Capacitors Basic concept Common examples Field Energy Dielectrics

Capacitors • • • Basic concept Common examples Field Energy Di-electrics “Circuit” diagrams Phys 208, Fall 2006 Capacitors Sept. 25, 2006

f and U of charge distribution • Charge distribution Þ potential Þ store energy • f~Q • U~f. Q ~ Q 2 or f 2 • How to control & use ? Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Insulator vs Conductor • Insulator – Hard to control movement of charge – know r find f, U • Conductor – Control V – Q flows to equilibrium - repeatable – Given V find Q, U • Capacity 1 Farad = C/ V Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Capacitance • Formal • Practical – Choose simple shapes – Store energy “internally” (Net Q = 0) • C ~ d e 0 Phys 208, Fall 2006 or C ~ (A/d) e 0 Capacitors Sept. 25, 2006

Capacitor classification External Internal Useful Simple Not Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Voltage supplies and control • Batteries • Power supply – AC/DC • Another (bigger) capacitor TOY Note use of symbols Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Parallel Plate Fringe fields Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Cylindrical Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Spherical For “external” spherical cap, take b ® ¥. Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Energy stored in capacitor • Recall • For capacitor Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Field Energy Phys 208, Fall 2006 Capacitors Sept. 25, 2006

Circuit elements • Standard symbols • Idealized wires • Idealized supplies Phys 208, Fall 2006 Capacitors Sept. 25, 2006