Physics 1161 Lecture 09 RC Circuits

Time Constant Demo Which system will be brightest? Which lights will stay on longest? Which lights consume more energy? 2 Each circuit has a 0.5 F capacitor charged to 9 Volts. When the switch is closed: 1  = 2RC  = RC/2

Time Constant Demo Which system will be brightest? Which lights will stay on longest? Which lights consume more energy? 2 Each circuit has a 0.5 F capacitor charged to 9 Volts. When the switch is closed: 1  = 2RC  = RC/2 2 I=2V/R 1 Same U=1/2 CV 2

RC Circuits Checkpoint 1 & 3 2R C  R S2S2 Both switches are initially open, and the capacitor is uncharged. What is the current through the battery just after switch S 1 is closed? 1)I b = 0 2)I b =  /(3R) 3)I b =  /(2R) 4)I b =  /R S1S1 IbIb Both switches are initially open, and the capacitor is uncharged. What is the current through the battery after switch 1 has been closed a long time ?long time 1)I b = 0 2) I b = V/(3R) 3) I b = V/(2R) 4) I b = V/R

Practice! Calculate current immediately after switch is closed: Calculate current after switch has been closed for 0.5 seconds: Calculate current after switch has been closed for a long time: Calculate charge on capacitor after switch has been closed for a long time: R C E S1S1 R=10  C=30 mF E =20 Volts -

Practice Calculate current immediately after switch is closed: Calculate current after switch has been closed for 0.5 seconds: Calculate current after switch has been closed for a long time: Calculate charge on capacitor after switch has been closed for a long time: R C E S1S1 R=10  C=30 mF E =20 Volts  - I 0 R - q 0 /C = 0 I  - I 0 R - 0 = 0 I 0 =  /R After a long time current through capacitor is zero!  - IR - q ∞ /C = 0  q ∞ /C = 0 q ∞ =  C

Both switches are closed. What is the final charge on the capacitor after the switches have been closed a long time? IRIR S2S2 S1S1 + - R + - C 2R Q = 0 2. Q = C  /3 3. Q = C  /2 4. Q = C 

Both switches are closed. What is the final charge on the capacitor after the switches have been closed a long time? IRIR S2S2 S1S1 + - R + - C 2R Q = 0 2. Q = C  /3 3. Q = C  /2 4. Q = C 

Charging: Intermediate Times Calculate the charge on the capacitor 3  seconds after switch 1 is closed. q(t) = q  (1-e -t/RC )  C R1R1 S2S2 S1S1 IbIb R 1 = 20  R 2 = 40  ε = 50 Volts C = 100  F R2R2

Charging: Intermediate Times  C R1R1 S2S2 S1S1 IbIb Calculate the charge on the capacitor 3  seconds after switch 1 is closed. q(t) = q  (1-e -t/R 2 C ) = q  (1-e - 3  /(40  100  10 -6) ) ) = q  (0.53) Recall q  = ε C = (50)(100x10 -6 ) (0.53) = 2.7 x10 -3 Coulombs R 1 = 20  R 2 = 40  ε = 50 Volts C = 100  F R2R2

RC Circuits: Discharging KLR: ____________ Just after…: ________ – Capacitor is still fully charged Long time after: ____________ Intermediate (more complex) – q(t) = q 0 e -t/RC – I c (t) = I 0 e -t/RC C  R S1S1 + + I - - S2S2 q RC2RC t

RC Circuits: Discharging KLR: q(t) / C - I(t) R = 0 Just after…: q=q 0 – Capacitor is still fully charged – q 0 / C - I 0 R = 0  I 0 = q 0 /(RC) Long time after: I c =0 – Capacitor is discharged – q  / C = 0  q  = 0 Intermediate (more complex) – q(t) = q 0 e -t/RC – I c (t) = I 0 e -t/RC C  R S1S1 + + I - - S2S2 q RC2RC t

Checkpoint RC Circuits 5 After switch 1 has been closed for a long time, it is opened and switch 2 is closed. What is the current through the right resistor just after switch 2 is closed? 1)I R = 0 2)I R =  /(3R) 3)I R =  /(2R) 4) I R =  /R KLR: -q 0 /C+IR = 0 Recall q is charge on capacitor after charging: q 0 =  C (since charged w/ switch 2 open!) -  + IR = 0  I =  /R 2R C  R S2S2 S1S1 IRIR

After being closed for a long time, the switch is opened. What is the charge Q on the capacitor 0.06 seconds after the switch is opened? E = 24 Volts R = 4  C = 15 mF R C 2R E S1S q q q q 0

After being closed for a long time, the switch is opened. What is the charge Q on the capacitor 0.06 seconds after the switch is opened? E = 24 Volts R = 4  C = 15 mF R C 2R E S1S1 q(t) = q 0 e -t/RC = q 0 (e /(4  (15  )) ) = q 0 (0.368) q q q q 0