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lecture9 - Physics 2102 Gabriela Gonzlez Physics 2102...

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Physics 2102 Capacitors Physics 2102 Gabriela González

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Capacitors and Capacitance Capacitor: any two conductors, one with charge + Q , other with charge - Q +Q -Q Uses: storing and releasing electric charge/energy. Most electronic capacitors: micro-Farads ( μ F), pico-Farads (pF) -- 10-12 F New technology: compact 1 F capacitors Potential DIFFERENCE between conductors = V Q = C V -- C = capacitance Units of capacitance: Farad (F) = Coulomb/Volt
Capacitance Capacitance depends only on GEOMETRICAL factors and on the MATERIAL that separates the two conductors e.g. Area of conductors, separation, whether the space in between is filled with air, plastic, etc. +Q -Q (We first focus on capacitors where gap is filled by AIR!)

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Electrolytic (1940-70) Electrolytic (new) Paper (1940-70) Tantalum (1980 on) Ceramic (1930 on) Mica (1930-50 Variable air, mica Capacitors
Capacitors and Capacitance Capacitor: any two conductors, one with charge + Q , other with charge - Q +Q Uses: storing and releasing electric charge/energy. Most electronic capacitors: micro-Farads ( μ F), pico-Farads (pF) -- 10-12 F New technology: compact 1 F capacitors Potential DIFFERENCE between conductors = V Q = C V C = capacitance Units of capacitance: Farad (F) = Coulomb/Volt -Q

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Parallel Plate Capacitor +Q -Q What is the capacitance C? Area of each plate = A Separation = d charge/area = σ = Q/A Relate E to potential difference V: = d x d E V 0 A Qd dx A Q d 0 0 0 ε = = d A V Q C 0 = = E field between the plates: (Gauss’ Law) A Q E 0 0 σ = = We want capacitance : C=Q/V
Parallel Plate Capacitor -- example A huge parallel plate capacitor consists of two square metal plates of side 50 cm, separated by an air gap of 1 mm What is the capacitance? C = ε 0A/d = (8.85 x 10-12 F/m)(0.25 m2)/(0.001 m) = 2.21 x 10-9 F (small!!) Lesson: difficult to get large values of capacitance without special tricks!

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Isolated Parallel Plate Capacitor A parallel plate capacitor of capacitance C is charged using a battery. Charge = Q, potential difference = V.
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lecture9 - Physics 2102 Gabriela Gonzlez Physics 2102...

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