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I have been asked to calculate the inductance of a double tape line :

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Now we know that $$ N\phi = LI$$ So let's calculate the magnetic field by assuming an amperian loop like this:

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$$\int B.dl = \mu_oI$$

But it seems to me that the current in the upper strip only contributes to the magnetic field though i know that the value of $B$ i get is the net magnetic field. How do we explain that the current in the lower strip also contributes to the magnetic field?

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    $\begingroup$ Consider each tape individually. The only difference is that the current is running in opposite directions in the two tapes, so each tape with its current produces a magnetic field; it's just that the magnetic field goes clockwise around one and counterclockwise around the other. $\endgroup$
    – S. McGrew
    Commented Apr 5, 2019 at 14:52

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