Okay I've got a fields exam coming up so like a good boy I've been practising with past papers but there is this one question that is driving me batty(adsbygoogle = window.adsbygoogle || []).push({});

a) Consider a long cylindrical co-axial capacitor with inner conductor radius a, outer conductor radius b, and a dielectric constant that varies with cylindrical radius K(r). Show that for the energy density the dielectic to be constant, K(r) must equal k/r^2.

b) Given that the capacitor is charged to voltage V, determine the electric field E(r) as a expression of V, r, a and b.

Okay part a I can sort of do by calculating the E field based on Gauss' Law and subbing into the expression for energy density. However this aproach requires that the charge density of the capacitor is constant throughout, which the question does not specify and seems a bit of a leap of faith.

part b I have no idea with, except it probably involves the boundary conditions of the E field and D.

Please help, this subject is starting to make quantum mechanics look easy

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# Homework Help: Fields help

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