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MAGNETISM Magnetism A Strangely Attractive Topic History #1 à Term comes from the ancient Greek city of Magnesia, at which many natural magnets were found. We now refer to these natural magnets as lodestones (also spelled loadstone; lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4. à Pliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron. History #2 à Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic property…and that such a rod when suspended from a string would align itself in a north-south direction. à Use of magnets to assist in direction-finding can be traced back to at least the eleventh century. Basically, we knew the phenomenon existed and we learned useful applications for it. We did not understand it. Scientists Can Be Famous, Too! Tesla Famous, continued Gauss The Concept of “Fields” Michael Faraday realized that ... A magnet has a ‘magnetic field’ distributed throughout the surrounding Finally, the Science à Not until 1819 was a connection between electrical and magnetic phenomena shown. Danish scientist Hans Christian Oersted observed that a compass needle in the vicinity of a wire carrying electrical current was deflected! à In 1831, Michael Faraday discovered that a momentary current existed in a circuit when the current in a nearby circuit was started or stopped à Shortly thereafter, he discovered that motion of a magnet toward or away from a circuit could produce the same effect. Let This Be a Lesson! à Joseph Henry (first Director of the Smithsonian Institution) failed to publish what he had discovered 6-12 months before Faraday The Connection is Made SUMMARY: Oersted showed that magnetic effects could be produced by moving electrical charges; Faraday and Henry showed that electric currents could be produced by moving magnets A Sheep in a Cow Costume? All magnetic phenomena result from forces between electric charges in motion. Looking in More Detail à Ampere first suggested in 1820 that magnetic properties of matter were due to tiny atomic currents à All atoms exhibit magnetic effects à Medium in which charges are moving has profound effects on observed magnetic forces For most of our discussions, we will assume the medium is empty space, which is a reasonable approximation of air in this context. MAGNETISM –THE ABILITY OF CERTAIN MATERIALS TO ATTRACT IRON, COBALT OR NICKEL. CLASSIFICATION OF MAGNETS NATURAL MAGNETS ARTIFICIAL PERMANENT MAGNETS ELECTROMAGNET NATURAL MAGNET -EARTH The Earth’s Magnetic Field We know Earth has a core of metal that rotates. This generates the so-called “Main Field” of the earth. Still some debate about exactly how this happens (e.g. why currents don’t dissipate). Movement of metal & charge in geodynamo creates field - more about this in a moment. NATURAL MAGNETS-LODESTONES LODESTONES BECAME MAGNETIZED BY LYING IN THE EARTH’S MAGNETIC FIELD FOR THOUSANDS OF YEARS THE ANCIENT GREEKS RECOGNIZED THE MAGNETISM OF LODESTONES MORE THAN 2500 YEARS AGO THE SPHERE OF ARCHIMEDES ARTIFICIAL PERMANENT MAGNETS HARD STEEL MAGNETIC COMPASS ALNICO HARD STEEL—HORSHOE MAGNET MAGNETIC COMPASS The magnetic compass was brought to Europe in the Middle Ages from the Chinese who had been using the compass for over 1500 years ALNICO—ALLOY OF: AL—ALUMINIUM NI—NICKEL CO—COBALT ALUMINUM CAN NOT BE MAGNETIZED ALNICO—SUPER STRONG MAGNETS Creating Magnets Methods used to create a magnet The picture shows that a screwdriver does not pick up paper clips. Creating Magnets Stroking using a permanent magnet However, after stroking the screwdriver with a magnet, the blade itself becomes magnetic and attracts the paper clips. Removing Magnetism Heat is one way to remove magnetism. Flux Magnetic lines of force Strongest at the poles LAWS OF MAGNETISM EVERY MAGNET HAS TWO POLES LIKE MAGNETIC POLES REPEL: UNLIKE POLES ATTRACT THE FORCE OF ATTRACTION OR REPULSION FOLLOWS INVERSE SQUARE LAW EVERY MAGNET HAS TWO POLESNORTH & SOUTH LIKE MAGNETIC POLES REPEL EACH OTHER UNLIKE POLES ATTRACT EACH OTHER THE FORCE OF ATTRACTION OR REPULSION BETWEEN TWO MAGNETS Pp F= k d2 THE FORCE IS PROPORTIONAL TO THE PRODUCT OF THE MAGNETIC POLE STRENGTHS DIVIDED BY THE SQUARE OF THE DISTANCE BETWEEN THEM NATURE OF MAGNETISM BREAKING A MAGNET WILL RESULT IN EACH FRAGMENT BECOMING A WHOLE MAGNET HEATING OR HAMERING STEEL IN PROXIMITY TO MAGNETIZE IT. A MAGNET –WILL STROKING OF IRON WITH MAGNET WILL MAGNETIZE IT GENTLY RUBBING A TEST TUBE OF IRON FILINGS NEAR A MAGNET WILL MAGNETIZE IT MAGNETIZATION AN ELECTRIC CHARGE SPINNING ON ITS AXIS WILL CREATE MAGNETIC FIELD MOVING CHARGED PARTICLE WILL INDUCE MAGNETIC FIELD THE SMALL MAGNET CREATED BY ELECTRON ORBIT IS CALLED A MAGNETIC DIPOLE ACCUMULATION OF MANY ATOMIC MAGNETS WITH THEIR DIPOLES ALIGNED CREATES A MAGNETIC DOMAIN THE MAGNETIC DIPOLES IN BAR MAGNET GENERATE IMAGINARY LINES OF THE MAGNETIC FIELD MAGNETIC LINES OF FORCE ARE CLOSE LOOPS MAGNETIC LINES OF FORCE EARTH MAGNETIC LINES OF FORCE BAR MAGNET THE STRENGTH OF MAGNETIC FIELD IS EXPRESSED IN SI UNIT TESLA 1 TESLA = 10,000 GAUSSES MAGNETIC ELEMENT TWO OR MORE ELECTRONS SPINNING IN THE SAME DIRECTION NUCLEUS A MAGNETIC ELEMENT CAN EXISTS EITHER IN MAGNETIZED OR NONMAGNETIZED STATE MAGNETIZED MAGNETIC MATERIAL NON-MAGNETIZED MAGNETIC MATERIAL IN NONMAGNETIC MATERIALS JUST AS MANY ELECTRONS SPIN IN ONE DIRECTION AS IN OTHER. THEIR OPPOSING MAGNETIC FIELDS CANCEL OUT NUCLEUS The Earth’s Field & Solar Wind The earth is continuously blasted by ionized particles blown off of the sun - called the “solar wind.” These fast particles generate their own field and interfere with that of the earth. Variations in both Main and External Fields cause the change in magnetic declination. Not to worry - near the earth, the Main Field dominates. CHARACTERISTICS OF THE LINES OF FORCE FROM NORTH TO SOUTH OUSIDE THE MAGNET IN THE SPACE FROM SOUTH TO NORTH INSIDE THE MAGNET IF IN THE SAME DIRECTION THEY WILL REPEL MAGNETIC FIELD IS DISTORTED BY MAGNETIC MATERIALS BUT NOT AFFECTED BY NON-MAGNETIC MATERIALS LINES OF FORCE LINES OF FORCE REPEL N N S S LINES OF FORCE ATTRACT S N N S NONMAGNETIC MATERIALS DO NOT AFFECT THE LINES OF FORCE OF MAGNETIC FIELD MAGNETIC MATERIALS LIKE IRON AFFECT THE LINES OF FORCE MAGNETIC INDUCTION (MAGNETIZATION) MAGNETIZATION –DOMAINS MOVE MAGNETIC PERMEABILITY EASE WITH WHICH A GIVEN MATERIAL CAN BE MAGNETIZED MAGNETIC RETENTIVITY MATERIAL RESISTANCE TO MAGNETIZATION SOFT IRON HIGH PERMEABILITY LOW RETENTIVITY HARD STEEL LOW PERMEABILITY HIGH RETENTIVITY “EASY COME—EASY GO” MAGNETIC CLASSIFICATION OF MATTER FERROMAGNETIC PARAMAGNETIC MATERIALS NONMAGNETIC MATERIALS DIAMAGNETIC MATERIALS FERROMAGNETIC MATERIALS –IRON, COBALT, NICKEL. STRONGLY ATTRACTED BY MAGNETS PARAMAGNETIC MATERIALS—PLATINUM. weakly ATTRACTED BY MAGNETS NONMAGNETIC MATERIALS—COPPER, WOOD, GLASS. THEY ARE NOT ATTRACTED TO MAGNETS. DIAMAGNETIC MATERIALS—BERYLLIUM & BISMUTH. weakly REPELLED and ATRRACTED BY MAGNETS DETECTION OF MAGNETISM ---COMPASS GEOGRAPHIC NORTH POLE IS MAGNETIC SOUTH POLE????? S. MAGNETIC N. GEOGRAPHIC N. MAGNETIC S. GEOGRAPHIC SMALL MAGNETS ELECTROMAGNETS TEMPORARY MAGNETS PRODUCED BY MEANS OF ELECTRIC CURRENT. ELECTROMAGNET MRI ELECTROMAGNET Electromagnetism Electromagnets A large electromagnet used to lift scrap metal A 30-foot diameter electromagnet used in research for a superconductor Electromagnets The source of electromagnetism is electricity (current) flowing through a coil of wire. Electromagnets A coil of wire can be made into a magnet by passing an electric current through it. Without electricity, there is no magnetic force Electric current applied to a coil creates a magnetic field Electromagnetic Strength How can an electromagnet be made stronger? Increase the number of coils. Increase the electric current flowing through the coil. Add an iron core in the center of the coil. Electromagnetic Strength The left hand rule to determine North Pole NORTH POLE SOUTH POLE Making a Magnet An example of a very strong magnet Making a Magnet The coil of wire has been attached to a 12 volt DC power source making an extremely strong magnet. The magnet is strong enough to lift this 50 pound engine cylinder head. Saturation Saturation of a magnetic field occurs when an increase of electric current flow does not increase the strength of the magnetic field as shown by the graph. The knee of the curve is the point that saturation occurs. Y Saturation Knee Y- Axis Increasing Magnetic Force X X- Axis Increasing Electric Current MAGNETIC COIL LOSSES Copper losses - resistance of copper coils. Hysteresis - magnetic flux inducing a current in the core. Eddy currents - magnetic polarity of the iron core is rapidly changed by an AC current causing friction and heat between the molecules ELECTROMAGNETIC DEVICES Electromagnetic Devices Electromagnetic Devices