# Homework Help: Electric Field in and outside a cylinder

1. Feb 27, 2010

### reising1

An infinitely long cylinder has charge density lamda and has a radius R.

What is E when r < R and when r > R.

I know the formula for an infinite line of charge is
E = (lamda) / (2 pi epsilon not * r)

Thanks!

2. Feb 27, 2010

### xcvxcvvc

do you know Gauss' law?

3. Feb 27, 2010

### reising1

Yes. Flux = q enclosed / Epislon

4. Feb 27, 2010

### xcvxcvvc

Hah. Well, have you tried to apply Gauss' law to this problem? The forum rules forbid posters from doing your homework for you. Instead, we are here to guide you. You won't get any help without posting the source of your confusion or posting work.

"here is the problem, thanks" is not acceptable

5. Feb 27, 2010

### reising1

Okay well I know that the integral of E dotted with dA is the flux, which can be simplifed to EA due to the symmetry of the problem. And the area would be equivlant to the pi r squared times length. And q enclosed would be lamda times L.