E&M-The magnetic field

© 2012   Fernando Caracena

Fig. 1. A magnetic field forms around an electric current in a way that deflects a magnet at right angles around the wire in direction that a right handed screw has to be turned to advance in the direction of the electric current (from positive toward negative).

On 21 April 1820, Hans Christian Oersted made a surprising discovery as he gave a public lecture on electricity. He was demonstrating the heating of a straight wire when an electric current flows through it. He would be also demonstrating magnetic effects on a compass needle as a separate experiment. The compass happened to be near the wire of his first experiment. Its response to the electric current was a surprise  gave everyone, especially Oersted: when electric current ran through the wire, the compass was deflected perpendicular to the wire always along the tangent to a circle in a plane perpendicular to the wire. [Actually, there would have been a small distortion introduced by the Earth's own magnetic field.] The direction of the magnetic field (as identified by the compass) was in the direction the the fingers on your

Fig. 2. The magnetic field about a bar magnet is imaged by the alligment of iron filings sprinkled on top of a sheet of paper placed over it. From the Wiki Commons.

right hand would would point if you grasped the wire with your right hand, the thumb extended along the wire in the direction of the current. This was the first recorded effect of electric currents' generating a magnetic field, the electromagnetism that is part of the functioning of our modern world. (For some applications see here.)

Ordinary magnetism, as understood before Oersted's discovery, was that of magnetic lines of force of permanent magnets such as shown in Fig.2. The lines of magnetic force were imagined to emerge from the north-seeking pole (N) of the bar magnet, and sink into the south-seeking pole (S) in a manner similar to the way the lines of electric force form about an electric dipole. However, subsequent experimentation showed that there was no analog for electric charge in magnetism (see magnetic monopoles).



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