Let's say I want to calculate the magnetic field at a distance d from the(adsbygoogle = window.adsbygoogle || []).push({}); centerof a wire of finite length L, carrying a current I. Why would it be wrong to apply Ampere's law to a circular path of radius d centered on the wire, and say that the integral of B.dl is simply B times 2pi*d? (obviously it gives the wrong answer...)

Is the magnetic field not constant along this circular path? I would say so - the problem obviously has cylindrical symmetry.

Is it not parallel to the path at all points? I would think so - from Biot-Savart's law applied to every small element of the wire.

So where is the mistake in this logic?

Thanks,

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# Magnetic field of a finite-length wire

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