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- Thread starter kasse
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OK, thanks. Btw, isn't the last calculation wrong here: http://planetphysics.org/encyclopedia/QuarterLoopExampleOfBiotSavartLaw.html

There should be no "pi" in the answer if I'm right.

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For example, a straight wire is symmetric about it's center axis, a sheet is symmetric, a solenoid is symmetric, etc...

Some arbitrary wire is not symmetric however and Ampere's law can generally not be used there.

Also if you only have a short solenoid or wire for example (a short length in relation to it's radius) then Ampere's law does not give a very accurate answer either, because it ignores the fact that the magnetic field lines curve away from the wire at the ends; it assumes the magnetic field is nearly constant.

About the link, if the calculation up until the integral is correct then yes, the pi should cancel indeed. I haven't checked what they did before that though, but I think it's correct (so the pi is wrong).

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