Please put away everything except
a pen/pencil and a calculator (if
you brought one).
!
No talking, and no using phones/computers
during Quiz! If you finish early, please sit
quietly until time is up.
Sourav Bhabesh
Kazage Utuje
Satvik Wani
Lecture 8.2 :!
Circuits Continued
Lecture Outline:!
Real Batteries!
Resistors in Parallel!
Grounding!
RC Circuits!
!
Textbook Reading:!
Ch. 31.4 - 31.9
March 5, 2015
2
Announcements
!
•Exam #2 on Thursday, March 19, in class.
Covers Ch. 28, 29, 30, 31.
Don’t forget your calculator and one sheet of notes.!
•HW #7 due on Tuesday, March 17 at 9am.!
•Recitations will be held as usual tomorrow (Friday), in case you
were wondering.
3
Last Lecture...
To analyze a circuit, we apply Kirchoff’s Laws and Ohm’s Law:
Kirchoff’s Junction Law:
Iin =
Kirchoff’s Loop Law:
Iout
Vloop =
Ohm’s Law:
V
I=
R
4
( V )i = 0
Last Lecture...
Battery delivers power to a circuit, which is subsequently
“used up” by the elements of the circuit.
Pbat = IE
5
Last Lecture...
How do resistors in series add?
Req = R1 + R2 + · · · + RN
Resistors in series all have the same current.
6
Real Batteries
Batteries in real life have some built-in internal resistance (r), which
causes their measured voltage (∆Vbat) to be less than their EMF.
7
Real Batteries
What happens to the lightbulbs if we “short out” this circuit? What
happens to the current in the circuit?
8
Resistors in Parallel
The resistors below are arranged in Parallel. They have the same potential
difference across them.
What is the equivalent resistance for these parallel resistors?
9
Resistors in Parallel
10
Clicker Question #1
Bulbs A and B are equally bright when the switch is open. !
What happens to the brightness of bulbs A and B after the switch is closed?
A. Both get brighter!
B. Both get dimmer!
C. A gets brighter, B gets dimmer!
D. A gets dimmer, B gets brighter
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Clicker Question #2
12
About those Lightbulbs
“The filament in a lightbulb functions as a resistor as long as it is glowing,
but the filament is slightly nonohmic because the value of its resistance
when hot is larger than its room-temperature value.” - Textbook
13
Resistors in Parallel
Example: What is the current through each resistor in this circuit?
3#
59 #
14
18 #
Clicker Question #2
Which resistor dissipates more power?
A. The 9 Ω resistor.
B. The 1 Ω resistor.
C. They dissipate the
same power.
15
Clicker Question #3
When the switch closes, the battery current
A. Increases.
B. Decreases.
C.Stays the same.
16
Resistors in Parallel
Voltmeters are used to measure potential difference between two points in a
circuit. They are placed in parallel with the two points of interest, and the
voltmeter must have a very high resistance.
17
Grounding
We ground (i.e. - connect to earth) circuits to establish an absolute potential
value (relative to the 0V of the earth), and for safety.
What is the potential at point a and point b?
Ground
18
Grounding
Safety?
19
RC Circuits
So far we’ve considered capacitors and resistors separately from each
other...let’s put them together.
0
RC Circuit, with open switch.
Current flows after the switch is closed.
20
RC Circuits
Want to solve for the current in the circuit, and the charge on the capacitor,
at all times after the switch is closed.
21
RC Circuits
22
RC Circuits
We can also show how the capacitor in the circuit below gets charged if it is
initially uncharged.
23
Reminders
!
•Recitations held tomorrow (Friday) as usual.!
•Exam #2 on Thursday, March 19.!
•HW #7 due on Tuesday, March 17.!
•Enjoy your Spring Break!!
24