Today 331

1
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.

2
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 capacitor

3
Exam 3

• Parallel plate problem
• Conceptual circuit problem (bulbs)
• Resistor circuit problem (find current and
  voltage)
• Capacitor problem (with resistors?)

4
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

5
Kirchhoffs 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.

6
Homework
100?
R120?
30?F
R120?
15V
7
Capacitors with Resistors
Will current flow when the switch is closed?
Yes, but only for an instant until the capacitor
is charged.
Yes, but it will take longer to charge the
capacitor.
8
Capacitors with Resistors
Describe current and voltage long after the
switch has been closed.
No current, 12 V across the capacitor.
Loop rule still applies!V IR for resistors
still applies!
No current, 12 V across the capacitor, zero V
across the resistor.
9
Capacitors with Resistors
Describe current and voltage long after the
switch has been closed.
2 Amps through the battery and both resistors.
2?
12 V
Loop rule still applies!V IR for resistors
still applies!
6?F
4?
4 V across 2? and 8 V across the capacitor and 4?
10
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
11
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
12
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
13
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
14
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
15
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
16
Charging Capacitors in Series
The same amount of charge that enters one side of
a capacitor, leaves the other.
17
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)
Current will flow until the sum of the voltages
across the capacitors equals the battery voltage.
(loop rule)
This is true even if they are not of equal
capacitance!
18
Capacitors in Series
Find the charge on each capacitor and the voltage
across each capacitor. The battery is 30V.
They are in series so the charge on each is the
same.
25?F
1
Capacitance means coulombs per volt so the one
with twice the capacitance has half the volts.
50?F
2
V1 20V, V2 10V, Q1 500?C, Q2 500?C
19
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.
Q1 750?C, Q2 1500?C
20
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.

21
Ohms 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.