Magnetism Whenever electrical current flows, a measurable effect called magnetism is produced in the vicinity of that current. For straight conductors, the magnetic field circles around the current in a direction dictated by the Right-Hand Rule ( RHR ). There are many versions of this rule, and the best one for this first situation is to give the thumbs-up sign with the thumb extended in the direction of the current. The fingers then wrap in the direction that the B-field (magnetic field) lines encircle the current. In fact, if you place a magnetic compass near a wire, it will align with B. The magnitude for such a current is given by Ampère’s Law , the magnetic-field equivalent of Gauss’s Law for electric fields. The equation is B= µ ₀ I in / l where B is the magnetic field density measured in teslas ( T ), µ (“mu-nought) is a constant called the permeability of free space ( 4 π × 10 ⁻⁷ ), I in refers to the current within the Amperian surface (which is just a circular loop this time), and
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