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Lodestones • Natural Magnets • Magnetite, Fe3O4 (an oxide of iron) • Ancient civilizations (Greek 590 BCE, Chinese 2600 BCE) realized that these stones would cling to iron tools. • A suspended, pivoting lodestone always pointed along the NorthSouth axis • Magnetite crystals have been found in living organisms Magnetic Poles • Magnets produce a force on other objects • Poles are regions where the magnetic force is the strongest • Like magnetic poles repel. • Opposite magnetic poles attract. • Most magnets have two poles (dipole), but can have three or more! 1 Have we ever seen a monopole? • Monopole: piece of a magnet that is simply a north pole or a south pole • Many have tried to isolate a monopole by breaking magnets in half. • No matter how we break a magnet, the pieces are always dipoles! • No one has ever found a monopole (but that doesn't mean that they aren't still trying) Magnetic Field Mapping with TestCompass Field Lines Form Closed Loops Field Mapped by Iron Filings 2 Magnetic Field Every magnet establishes in the space surrounding it, a magnetic field (Bfield) Map field with a testcompass Direction of field is direction in which the testcompass needle will point at that location. Draw field lines so that compass always points tangent to the field lines. Field lines point from N to S outside the magnet Field lines point from S to N inside the magnet Field lines form closed loops Field lines never intersect N S Earth’s Magnetism • Magnetic field has reversed direction ~300 times in the past 170 million years • Magnetic poles wander! • Magnetic & geographic poles not the same. • Magnetic declination: 11.5° • What’s strange about this picture? à 3 Electrodynamics: The Study of Electromagnetism Magnetism is caused by charge in motion. Charges at rest have just an electric field But, when they move, they generate both an electric field and a magnetic field Electrons function as a subatomic dipole Electron “spin” is shown in energy levels Electrons existing in pairs: Bfields cancel Electron “orbit” around nucleus Random “orbits” of electrons: Bfields cancel Magnetic Domains sizeusually between 106 and 103m • Domain: region where many atomic dipoles are aligned • Usually aligned randomly and effects cancel • Place ferromagnetic material in strong Bfield • Entire domains realign with applied field • Creates "permanent" magnets (at least until domains lose overall alignment) S Domains are not aligned Substance is Permanently Magnetized Electrons in domains align with applied field 4 Materials: ferromagnetic: domains are very magnetic, however the material will not be magnetic by itself. when placed in an external field, it will display strong magnetic properties and act like a magnet until removed from the field. Paramagnetic: Material will weakly become magnetic when in the presence of an external field. domains will align with magnesium, molybdenum, lithium the external field. Diamagnetic: Material will weakly become magnetic when in the presence of an external field. Domains will align in a direction opposite of the external field. Repelled copper, silver, and gold Can look at individual charges or electric current in a wire Direction of current, or motion of positive charge, determines direction of the magnetic field. Use right hand rules for analysis and direction. 5 First Right Hand Rule: thumb points in direction of current, fingers curl in direction of magnetic field note compass readings. Use for currentcarrying wire. Fig 19.15b, p.678 Slide 21 Diagramming 3D Magnetic Fields • Not everybody is an artist. • Use 2D images to draw 3D field vectors. • If field points perpendicularly into the page or X X X X board, use X X X X • If field points perpendicularly out of the page • • • • or board, use • Otherwise, draw the lines neatly. • Don’t forget, field lines are vectors! B I 6 Sketch the magnetic fields around each of the four sides of the current carrying loop. I X X X I X X Magnetic field of a long straight wire B I • • • • • B: magnetic field strength (Teslas T) I: current (amperes A) r: radius from wire (meters m) μo: permeability constant in a vacuum μo = 4π x 107 T·m/A • What is the shape of this magnetic field? 7 The magnetic field for a loop of wire is found by the equation : radius of the loop A loop of wire with a radius of 5cm has a current of 4A traveling through it. What is the strength of the magnetic field at the center of the loop? 2nd Right Hand Rule Fingers curl in direction of current, thumb points to direction of magnetic field. Use for current carrying loop or solenoid coil. B 8