Magnetic Field of a Current Loop
Field at Center of Current Loop
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The form of the magnetic field from a current element in the Biot-Savart law becomes 
which in this case simplifies greatly because the angle =90 ° for all points along the path and the distance to the field point is constant. The integral becomes 
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The current used in the calculation above is the total current, so for a coil of N turns, the current used is Ni where i is the current supplied to the coil.
The Earth's magnetic field at the surface is about 0.5 Gauss.
Discussion of current loop Index