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Lecture12

# Lecture12 - Capacitors store energy Requires energy to...

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Physics 42 February 8, 2006 Physics 42 Lecture 12 Announcements Exam I in recitation today Lab 4 next Monday: capacitance Next Topic Electrostatic energy storage, capacitance, simple circuits Electrostatic energy storage in charged systems Some example calculations Relating electrostatic energy storage to electric potential Capacitance Definition Example calculations

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Physics 42 February 8, 2006 Introduction to Electrostatic energy storage For example consider assembling three charges by starting with one fixed at rest (Q) and bringing two more (q 1 and q 2 ) from far away. After the object is assembled there is electrostatic energy stored in the system. Q W 1 W 2 W ext = W 1 +W 2 W ext = U ES = kQq 1 /r 1 + kQq 2 /r 2 + kq 1 q 2 /r 12 q 2 q 1 q 2 q 1 Q r 2 r 1 r 12
Physics 42 February 8, 2006 Capacitance A capacitor stores charge, usually + Q and - on separate conductors Capacitance is the amount of charge per unit potential difference Everything has some capacitance! Unit is the Farad =Coulomb/Volt A Farad is enormous! Typical capacitors pF through μ F

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Unformatted text preview: Capacitors store energy Requires energy to separate charge Release energy when discharged Can be dangerous! C " Q V + − +Q − Q Physics 42 February 8, 2006 Capacitors are electrical components designed to efficiently store charge and therefore electrostatic potential energy Physics 42 February 8, 2006 Capacitors are rated by their capacitance and equally important the maximum voltage they can sustain before electrical break down Physics 42 February 8, 2006 Parallel Plate Capacitor + − +Q − Q s Area A Capacitance is only a function of geometry! (generally true) Large A and small s give large capacitance. E = " # Q = A " V = Es = \$ s " V = Q A s C " Q V = Q Qs A = A s = 8.85 # 10 \$ 12 F / m = 8.85 pF / m Physics 42 February 8, 2006 An ideal parallel plate capacitor C = ε ο A/d Physics 42 February 8, 2006 A real parallel plate capacitor C ~ ε ο A/d fringe ﬁelds E ~ constant...
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Lecture12 - Capacitors store energy Requires energy to...

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