# Electric field

1. Mar 27, 2005

### colonel

Suppose there is a positively charged wire inside a negatively charged cylinder. What would be the net external electric field? Would I first need to combine the charges, or can I just combine the seperate fields?

2. Mar 27, 2005

### Andrew Mason

You would draw a Gausian cylinder around the charged cylinder and determine the enclosed charge. Then you would apply Gauss' law to determine the flux through that surface:

$$\oint E\cdot dA = \frac{q}{\epsilon_0}[/itex] where q is the enclosed charge. To find q, just add the charges (negative subtracts from positive). If they sum to 0, there is no external field. Since E would be the same at all points on the Gausian surface, and since the area of the cylinder is $A = 2\pi RL$ the external field E would be: [tex]E = \frac{q}{2\pi\epsilon_0RL}$$

AM