013 #4
    i think i have found it!! :D
    narrow minded me.. tonight.. because tired.. mixing the formulas..
    the true formula is :
    C= 2pii*eps0*L/ln(b/a)
    then q=c*v
    and then E=q/eps0*A(gauss law)!
    and i got the last choice 14.4kV/m!

    Just one more thing.. can anyone track me if it is correct?? you gave me this procedure,did i used it in the right way?

    Evening! and thanks a lot!
    Last edited: Apr 25, 2013
  6. Apr 25, 2013 #5


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    Homework Helper

    It is correct. :smile:

  7. Apr 25, 2013 #6
    good morning ehild :)
    Thanks a lot!!
    wish me luck please.. my exam is in 3h (Y) :P
    have a nice day!
  8. Apr 26, 2013 #7


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    Homework Helper

    Good luck! By the way, do not forget to use parentheses in your equations. You forgot them in your post.

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