The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire. The direction of the magnetic field is perpendicular to the wire and is in the direction the fingers of your right hand would curl if you wrapped them around the wire with your thumb in the direction of the current.
The magnetic field of an infinitely long straight wire can be obtained by applying Ampere's law. The expression for the magnetic field is
The earth's magnetic field is about 0.5 gauss. The permeability of free space is:
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