# Charge Volume Density To Charge Per Unit Length

1. Sep 15, 2013

### Baumer8993

1. The problem statement, all variables and given/known data
http://imgur.com/VzZjSEH

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

2. Relevant equations
Surface area, and volume of a sphere.

3. 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?

2. Sep 16, 2013

### rude man

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.

3. Sep 16, 2013

### vela

Staff Emeritus
Calculate the total charge Q of a length $l$ of the insulator. Then divide Q by $l$.