|MadSci Network: Physics|
Juan, I am disappointed to tell you that this is a fundamental truth discovered by experiment. Danish physicist Hans Christian Oerstad is credited with discovering the field direction. (See Isaac Asimov: The New Intelligent Man's Guide to Science.) Oerstad observed the field makes circles around the charge -- the circle plane being perpendicular to the velocity vector. The easiest way to make moving charges is to pass an electric current through a wire. In 1819, French physicists Biot and Savart experimentally derived the law bearing their name relating the magnetic field strength outside a thin current-carrying wire to the current in the wire and the distance from the wire. No current -- no magnetic field. (See almost any physics text covering magnetodynamics with the Biot-Savart law in the index.) At that time, recognize we hadn't yet discovered the electron! That discovery did not come until the 1890's. We now know electric current in a wire is moving electrons. (How does it feel knowing something Benjamin Franklin didn't know?) So summing up, it's because experimenters found both to be so. Larry Skarin
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