ID: B
5
UCSD Physics 2B
Unit Exam 3B
Current, Resistance, Circuits
Answer Section
MULTIPLE CHOICE
1. ANS: A
when all resistors are identical
R
SER
=
NR
=
3
×
15
Ω=
45
Ω
TOP: SERIES
2. ANS: B
R
=
ρ
L
A
=
s
s
2
⇒
=
Rs
=
1320
Ω
()
17.2
×
10
−
2
m
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
=
227
Ω•
m
TOP: RESISITIVITY
3. ANS: C
The middle lamp is "shorted out" and therefore has no current through it since the potential difference across
it is zero. At the same time, the total resistance of the circuit is lowered so more current flows through the
remaining bulbs. You can also see this by recognizing the circuit as a voltage divider. When the middle bulb
is shorted, the battery voltage is divided between only two remaining bulbs, so they are brighter than when
the voltage was divided by 3.
TOP: DEMO
4. ANS: A
Each differential area element
dA
of a circle (cross section) is a ring of thickness
dr
and length
(circumference) 2
π
r
. The area of this ring is the product,
dA
=
2
rdr
I
=
J
•
∫
=
Jr
2
0
R
∫
=
2
Kr
3
dr
0
R
∫
=
2
K
R
4
4
=
1
2
3.14
12.2
A
/
m
4
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
16.6
×
−
2
m
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
4
=
14.5
mA
TOP:
CURRENT DENSITY + INTEGRATION
5. ANS: B
Resistance is
directly
proportional to length.
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 Summer '08
 schuller
 Current, Resistance, Resistor, Electrical resistance, dr dr dr, ~Á ~Á ~Á

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