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p2020chap05 - PHYS-2020 General Physics II Course Lecture...

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PHYS-2020: General Physics II Course Lecture Notes Section V Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3
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Abstract These class notes are designed for use of the instructor and students of the course PHYS-2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille.
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V. Magnetism A. Magnets. 1. Ancient people discovered a rock that when placed close to a sim- ilar rock, the rocks would move either closer or farther apart from each other = these rocks were called loadstones or magnets . 2. Like poles repel each other and unlike poles attract each other. a) Magnets have to be asymmetrical for this to work = preferably the shape of a bar. b) One pole is called north (N), the other south (S). 3. Like the electric field ( vector E ), magnets have a magnetic field called the vector B -field. N S a) The B -field points away from the north pole (hence is analogous to a + electric charge). b) The B -field points towards from the south pole (hence is analogous to a - electric charge). 4. Unlike the electric field, there are no monopoles in magnetism = magnets are always dipoles (2 poles). V–1
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V–2 PHYS-2020: General Physics II + E-field monopole field (1 pole) + - E-field dipole field (2 poles) B. Magnetic Field of the Earth. 1. The Earth has a magnetic field! a) Currently, the magnetic N pole corresponds to the S ge- ographic pole (which is why your compass’s N points to geographic N, remember, like poles repel, opposites at- tract). b) The magnetic S pole corresponds to the N geographic pole. c) The magnetic axis is not aligned with the spin axis! 2. Devices used to measure the direction of the magnetic field are called compasses . The designation ‘N’ and ‘S’ for either pole on a compass was assigned based on which end pointed to the Earth’s north pole = ‘N’ seeks Earth’s North pole. 3. The Earth’s magnetic field is generated by electric currents in the liquid outer core of the planet which is composed of mostly nickel and iron. The electric currents arise from the Earth’s rota- tion. The details of the geomagnetic field source is still not well understood.
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Donald G. Luttermoser, ETSU V–3 4. The magnetic field of the Earth flips polarity ( i.e. , reverses di- rection) in random intervals over time. a) It has flipped 171 times over the past 76 million years. The average period between flips is about 10 4 to 10 5 years (with a peak average period of 200,000 years). b) The last flip occurred 780,000 years ago. c) We know this from the volcanic rocks near the Mid-Atlantic Ridge. As the sea-floor spreads as mantle material flows upward at that location, the rock solidifies when it hits the water. The metallic compounds in the lava “freeze” the direction of the magnetic field as the liquid lava turns to solid basalt rock.
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