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p240_ct8_w12+jan+31

# p240_ct8_w12+jan+31 - Physics 240 Winter 2012 Lecture#8 My...

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Physics 240 Winter 2012 Lecture #8 Dr. Dave Winn 2405 Randall Lab My Office Hours (1416 Randall Lab) Monday 1pm-3pm Tuesday 1pm-3pm Thursday 2pm-4pm (Prof. Gidley) Learning Assistant Office Hours Tuesday 4-8pm Thursday 4-6pm Also in chat room Tue/Thu evening!

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The two conductors a and b are insulated from each other, forming a capacitor. You increase the charge on a to +2 Q and increase the charge on b to –2 Q , while keeping the conductors in the same positions. As a result of this change, the capacitance C of the two conductors A. becomes 4 times great. B. becomes twice as great. C. remains the same. D. becomes 1/2 as great. E. becomes 1/4 as great. Q24.1
A 12– m F capacitor and a 6– m F capacitor are connected together as shown. If the charge on the 12– m F capacitor is 24 microcoulombs (24 m C), what is the charge on the 6– m F capacitor? A. 48 m C B. 36 m C C. 24 m C D. 12 m C E. 6 m C Q24.4 a b 12 m F 6 m F

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Introduction to capacitors • As we begin to explore the use of electricity, we need tools. One basic tool is a storage device; the “capacitor” • Capacitors store charge and energy • “Capacitance” is a measure of the economy with which an object stores charge
The basic idea Generally, capacitors separate positive and negative charge on two separate parts Capacitance is a measure of how much charge can be stored on these objects at a given potential difference C = Q/V C = charge stored / work to store it each unit charge +Q -Q E

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The basic idea • C = Q/V • To increase C, you generally make the object large (more Q can be stored) and move the parts closer together (less potential difference between them) +Q -Q E
The archetype: parallel plates • Two charge plates, area A (~L 2 ), charge +Q and –Q, separation d • If d << L then we know: • E = / 0 =(Q/A 0 ) V +- = Ed • Call this potential change V • V = (Q/A 0 )d • Q = (A 0

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p240_ct8_w12+jan+31 - Physics 240 Winter 2012 Lecture#8 My...

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