# Electric Feild help

1. Sep 4, 2004

### dink

I just recently began the Electricity and Magnetism course. Having said that I'm requesting help with a general plan of attack for two homework problems, that is, conceptually understanding how to use the math on these problems.

First problems states there is a hollow cylinder of length h and radius R, determine the electric feild at point d to the right of the cylinder (this is the axis through the center). It also states to think of the cylinder as a collection of rings and gives me nearly the same problem solved for a single ring. My question is how would I adapt the results of the ring?

My second problem is a uniformly charged disk of radius R and charge Q. This also has the electric feild calculated along the axis through the disk. The problem asks show that the electric feild at distances x that are large compared with R approaches that of a point charge. How would I go about grappling this problem?

What I'm essentially asking is a gameplan for these problems like, do I integrate and if so along what axis. Thanks for any help you can give me.

2. Sep 4, 2004

### e(ho0n3

This is a basic calculus problem. You know what the electric field is at a point d from the center of the ring, so all you need to do is sum for the collection of rings (i.e. integrate).

Easy. Find the equation for the electric field at a distance r from the center of the disk (another calculus problem). Then, take the limit as r approaches infinity and compare the result with the electric field equation for a point charge.

Hope that helps.

3. Sep 4, 2004

### dink

Thanks, I think I have something to turn in at the least now. :p