p240_ct2_f07 - Physics 240 Fall 2007 Lecture #2 Dr. Dave...

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Physics 240 Fall 2007 Lecture #2 Dr. Dave Winn winn@umich.edu
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Electromagnetic Interactions • Holds together nuclei and electrons in atoms • Binds atoms together in chemistry (polar, hydrogen, Van der Waal’s, covalent bonding). • Underlies all forces in everyday life (muscular, air pressure, elastic, thermal expansion…) • Gravity is the only exception in everyday life
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Electric Charge • A fundamental property of matter (like mass) • Two kinds of charge, + and – (unlike mass!) • Normal matter, though made of many charges, is neutral 1. Charge is ‘quantized’, appearing only in discrete chunks (q e ) 2. Charge is conserved 3. Charge moves with varying difficulty through different materials: conductors and insulators Slide
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Charges can be created, but only in +/- pairs, so that no net charge is created! e + e - e + e - This is an old ‘bubble chamber’ image which shows the paths of charged particles. They curve because the chamber is in a large magnetic field.
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Consider the following sequence. Bring a positively charged rod near a conducting sphere. Attach the sphere to ground with a conducting wire. Remove the wire, then remove the charged rod. What is the final state of the conducting sphere? 1. Neutral 2. Positively Charged 3. Negatively Charged
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ Conductor Foil Ball The picture above shows a conducting rod sitting on an insulating glass stand. To the right a light foil ball hangs on a string. We then bring a negatively charged insulator up on the left without contacting the conductor. What happens to the foil ball in the system when we first bring the charged rod close? 1. The foil ball hangs straight down 2. The foil ball is attracted to the rod 3. The foil ball is repelled from the rod
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Electrical experiments were all the rage in the 18 th century: Ben Franklin: “For my own part, I never was before engaged in any study that so totally engrossed my attention and my time as this has lately done; for what with making experiments when I can be alone, and repeating them to my Friends and Acquaintances, who, from the novelty of the thing, come continually in crowds to see them, I have, during some months past, had little leisure for anything else.”
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Demonstrations
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p240_ct2_f07 - Physics 240 Fall 2007 Lecture #2 Dr. Dave...

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