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Physics 2112 Unit 11

Physics 2112 Unit 11. Today’s Concept: RC Circuits. RC Circuit (Charging). a. C. b. V battery. R. C. a. V battery. R. b. Capacitor uncharged, Switch is moved to position “ a ”. Kirchoff’s Voltage Rule. Differential Equation. Unit 11, Slide 2.

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Physics 2112 Unit 11

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  1. Physics 2112Unit 11 Today’s Concept: RC Circuits

  2. RC Circuit (Charging) a C b Vbattery R C a Vbattery R b Capacitor uncharged, Switch is moved to position “a” • Kirchoff’s Voltage Rule • Differential Equation Unit 11, Slide 2

  3. What is the charge on the capacitor 1second after the switch is closed?What is the current through the resistor 1 second after the switch is closed? Example 11.1 (Charging Capacitor) 10V R1 = 1.2MW C1 = 1uF • Conceptual Idea: Use charging equations for a capacitor. • Note: • e-1 = 0.37 • 1-e-1 = 0.63 • Plan: • Find time constant, t • Find Qf and Io • Put in t=1sec

  4. What is the charge on the capacitor 10 seconds after the switch is closed?What is the current through the resistor 10 seconds after the switch is closed? Example 11.2 (Charging Capacitor) 10V R1 = 1.2MW C1 = 1uF • Conceptual Idea: Use charging equations for a capactor. • Plan: • Find time constant, t • Find Qf and Io • Put in t=1sec

  5. RC Circuit (Discharging) a C - + I b Vbattery R V a C b Vbattery R Capacitor has q0=CV, Switch is moved to position “b” • Kirchoff’s Voltage Rule • Differential Equation -I Unit 11, Slide 5

  6. The switch is held in position 1 for a long time and the capacitor becomes fully charged. It is then flipped to position 2.What is the charge on the capacitor 2 seconds after the switch is flipped? Example 11.3 (Discharging Capacitor) 10V R1 = 1.2MW C1 = 1uF 1 2 • Conceptual Idea: Use discharging equations for a capacitor. • Plan: • Find time constant, t • Find Qo • Put in t=2sec

  7. How do Exponentials Work? “Fraction of initial charge that remains” Not all curves that look like are exponential!! “How many time constants worth of time that have elapsed” Unit 11, Slide 7

  8. RC = 2 Time constant: t=RC The biggertis, the longer it takes to getthe same change… RC= 1 Unit 11, Slide 8

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