# Clarification on an electric fields solution

1. Jan 20, 2012

### Allenman

This isn't actually a homework problem. I just had a question about the solution they provided.

1. The problem statement, all variables and given/known data

2. Solution given in solutions manual

3. My question

When they convert dE into cylindrical coordinates why does the radius "a" lose its exponent?

dE = $\frac{\kappa\lambda\delta l}{a^{2}}$ = $\frac{\kappa\lambda\delta\theta}{a}$

I have to be missing something simple, I just know it...

2. Jan 20, 2012

### SammyS

Staff Emeritus
It's because $d\ell=a\cdot d\theta\,.$

3. Jan 20, 2012

### Allenman

Does that come from the arc length formula?

Thank you

4. Jan 20, 2012

### SammyS

Staff Emeritus
Yes it does.