# Electric field

1. Oct 26, 2009

### nns91

1. The problem statement, all variables and given/known data

1. A portion of an infinitely long, concentric cable. The inner conductor carries a charge of 6 nC/m; the outer conductor is uncharged. The radius of the inner conductor is 1.5 cm, of the outer conductor is 6.5cm and of the inner surface of the outer conductor is 4.5 cm.

Find the surface charge densities on the inside and the outside surface of the outer conductor

2. Relevant equations

density= Q/A

3. The attempt at a solution

1. So I have found that the Electric Field in 1.5<r<4.5 and r>6.5 is E= 108/r (N/C) and Electric Field in 0<r<1.5 and 4.5<r<6.5 is E=0.

So area of the cylinder is 2*pi*L. Thus

Density= Q/A = Q/(2*pi*r*L). However, since this cable is infinitely long, how do I get L then ?

2. Oct 26, 2009

### Andrew Mason

The "infinitely long" cable means the field lines have no component along the length of the cable ie. they are all perpendicular to the long axis of the cable.

This is a Gauss' law problem. Imagine a Gaussian ring of width dL with the outer surface in the middle of the outer conductor (ie. where the field must be 0), and with the inner surface inside the inner ring (ie where the field is also 0). Since there are no lateral components to the field, you only have to concern yourself with the inner and outer surfaces of this ring.

$$\int \vec E \cdot dA = \frac{q_{encl}}{\epsilon_0}$$

What is the LS of this equation? What is the enclosed charge? From that, you can work out what the charge density must be on the inner surface of the outer conductor (you know the charge density of the inner conductor so you can work out what the charge density on the outer conductor must be in order to achieve the total enclosed charge).

AM