Lecture_09_(Chap.17)

Lecture_09_(Chap.17) - Announcements Exam 1 on Feb 15...

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Announcements Exam 1 on Feb 15 (Tuesday), 8-9:30 PM, PHYS 202,203. Covers materials from Chap.14 – 17. Practice exam is posted on the course home page. rib sheet provided As same as that in the practice exam Crib sheet provided. As same as that in the practice exam. No calculator is needed. There will be a lab session on Tuesday as usual. HW Chap.17 due on 2/15 (Tuesday) by 5 PM. 1
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Question A and B are at equal distance from the center of the sphere. 2
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Potential in Metal +Q 1 -Q 1 1 mm In static equilibrium Insert a 1 mm thick metal slab into the center of the capacitor. etal slab polarizes and has charges +Q hat are the charges d Metal slab polarizes and has charges +Q 2 and -Q 2 on its surfaces. 0 1 1 / A Q E 0 2 2 / A Q E What are the charges Q 1 and Q 2 ? Now we have 2 capacitors instead of one f Q 2 =Q 1 E inside metal is zero d = 3 mm i VE d l  4V Charges +Q 2 and Q 2 V left V right 2000 V/m 0.001 m 2 V V = 4 V There is no “ conservation of potential” ! V inside metal slab is zero! 3
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Potential Difference in an Insulator Electric field in capacitor filled with insulator: E et = E lates +E ipoles net plates dipoles E plates = const (in capacitor) E dipoles,A E dipoles =f(x,y,z) B B A A VE d l  Travel from A to B : E dipoles is sometimes parallel to dl , and sometimes antiparallel to dl, depending 123 45 on the path. 4
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Potential Difference in an Insulator Instead of traveling through inside – travel outside from B to A: A B E dipoles, average l d E 0 Ed l B 0 BA B A A VV V E d l   Effect of dielectric is to reduce the potential difference. 5
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Dielectric Constant Electric field in capacitor filled with insulator: E net = E plates -E dipoles E plates E net K Q / A E plates 0 E net Q / A K 0 K – dielectric constant 6
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Dielectric Constant Inside an insulator: E E applied net Dielectric constant for various insulators: K vacuum 1 (by definition) air 1.0006 typical plastic 5 NaCl 6.1 ater 0 water 80 strontium titanate 310 7
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Potential Difference in a Capacitor with Insulator B VE d l
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Lecture_09_(Chap.17) - Announcements Exam 1 on Feb 15...

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