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RC Circuits

RC Circuits. 20-13. Charging a Capacitor. Circuit with a capacitor, battery, and resistor Initially capacitor is uncharged When battery connected current flows to charge capacitor When capacitor is fully charged, no current will flow. Charging a Capacitor. Loop Rule V = q/C + IR

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RC Circuits

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  1. RC Circuits 20-13

  2. Charging a Capacitor • Circuit with a capacitor, battery, and resistor • Initially capacitor is uncharged • When battery connected current flows to charge capacitor • When capacitor is fully charged, no current will flow

  3. Charging a Capacitor • Loop Rule • V = q/C + IR • Solve for I • I = V/R – q/RC • I is rate of change of q • Differential Calculus says

  4. Charging a Capacitor • RC =  (time constant) • CV = Q (maximum charge)

  5. Discharging a Capacitor • The battery is disconnected • The capacitor acts like a battery supplying current to the circuit • Loop Rule • IR = q/C • I = q/RC

  6. Discharging a Capacitor

  7. Conceptual Example • Most cars have intermittent windshield wipers. How do capacitors help with that? • The wipers are part of an RC circuit the time constant of which can be varied by selecting different R’s. The brief time that the wipers remain on and the time they are off are determined by the value of the time constant.

  8. Example • An uncharged capacitor and a resistor are connected in series to a battery. If V = 12 V, C = 5 F, and R = 8x105 . Find the time constant, max charge, max current, and charge as a function of time.

  9. Practice Problems • Discharge your knowledge by completing these problems • 612 CQ 16, P 95 – 97 • Total of 4 Problems

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