Electric Fields in Coaxial Cables: Understanding Bound Charge Densities

[PhysicsPrincess]

Homework Statement

http://postimg.org/image/vzhqi8er5/

Homework Equations

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.

 

[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.