## clarification on an electric fields solution

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...

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Mentor
 Quote by 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...
It's because $d\ell=a\cdot d\theta\,.$

 Does that come from the arc length formula? Thank you

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Yes it does.