I know that \oint_{C}\mathrm{d}\vec{l} = 0, for any closed curve C.
But when i try to calculate the integral around the unit circle in polar coordinates, I get a result different from zero.
Here is my approach : \oint_{C}\mathrm{d}\vec{l} = \int_{0}^{2\pi}\hat{\phi}\mathrm{d}\phi =...