Electric potential

1. Apr 25, 2013

universitypq98

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
b)infinity
c)7.7 kv/m
d)14.4kV/m

2. Relevant equations

C= k*L/ln(b/a)
q=C*V
E=V/d
E=k*q/R^2

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

Last edited: Apr 25, 2013
2. Apr 25, 2013

ehild

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.

ehild

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3. Apr 25, 2013

universitypq98

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

4. Apr 25, 2013

universitypq98

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
5. Apr 25, 2013

ehild

It is correct.

ehild

6. Apr 25, 2013

universitypq98

good morning ehild :)
Thanks a lot!!
wish me luck please.. my exam is in 3h (Y) :P
have a nice day!

7. Apr 26, 2013

ehild

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

ehild