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capacitors 2 - Last Lecture Q = CV How to calculate...

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1 1 Capacitors – II This lecture: HRW 25.4-25.8 For next time: review After the exam: HRW 26.1-26.5 Exam-related problem session, next Monday, here at 4:40-6:20. 2 Last Lecture Q = CV How to calculate capacitance: compute V(Q) and read off the proportionality constant Today Combinations of capacitors: Series Parallel Energy storage in capacitors • Dielectrics 3 A B C D C 1 C 2 Q 1 Q 2 C eq Q tot Capacitors in Parallel What is the equivalent capacitance of two capacitors in parallel ? Capacitors in parallel add. The parallel capacitors need not have the same charge on their plates. Q tot = Q 1 + Q 2 C eq V = C 1 V + C 2 V For many capacitors in parallel: = = n eq i i 1 C C eq 1 2 C = C + C A conducting wire is an equipotential Ä Capacitors in parallel must have the same potential difference across their plates V AB = V CD = V 4 10 μ F 30 μ F 20 μ F 120V Q = CV; V = 120 V Q 1 = (10 μ F)(120V) = 1200 μ C Q 2 = (20 μ F)(120V) = 2400 μ C Q 3 = (30 μ F)(120V) = 3600 μ C For capacitors in parallel, the largest capacitor holds the most charge. Parallel Capacitor Example In the circuit to the right, what is the charge on each capacitor? What is the equivalent capacitance of this circuit?
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