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Unformatted text preview: Physics 240 Fall 2007 Lecture #10 Dr. Dave Winn 2405 Randall Lab winn@umich.edu First exam tonight, October 4, 8:009:30 Exam details Discussion instructors Eckhause : take the exam in Chemistry 1800. Discussion instructor Bobkov : go to Dennison 182. Discussion instructors Winn : go to 170 Dennison Discussion instructor: Campbell : go to 1210 Chemistry Capacitors Arrangements of conductors which store charge and energy Capacitance measures efficiency of charge storage: C = Q/V Determined by geometry of device; typically some size scale times To increase C, make storage area large, keep positive and negative charge close.. Combining capacitors in parallel simply adds more capacitance: C eq = C i Combining capacitors in series reduces the 1/C eq = 1/C i Limits for capacitors in series Interesting limits for two capacitors in series: When C 1 = C 2 = C, then C eq = C/2, the capacitance is cut in half When C 1 >> C 2 , then C eq C 2 ; the equivalent capacitance is the smaller capacitor 2 1 2 1 C C C C C eq + = 2 1 1 2 1 2 2 1 1 1 1 C C C C C C C C C eq + = + = Consider the case of a capacitor with a spacing 3d. A metal plate of thickness d is inserted in the center of the capacitor, leaving two gaps of width d. This is case A. Now a wire is attached to short out the top gap and make case B. Which has larger capacitance? d d A B 1. Case A 2. Case B 3. They are the same A is two capacitors with spacing d in series. B is one capacitor with spacing d. More complex arrangements To understand a more complex arrangement, break it into pieces First the series bit Then the parallel parts 2 1 2 1 12 C C C C C eq + = 3 2 1 2 1 123 C C C C C C eq + + = C eq 12 If C 1 =C 2 =C 3 , which of these four arrangements has the largest total capacitance?...
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This note was uploaded on 04/04/2008 for the course PHYSICS 240 taught by Professor Davewinn during the Fall '08 term at University of Michigan.
 Fall '08
 DaveWinn
 Physics

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