# Lect08 - Physics 212 Lecture 8 Today's Concept Capacitors...

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Physics 212 Lecture 8, Slide 1 Physics 212 Lecture 8 Today's Concept: Capacitors Capacitors in a circuits, Dielectrics, Energy in capacitors

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Physics 212 Lecture 8 Music Who is the Artist? A) Ricky Skaggs B) Jerry Garcia & David Grisman C) Tony Rice D) Steve Earle E) Mike Marshall & Darol Anger Theme of the Week: Mandolin Players Extraordinaire Young (Chris Thile) & Old (David Grisman)
Physics 212 Lecture 8, Slide 3 Some Exam Stuff Exam next Wednesday at 7:00 (Sep. 21) Covers material through lecture 8 Bring your ID Conflict exam at 5:15 – sign up in your gradebook by 10:00 TONIGHT If you have class conflicts with both of these, please email Dr. Park ( [email protected] ) by 10:00 tonight. Where do I go? EXAM ROOMS (by discussion section) Preparation: no in class review :( Practice exams Worked examples Exam Preparation Exercises Extra Office Hours

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Physics 212 Lecture 8, Slide 4 Your Comments 05 i thought i understood till the 2nd and 3rd checkpoints” Please, explain last checkpoint question (capacitors with and without a dielectric). Everything else was fine :) WILL DO See Calculation at End Will Do.. They were the hardest It seems like a lot of variables are inverses of each other... Can you clarify on this? An overview would be very helpful. How do different situations (e.g. capacitors in series, parallel, presence of dielectric, etc.) affect the charge, voltage, and energy of the circuit. ” “It's hard to keep all of the symbols and letters separate, and even harder to remember the "special cases" when variables stay they same or are changed.” I would like to go over some more examples with different types of capacitors set in circuits with both parallel and series connections.“ Detail detail detail, and some worked calculations would be “ We’ll summarize main points. “I worry that I have reached the capacity of my brain, I guess I can forget Chem to make some room.”
Physics 212 Lecture 8, Slide 5 C V + Q -Q Q Q This “Q” really means that the battery has moved charge Q from one plate to the other, so that one plate holds +Q and the other -Q . C V Q=VC Simple Capacitor Circuit 8

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Physics 212 Lecture 8, Slide 6 By adding a dielectric you are just making a new capacitor with larger capacitance (factor of κ ) Dielectrics C 0 V Q 0 =VC 0 C 1 = κ C 0 V Q 1 =VC 1 11
Physics 212 Lecture 8, Slide 7 V Q 1 = C 1 V Q 2 = C 2 V C 2 C 1 Parallel Capacitor Circuit Key point: V is the same for both capacitors Key Point: Q total = Q 1 + Q 2 = VC 1 + VC 2 = V(C 1 + C 2 ) C total = C 1 + C 2 Q total Q total 14

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Physics 212 Lecture 8, Slide 8 Series Capacitor Circuit Q Q Q +Q +Q -Q -Q V C 1 C 2 V 1 V 2 Q V Q Key point: Q is the same for both capacitors Also: V = V 1 + V 2 Q=VC total Key point: Q = VC total = V 1 C 1 = V 2 C 2 Q/C total = Q/C 1 + Q/C 2 1 C total 1 C 1 1 C 2 + = 17
Physics 212 Lecture 8, Slide 9 Checkpoint 1 C C C C C 1/C total = 1/C + 1/C = 2/C C total = C/2 C total = C C total = 2C Which has lowest total capacitance : 18

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Physics 212 Lecture 8, Slide 10 Checkpoint 2 C total = C Which has lowest total capacitance: C C C C C C left = C/2 C right = C/2 C total = C left + C right C total = C 20 Same
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