# Charge Volume Density To Charge Per Unit Length

## Homework Statement

http://imgur.com/VzZjSEH

I took a screen shot so I did not leave anything out.

## Homework Equations

Surface area, and volume of a sphere.

## The Attempt at a Solution

I am trying to find the linear charge density from the charge volume density. I got it explained from a friend. It was right, but made no sense. He did it by taking the volume density, and multiplying it by the charge volume density by pi, and (outer r2 - inner r2).

So he did -657 * pi * (.0422 - .0262)

Can someone explain to me how this is right?

rude man
Homework Helper
Gold Member
It's the total charge per unit length due to the outer insulator. Charge = voume charge density x volume. The volume of the insulator is pi(r2^2 - r1^2) x 1 unit length. Multiply by the charge density rho and you have your answer.

Of course, the complete answer must include the linear charge density of the wire as well.

vela
Staff Emeritus