Friday, 4 September 2015

electromagnetism - Doubt regarding Ampere's Circuital Law


The Ampere's Circuital law states


$$\oint B\cdot d\ell~=~ \mu_0I$$



We can use it to derive the magnetic field of an infinitely long current carrying wire easily. My question is, why does the wire need to be infinitely long? I know it has something to do with $B$ being constant and tangential to the loop at every point for easy evaluation of the integral, but I can't find an explanation to my question.




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