# Griffiths 4.21

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1. Oct 11, 2015

### PhysicsPrincess

1. The problem statement, all variables and given/known data
http://postimg.org/image/vzhqi8er5/

2. Relevant equations

3. The attempt at a solution
I understand all the calculations here - http://www.physicspages.com/2012/10/18/coaxial-cable-with-dielectric/
I have one issue that is bugging me though - if λ charge density is distributed over the inner conductor, why can it be assumed that the same -λ is distributed over the outer shell? Aren't there any bound charge densities involved here?

Thank you.

2. Oct 11, 2015

### BvU

With bound charge densities you mean the polarization ? That is involved. That's why (for $\epsilon_r > 1$) C is greater than for the same coax without the dielectric. But the $\lambda$ are the same: polarization has no total charge.