# Homework Help: Wire in Field

1. Feb 11, 2005

### Soaring Crane

A very long straight wire has charge per unit length 1.49×10-10 C/m.

At what distance from the wire is the magnitude of the electric field equal to 2.49 N/C?

Use 8.85×10-12 C^2/(N*m^2) for the permittivity of free space and use pi = 3.1416 for E = (lambda)/(2*pi*permittivity of free space*r).

Given:
E = 2.49 N/C

I have no idea what to do with this formula. Why is wavelength needed and what is the variable we are trying to find? I am lost.

Thanks.

2. Feb 11, 2005

### Staff: Mentor

In this formula, lambda stands for the "charge per unit length", not wavelength. You are trying to find the distance from the wire, which is "r".

3. Feb 11, 2005

### dextercioby

$\lambda$ is not a wavelength,it's the linear charge density and it's measured in C/m.
Plug in all the numbers in the formula & see what happens...

Daniel.