L18_Viet_Magnetism_Magnetic Fields and Forces

# L18_Viet_Magnetism_Magnetic Fields and Forces - Physics 122...

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Physics 122 Electricity and Magnetism Lecture 18 Magnetism, Magnetic Fields and Forces

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05/06/09 Physics 122 - Lecture 18 2 Tape a bar magnet to a cork and allow it to float in a dish of water. The magnet turns and aligns itself with the north-south direction. The end of the magnet that points north is called the magnet’s north-seeking pole , or simply its north pole . The other end is the south pole . Experiments with Magnetism: Experiment 1
05/06/09 Physics 122 - Lecture 18 3 Experiments with Magnetism: Experiment 2 Bring the north poles of two bar magnets near to each other. Then bring the north pole of one bar magnet near the south pole of another bar magnet. When the two north poles are brought near, a repulsive force between them is observed. When the a north and a south pole are brought near, an attractive force between them is observed.

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05/06/09 Physics 122 - Lecture 18 4 Experiments with Magnetism: Experiment 3 Bring the north pole of a bar magnet near a compass needle. When the north pole is brought near, the north-seeking pole of the compass needle points away from the magnet’s north pole. Apparently the compass needle is itself a little bar magnet.
05/06/09 Physics 122 - Lecture 18 5 Experiments with Magnetism: Experiment 4 Use a hacksaw to cut a bar magnet in half. Can you isolate the north pole and the south pole on separate pieces? No. When the bar is cut in half two new (but weaker) bar magnets are formed, each with a north pole and a south pole. The same result would be found, even if the magnet was sub-divided down to the microscopic level.

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05/06/09 Physics 122 - Lecture 18 6 Experiments with Magnetism: Experiment 5 Bring a bar magnet near an assortment of objects. Some of the objects, e.g. paper clips, will be attracted to the magnet. Other objects, e.g., glass beads, aluminum foil, copper tacks, will be unaffected. The objects that are attracted to the magnet are equally attracted by the north and south poles of the bar magnet
05/06/09 Physics 122 - Lecture 18 7 Experiments with Magnetism: Experiment 6 Bring a magnet near the electrode of an electroscope. There is no observed effect, whether the electroscope is charged or discharged and whether the north or the south pole of the magnet is used.

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05/06/09 Physics 122 - Lecture 18 8 Conclusions from Experiments 1. Magnetismis not thesameas electricity. Magnetic poles aresimilar to charges but have important differences . 2. Magnetismis a long range force . Thecompass needleresponds to the bar magnet fromsomedistanceaway. 3. Magnets have two poles , “north” (N) and “south” (S). Likepoles repel and oppositepoles attract. 4. Poles of a magnet can beidentified with a compass . A north magnet pole (N) attracts thesouth-seeking end of thecompass needle(which is a south pole). 5. Somematerials (e.g., iron) stick to magnets and others do not. Thematerials that areattracted arecalled magnetic materials . Magnetic materials areattracted by either pole of a magnet. This is similar in someways to theattraction of neutral objects by an electrically charged rod by induced polarization.
05/06/09 Physics 122 - Lecture 18 9

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