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# zhang19 - 2/15/2010 Magnetism Loud speaker Credit card...

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2/15/2010 1 Magnetism Loud speaker Credit card Video/audio tape; floppy disk ; Hard drive Electric generator/motor Bullet train Hybrid cars All have magnets!

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2/15/2010 2 A magnet always has two poles , called north and south , at the same time Like poles repel, unlike attract each other - Similar to electric charges Magnetic poles cannot be isolated: If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole - Different from electric charges A compass is a small piece of magnetic pointer free to align itself with a magnetic field A magnet produces a magnetic field A vector quantity; symbolized by Direction is given by the direction a north pole of a compass needle points in that location Magnetic field lines can be used to show the field, as traced out by a compass, would look In addition to permanent magnets, moving charge (electric current) also includes a magnetic field (discussed later) B B
2/15/2010 3 Similar to the field of a positive and a negative charge (electric dipole) Arrows give direction Density gives strength x x x x x x x x INTO Page • • • • • • • • OUT of Page points field B points field B Iron filings are also often used to show the pattern of the magnetic field lines Compared to field lines of electric charges

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2/15/2010 4 The Earth’s magnetic field resembles that of a huge magnetic bar The Earth’s geographic north pole corresponds to a magnetic south pole ; its geographic south pole corresponds to a magnetic north pole The earth’s magnetic filed direction reverses every few million years When a charged particle is moving through a magnetic field, a magnetic force acts on it (electric force does not require moving of charge) Maximum when the charge moves perpendicularly to the magnetic field lines Zero when the charge moves along the field lines v v v B 0 F maximum F ? F
2/15/2010 5 Magnitude of the force Can be used to determine the magnitude of the magnetic field: Obviously, if sin qvB F sin qv F B v B 0 ) 0 sin( , 0 qvB F max 0 ) 90 sin( , 90 F qvB qvB F 1 ) 90 sin( 0 ) 0 sin( o o ) , measure , known are , q (if B F ,v The direction of magnetic force is perpendicular to both B and v Right-hand-rule: Point (to the direction of v) curl (to the direction of B) Thumb points to the direction of F Only true for positive charge For negative charges, use the same rule, but reverse the direction of force at the end sin qvB F • • • • • • • • page - of - out field B

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2/15/2010 6 Magnetic force is always perpendicular to the velocity of the charge particle so it can only change the direction, not the speed, of the particle
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## zhang19 - 2/15/2010 Magnetism Loud speaker Credit card...

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