# lecture07 - Physics 102 Lecture 7 RC Circuits Recall First...

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RC Circuits Physics 102: Lecture 7

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Recall …. First we covered circuits with batteries and capacitors series, parallel Then we covered circuits with batteries and resistors series, parallel Kirchhoff’s Loop and Junction Relations Today: circuits with batteries, resistors, and capacitors
RC Circuits RC Circuits RC Circuits Charging Capacitors Discharging Capacitors Intermediate Behavior

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RC Circuits RC Circuits Circuits that have both resistors and capacitors : With resistance in the circuits, capacitors do not charge and discharge instantaneously – it takes time (even if only fractions of a second). R K C S + + + R Na R Cl ε K ε Na ε Cl
Capacitors Capacitors Charge (and therefore voltage) on Capacitors cannot change instantly: remember V C = Q/C Short term behavior of Capacitor : If the capacitor starts with no charge, it has no potential difference across it and acts as a wire If the capacitor starts with charge, it has a potential difference across it and acts as a battery . Long term behavior of Capacitor : Current through a Capacitor eventually goes to zero . If the capacitor is charging, when fully charged no current flows and capacitor acts as an open circuit . If capacitor is discharging, potential difference goes to zero and no current flows .

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Charging Capacitors Charging Capacitors Capacitor is initially uncharged and switch is open. Switch is then closed. What is current I 0 in circuit immediately thereafter? What is current I in circuit a long time later? ε R C S
Charging Capacitors: t=0 Charging Capacitors: t=0 Capacitor is initially uncharged and switch is open. Switch is then closed. What is current I 0 in circuit immediately thereafter? Capacitor initially uncharged Therefore V C is initially 0 Therefore C behaves as a wire (short circuit) ε – I 0 R = 0 » I 0 = ε /R ε R C S R ε Opposite sign convention compared to your handouts!

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lecture07 - Physics 102 Lecture 7 RC Circuits Recall First...

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