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Physics 106 Lesson #19 Magnetism Dr. Andrew Tomasch 2405 Randall Lab [email protected] Review: Engineering Trade Studies • When a number of possible engineering solutions exist to meet a given need or goal, engineers perform a “trade study”. • This is a useful approach whenever a choice is to be made between a number of possible options. • The benefits, disadvantages and costs are quantified for each option and decisions are then made by “trading off” advantages against disadvantages for the various options based on a goal such as minimum cost, safety, or maximum reliability. Magnets • All magnets have two poles: North and South • Like poles repel each other; Unlike poles attract each other • Magnetic poles are always found in pairs • Isolated magnetic poles have never been found Magnetic Forces Electricity and Magnetism • Originally thought to be independent phenomena • Like gravity, both have actionat-a-distance force laws • Faraday and others believed in the unity of nature • Maxwell proved that they were both aspects of a single phenomenon: Michael Faraday Electromagnetism James Clerk Maxwell The Magnetic Field The magnetic field is a vector and denoted by the letter B B • The north pole of a compass needle is a magnetic north pole • The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle (test magnet) placed at that point Magnetic Field Lines • Magnetic field lines surround magnets • The magnetic field is always tangent to the magnetic field line • The number of lines per unit area (density) is proportional to the magnetic field strength • Outside of the magnet, magnetic field lines point away from north poles and toward south poles B Iron Filings: Visualizing Magnetic Field Lines The magnetic field lines inside a bar magnet run from south pole to north pole. The Earth’s Magnetic Field • The Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth • Caused by the “dynamo effect”, the rotation of a partially ionized outer core → internal electric current! • The north pole of a compass needle is attracted to the geographic North Pole, which is therefore a magnetic south pole South magnetic pole Magnetic Field Produced by a Long, Straight Wire Demonstration: Oersted Experiment Field Direction for a Straight Wire • The magnetic field due to the current in a long straight wire has circular field lines around the wire • The direction of the field is given by a right hand rule Electromagnets • Current flowing in a loop of wire creates a magnetic field • The Current loop can be visualized as a “phantom bar magnet” N S = N S Which End is the North Pole? • Another Right Hand Rule! N S The Field of a Solenoid Current Into Page B Current Out of Page Inside a solenoid (away from the ends) the magnetic field is constant.