# mathematical transition of an integral

• April 18th 2011, 03:21 AM
transgalactic
mathematical transition of an integral
YouTube - Module 1_5_7 | MIT 8.02SC Physics II: Electricity and Magnetism, Fall 2010
(this posted has latex errors i dont know why)
here is a print screen of the post
http://i54.tinypic.com/2zi0kex.jpg
in here in the end
i got an arch that startes at A and end in B
he wrote this integral
$
\intop_{a}^{d}\frac{{dl}}{r^{2}}
$

but point a and be are not values that i can put in
if my function depends on "r" then i need to change dl to be dr expression somehow
?
i think that the integral should go from 0 to pi
and dl should be d theta expression (instead of dr)
and dl =r*d(theta)
R is constant
i get
$
\intop_{0}^{2\pi}\frac{{Rd\theta}}{R^{2}}
$

i get 2pi/R
this resolt is different from the resolt he gets
why?
• April 18th 2011, 12:04 PM
running-gag
He's getting 2piR/R² which is 2pi/R, there is no difference