1. The problem statement, all variables and given/known data: Co-axial cable, relative permittivity, capacitance, internal energy A long straight co-axial cable of length 1 consists of an inner conductor of radius r1 and a thin outer conductor or radius r2. The dielectric between the conductors has a relative permittivity εr. (a) Find the strength of the electric field E(r) between the conductors (r1 < r < r2) for equal and opposite charges ±Q on the conductors. (b) Calculate the total potential difference V between the conductors if they were to carry equal and opposite charges ±Q. 2. Relevant equations: Gauss' Law ∫E.ndA = Q/εr dV=-∫E.dl 3. The attempt at a solution: Part a) I'm not sure if i'm being really stupid here but are you able to work this question out like this? first find the electric field produced at the inner and outer surface of the outer and inner conductors respectively and then sum them together?: for the inner cable: ∫E.ndA = Q/εr ⇒ E(2πr1l)=Q/εr ⇒ (l=1) ⇒ E = Q/2πr1εr For the outer cable: Used same method as before but when taking into account the normal I got the same answer but with r2 in place of r1 1 E=2πr2εr Then I summed these together to get: E=(Q/2πεr)(1/r1+1/r2) I thought this was correct but i am not doubting myself as I am struggling to do part b). Part B) To complete this part of the question surely i need my expression for a) to be in terms of an arbitrary value r rather than r1 and r2. Any help would be hugely appreciated as i've been going round in circles with this for a while now.