What is Electromagnetism

Chapter 14.2 Electromagnetism

Lessons In Electric Circuits Volume I – DC Book
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Lessons In Electric Circuits Volume I – DC Book

  • 14.2. ELECTROMAGNETISM465• Lodestone(also called Magnetite) is a naturally-occurring ”permanent” magnet mineral.By ”permanent,” it is meant that the material maintains a magnetic field with no externalhelp. The characteristic of any magnetic material to do so is called retentivity.• Ferromagneticmaterials are easily magnetized.• Paramagneticmaterials are magnetized with more difficulty.• Diamagneticmaterials actually tend to repel external magnetic fields by magnetizing inthe opposite direction.14.2ElectromagnetismThe discovery of the relationship between magnetism and electricity was, like so many otherscientific discoveries, stumbled upon almost by accident. The Danish physicist Hans ChristianOersted was lecturing one day in 1820 on the possibilityof electricity and magnetism beingrelated to one another, and in the process demonstrated it conclusively by experiment in frontof his whole class! By passing an electric current through a metal wire suspended above a mag-netic compass, Oersted was able to produce a definite motion of the compass needle in responseto the current. What began as conjecture at the start of the class session was confirmed as factat the end. Needless to say, Oersted had to revise his lecture notes for future classes! Hisserendipitous discovery paved the way for a whole new branch of science: electromagnetics.Detailed experiments showed that the magnetic field produced by an electric current isalways oriented perpendicular to the direction of flow. A simple method of showing this rela-tionship is called the left-hand rule. Simply stated, the left-hand rule says that the magneticflux lines produced by a current-carrying wire will be oriented the same direction as the curledfingers of a person’s left hand (in the ”hitchhiking” position), with the thumb pointing in thedirection of electron flow:IIIIThe "left-hand" ruleThe magnetic field encircles this straight piece of current-carrying wire, the magnetic fluxlines having no definite ”north” or ”south’ poles.While the magnetic field surrounding a current-carrying wire is indeed interesting, it isquite weak for common amounts of current, able to deflect a compass needle and not much