Lecture 8.2 :
Circuits Continued
Lecture Outline:
Real Batteries
Resistors in Parallel
Grounding
RC Circuits
Textbook Reading:
Ch. 31.5 - 31.9
March 7, 2013
1
Announcements
•Homework #7 due on Monday, March 18 in Mastering Physics
(don’t forget about this over Spring Break).
•Quiz #4 on March 21 (Thu. after Spring Break).
Covers Ch.30-31
material.
•Prof. Hubisz will be handling lectures the week after Spring Break.
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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
3
�
(∆V )i = 0
Last Lecture...
Battery delivers power to a circuit, which is subsequently
“used up” by the elements of the circuit.
Pbat = IE
4
Last Lecture...
How do resistors in series add?
Req = R1 + R2 + · · · + RN
Resistors in series all have the same current.
5
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.
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Real Batteries
What happens to the lightbulbs if we “short out” this circuit? What
happens to the current in the circuit?
7
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?
8
Resistors in Parallel
9
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
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Resistors in Parallel
Example: What is the current through each resistor in this circuit?
3Ω
59 Ω
12
18 Ω
Clicker Question #2
Which resistor dissipates more power?
A. The 9 Ω resistor.
B. The 1 Ω resistor.
C. They dissipate the
same power.
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Clicker Question #3
When the switch closes, the battery current
A.
B.
C.
Increases.
Decreases.
Stays the same.
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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.
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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
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Grounding
Safety?
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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.
18
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.
19
RC Circuits
20
RC Circuits
We can also show how the capacitor in the circuit below gets charged if it is
initially uncharged.
21
Reminders
•Homework #7 is due the Monday (March 18) after
Spring Break. Don’t forget about it!.
•Quiz #4 is scheduled for the Thursday after Spring
Break (March 21).
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