Download 3-31 Circuits, Current and Potential, Capacitors

Today 3/31
Circuits
Current
Potential (same as always)
Capacitance (energy and in circuits)
3/31 “Circuits 4”
Due Thursday 4/3
Exam 3 Thursday, 4/3 5-7 pm 116 Witmer
Other room to be announced.
HW:
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
R
product RC. Can also
C
discharge the capacitor
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
100
R1=20
30F
15V
R1=20
Capacitors with Resistors
Will current flow when the switch is closed?
Yes, but only for an instant until
the capacitor is charged.
12 V
6F
12 V
6
6F
Yes, but it will take longer to
charge the capacitor.
Capacitors with Resistors
Describe current and voltage long after the switch has
been closed.
No current, 12 V across the capacitor.
12 V
12 V
6F
Loop rule still applies!
V = IR for resistors still applies!
6
No current, 12 V across the capacitor,
zero V across the resistor.
6F
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
6F
4
Loop rule still applies!
V = IR for resistors still
applies!
4 V across 2 and 8 V across the
capacitor and 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
Current will
the same charge on
flow until the
them. (what goes
sum of the
around, comes
voltages across
around)
the capacitors
equals the
This is true even if
battery
they are not of
voltage.
equal capacitance!
(loop rule)
Capacitors in Series
Find the charge on each capacitor and the voltage across
each capacitor. The battery is 30V.
1
25F
2
50F
Q
C
V
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.
V1 = 20V, V2 = 10V, Q1 = 500C, Q2 = 500C
Capacitors in Parallel
Find the charge on each capacitor and the voltage across
each capacitor. The battery is 30V.
1
2
25F
50F
Q
C
V
They are in parallel so the voltage
across each is the same, each equal
to 30V.
Q1 = 750C, Q2 = 1500C
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.