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Prove. The magnetic field intensity at the center of a flat circular loop of wire of radius r carrying current I is given by

Proof. Let us use Biot’s formula

where θ is the angle between the direction of ds and the line from ds to P. See Fig. 1.

We note that all the dB vectors at point P arising from all incremental elements ds on the wire coincide. This means we can do an algebraic sum instead of a vector sum to sum the dB’s. We also note that

∑ds = circumference of circle = 2πr

and that

sin θ = sin 90^{o} = 1 for all elements ds.

Integrating 2) over the loop C

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