Transcript r 36 lec
R Ch 36 Magnetism pg 1 Text Qs pg 575 RQ 1, 2, 4, 5, 7-9,12,13,19 R Ch 36 Magnetism pg 2 • Magnets are essential for modern life, they are used in generators, motors, lights etc. • Originally called loadstones they were found naturally in Magnesia, Greece about 2,000 yrs ago • They contain a form of iron called Magnetite • Magnetism & electricity were linked in 1820 by Hans Oersted while playing with a compass and a current carrying wire R Ch 36.1 Magnetic Poles pg 3 • • • • Magnets have these properties; 1) are a no touch force 2) attract or repel 3) their effect follows the inverse square law for distance • 4) Magnetic force = mag. Pole strength x mag. Pole strength / d2 • 5) Magnets always have two poles ( N & S ) R Ch 36.2 Magnetic Fields pg 4 • The space around a magnet is filled with a magnetic force field • 1) The direction of the field is N -> S • 2) The field is strongest at the poles • 3) Lines can be drawn to represent the field & the closer the lines the stronger the field R Ch 36.3 The nature of the Magnetic Field pg 5 • Magnetism is produced by the motion of an electric charge, magnets get their moving charges from the electrons moving around inside R Ch 36.3 The nature of the Magnetic Field pg 6 • Remember that electrons • 1) orbit the nucleus • 2) spin on their axis, stronger than orbit R Ch 36.3 The nature of the Magnetic Field pg 7 • When the spin & orbit match-up, the magnetic fields add • When the spin & orbit are opposite they cancel R Ch 36.3 The nature of the Magnetic Field pg 8 • Electron usually exist in pairs • When electrons are paired up, their spins are opposite they cancel • The canceling effects of the various spins & orbits are the reason most materials are not magnetic R Ch 36.3 The nature of the Magnetic Field pg 9 • Iron, cobalt & nickel all are magnetic because they have single electrons with spin that is not canceled by a partner electron • Iron is the strongest with four unpaired electrons, • cobalt next with three unpaired electrons, • nickel weakest with two unpaired electrons R Ch 36.3 The nature of the Magnetic Field pg 10 • Iron, cobalt & nickel are mixed with aluminum (for weight) to make common magnets R Ch 36.4Magnetic Domains pg 11 • Magnetic domains are clusters of atoms aligned with their N & S poles all pointed in the same direction. R Ch 36.4 Magnetic Domains pg 12 • Permanent magnets are made by placing iron in a strong magnetic field and; • 1) tapping the iron to help the domains align • 2) sliding a magnet along the iron to the domains align • Permanent magnets are destroyed by • 1) heating 2) dropping the magnet because it knocks the domains out of alignment R Ch 36.5 Electric Currents & Magnetic Fields pg 13 • Electric current produces a magnetic field because it is the movement of charges. • Looping wire around a piece of Iron will create an electromagnet that can be switched off and on. R Ch 36.5 Electric Currents & Magnetic Fields pg 14 • MRI (magnetic resonance imaging) machines do this by using a strong magnetic field to flip all of your electrons up and then letting them flip back to their natural position. • When your electrons flip back they move giving off a weak magnetic signal that the machine detects and uses to construct an image of your insides R Ch 36.6 Magnetic Forces on Moving Charge Particles pg 15 • Moving charged Particles behave weirdly in a magnetic field; • 1) static charge & static field = no interaction • 2) charge & field not moving relative to each other = no interaction • 3) Charge & magnetic field moving at right angles to each other = creating a force perpendicular to the magnet motion & the charge motion R Ch 36.6 Magnetic Forces on Moving Charge Particles pg 16 • This right angle motion is how TV sets bend the electrons that make the picture R Ch 36.7 Magnetic Forces on Current Carrying Wires pg 17 • Wires also feel the force of a magnet field if the current is moving through the wire and the wire is at a right angle to the magnetic field. • This is how motors, generators, speakers & microphones work R Ch 36.8 Meters to Motors pg 18 • Voltmeter – measures volts • Ammeter – measures amps (current) • Electric motors changed the world by replacing human & animal power. • Electric motors spin because a permanent magnet puts a force on an electromagnet by switching the poles to keep like poles together so they always repel and keep moving R Ch 36.9 The Earth’s Magnetic Field pg 19 • The earth has two North poles; • 1) Magnetic north (where a compass points) near Hudson’s Bay in Canada. It moves a little every year. • 2) True north is at the north pole. • In Chicago magnetic north & true north line up. • The magnetic field is not perfectly understood but it caused by the magma moving around inside the earth R Ch 36.9 The Earth’s Magnetic Field pg 20 • Even weirder and not understood; • 1) the earth’s magnetic field has reversed and the poles switched places. • 2) the magnetic field has switched off for long periods and compasses didn’t work R Ch 36.9 The Earth’s Magnetic Field pg 21 • The average time of the magnetic reversals is every one million years and the time is about up so the next switch may be soon. • The earth’s magnetic field has decreased 5% in the last 100 yrs. At this rate magnetic field will change in 2,000 yrs.