Today 3/31 Circuits

Current Potential (same as always) Capacitance (energy and in circuits)

HW: 3/31 “Circuits 4” Due Thursday 4/3

Exam 3 Thursday, 4/3 5-7 pm 116 WitmerOther room to be announced.

Lab: RC Circuits

RC Circuits (circuits with resistor and a capacitor in series)

Current flows only while the capacitor charges up. The time to charge up depends on the product RC. Can also discharge the capacitorC

R

Exam 3

Parallel plate problem Conceptual circuit problem (bulbs) Resistor circuit problem

(find current and voltage) Capacitor problem (with resistors?)

Loop Rule (voltage)

The sum of all voltages around any closed loop is always zero. (what goes up must come down) or VA,A = 0

!!must keep track of ups and downs!! (+/-)

Across capacitors, resistors, and batteries

Kirchhoff’s Rules, Junction

The sum of all currents at any junction is zero. (what goes in must come out) !!must keep track of ins and outs!! (+/-)

Current only flows into and out of capacitors while charging and discharging. No current after enough time has passed through capacitors.

Homework

R1=20100

R1=20

30F

15V

Capacitors with ResistorsWill current flow when the switch is closed?

12 V

6F

6

12 V

6F

Yes, but only for an instant until the capacitor is charged.

Yes, but it will take longer to charge the capacitor.

Capacitors with ResistorsDescribe current and voltage long after the switch has been closed.

12 V

6F

6

12 V

6F

No current, 12 V across the capacitor.

No current, 12 V across the capacitor, zero V across the resistor.

Loop rule still applies!V = IR for resistors still applies!

Capacitors with ResistorsDescribe current and voltage long after the switch has been closed.

12 V

6F

22 Amps through the battery and both resistors.

4 V across 2 and 8 V across the capacitor and 4

Loop rule still applies!V = IR for resistors still applies!

4

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other.

Charging Capacitors in Series

The same amount of charge that enters one side of a capacitor, leaves the other. Capacitors in series

will always have the same charge on them. (what goes around, comes around)

This is true even if they are not of equal capacitance!

Current will flow until the sum of the voltages across the capacitors equals the battery voltage. (loop rule)

Capacitors in Series

Find the charge on each capacitor and the voltage across each capacitor. The battery is 30V.

25F

50F

They are in series so the charge on each is the same.Capacitance means “coulombs per volt” so the one with twice the capacitance has half the volts.

V

QC

1

2

V1 = 20V, V2 = 10V, Q1 = 500C, Q2 = 500C

Capacitors in Parallel

Find the charge on each capacitor and the voltage across each capacitor. The battery is 30V.

They are in parallel so the voltage across each is the same, each equal to 30V.

V

QC

Q1 = 750C, Q2 = 1500C

25F 50F

1 2

Series and Parallel

Objects in series have the same current through them. This is why capacitors in series always have the same charge on them.

Objects in parallel have the same voltage across them.

Ohm’s law, loops & junctions

V = IR true for entire circuits as well as individual elements.

Voltage changes summed around any closed loop equal zero.

Current divides and combines at junctions like water in pipes. What enters the junction must also leave.