This preview shows pages 1–3. Sign up to view the full content.
ID: A
5
UCSD Physics 2B
Unit Exam 3A
Current, Resistance, Circuits
Answer Section
MULTIPLE CHOICE
1. ANS: B
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
∫
=
Kr
2
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
2
0
R
∫
=
2
Kr
3
dr
0
R
∫
=
2
K
R
4
4
=
1
2
3.14
122
A
m
4
Ê
Ë
Á
Á
Á
Á
Á
Á
ˆ
¯
˜
˜
˜
˜
˜
˜
166
×
10
−
2
m
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
4
=
1.45 k
A
TOP:
CURRENT DENSITY + INTEGRATION
2. ANS: D
This is a series combination. When all resistors are identical
R
SER
=
NR
=
3
×
33
Ω=
99
Ω
TOP: SERIES
3. ANS: D
This is basically a
unit conversion
problem if you think about it:
Power
=
energy
time
⇒
=
power
×
E
=
P
×
t
=
V
×
I
×
t
=
12.0
J
C
Ê
Ë
Á
Á
Á
Á
Á
Á
ˆ
¯
˜
˜
˜
˜
˜
˜
5.00
C
s
Ê
Ë
Á
Á
Á
Á
Á
Á
ˆ
¯
˜
˜
˜
˜
˜
˜
3.00 min
60 s
min
Ê
Ë
Á
Á
Á
Á
Á
Á
ˆ
¯
˜
˜
˜
˜
˜
˜
=
1.08
×
4
J
=
10.8k
J
TOP: ENERGY
4. ANS: B
Important to keep track of direction here. Since the protons and electrons are moving in the
same
direction, you must subtract the electron contribution to the flux since the charge is negative.
Calculate the number of particles times the charge/particle and then divide by time
I
=
Q
t
=
n
p
e
+
+
n
e
e
−
t
=
n
p
e
+
−
n
e
e
+
t
=
n
p
−
n
e
Ê
Ë
Á
Á
ˆ
¯
˜
˜
e
+
t
=
6.5
×
17
−
3.5
×
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
1.6
×
−
19
C
Ê
Ë
Á
Á
Á
ˆ
¯
˜
˜
˜
1
s
=
0.048
A
=
48m
A
TOP: CURRENT
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document ID: A
2
5. ANS: B
Each parallel pair has resistance R/2, so the total resistance of the circuit is
R
TOTAL
=
3
R
+
3
R
/2
()
=
4.5
R
=
225
Ω
.
Since the resistance between
A
and
B
(going around clockwise)
is
R
AB
=
2
R
+
2
R
=
3
R
, the
ratio
of the resistances is
3
R
4.5
R
=
2
3
.
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 04/04/2010 for the course PHYSICS 2B taught by Professor Lever during the Summer '09 term at UCSD.
 Summer '09
 Lever
 Current, Resistance

Click to edit the document details