# Homework Help: Proof that disk of charge = point charge when very far?

Tags:
1. Feb 25, 2016

### Brennen berkley

1. The problem statement, all variables and given/known data
Take the expression 21.11 (pictured below, specifically the bottom one) for the electric field above the center of a uniformly charged disk with radius R and surface charge density σ, and show that when one is very far from the disk, the field decreases with the same square of the distance as it would for a point charge, E ~ q/4πεx2, where q is the total charge on the disk.

2. Relevant equations

3. The attempt at a solution
I solved one like this pretty easily where you prove that a long wire acts as an infinite wire, but I've been looking at this one for about an hour and I'm stumped. I know I need to get x2 on the bottom of the equation without being inside a square root, but I don't know how. The only approximation I can think of is that when R is much bigger than x, √(x2 + R2 + 1) goes to 1, but then I don't have an x anymore.

2. Feb 25, 2016

### TSny

3. Feb 25, 2016

### haruspex

I think you mean x much larger than R.
Do you know the binomial expansion of (1+t)a for small t?

Edit: must learn to post more quickly.

4. Feb 25, 2016

### Brennen berkley

Yes, I meant if x is much larger than R then √(R2/x2 + 1) goes to 0 (hopefully I said it right that time). I haven't done binomial expansions, for a while so I'll review those, thanks.

Last edited: Feb 25, 2016
5. Feb 25, 2016

### Brennen berkley

Ok I'm still stuck. I used σ = q/πR2 and did the binomial expansion and got this:

EDIT: that should be R2/2x2

6. Feb 25, 2016

### TSny

Note that $\frac{1}{\sqrt{1+t^2}} = (1+t^2)^{-1/2}$

7. Feb 25, 2016

### Brennen berkley

I see how to do it know, it's pretty simple, thanks.