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Physics 122
Sept. 28, 2010
Concluding chapter 18.
This lecture is the end of the
material that will appear on your 1
st
midterm exam.
September 30.
First lecture on Ch. 19 (magnetism)
ctober 4
Review session: Javits 103 6:50 :50 PM
October 4.
Review session: Javits 103, 6:50 8:50 PM
October 5. Second lecture on Ch. 19
October 5.
8:30 PM
Midterm exam on Ch. 1518.
October 6.
5:00 PM.
HW on Ch. 19 is due.
Room assignments for the midterm will be posted soon.
Question 1: how do you want review session?
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View Full Document A car battery has a voltage of 12 V when it is not under
load, but when it is providing 100 Amps of current to the
arter motor (load), its terminal voltage drops to 10 V.
starter motor (load), its terminal voltage drops to 10 V.
What is the internal resistance of the battery?
A car battery has a voltage of 12 V when it is not under
load, but when it is providing 100 Amps of current to the
starter motor, its terminal voltage drops to 10 V.
What is
e internal resistance of the battery?
the internal resistance of the battery?
E
= 12 V
When 100 Amperes are
lowing through
the
flowing through R
load
, the
voltage across R
load
is 10 V,
so the voltage across R
int
is 2
V.
R
t
= 2 V / 100 A = 0.02
Ω
.
int
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View Full Document For more complicated problems, we can use
Kirchhoff’s rules*:
1. Sum (algebraic) of currents entering a
junction is zero.
2. Sum of voltage differences around any loop
is zero.
*Not new physics: conservation of charge and conservative force.
Tactics:
Draw and label an arrow to represent each
unknown current.
Write node equations: sum of currents flowing into
each node – sum of currents flowing out = 0.
raw closed loops keeping track of starting point and
Draw closed loops, keeping track of starting point and
direction of each loop.
Write loop equations: sum of voltage differences
around each loop = 0.
Pay attention to signs.
1.
+V
batt
if traverse a battery from – to + terminal.
att
2.
–V
batt
if traverse a battery from + to – terminal.
3.
+IR
if traverse a resistor opposite current
I
.

traverse a resistor with current
4.
IR
if traverse a resistor with current
.
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View Full Document Problem:
What is the current through each resistor?
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