Concept Question
Assume in each of the figures below, the number of charges drawn represents the actual density of charges moving, and the arrows represent equal drift velocities for any moving charges. In which case is there the greatest current density going to the left?
J
 nq v d
A
-
+
+
-
+
-
+
+
-
-
B
-
+
+
-
-
+
+
+
-
-
C
-
+
+
-
-
+
+
+
-
D
-
+
+
-
-
+
+
+
-
-

Concept Question
Platinum has a temperature coefficient of wire at T = T
0

= 0.00392/

C. A
= 20.0

C has a resistance of R = 100.0

.
What is the temperature if the resistance changes to 103.9

?
A) 0

C B) 10

C C) 20

C D) 30

C
E) 40

C F) None of the above
R

R
0
 1
  
T

T
0

1
  
T

T
0
 
R

R
0
 
T

T
0
 
0.039
103.9
100.0
T

T
0

0.039



1.039
0.039
T T
0
9.95 C

30.0 C

9.95 C

Concept Question
Two “resistors” are connected to the same 120 V circuit, but consume different amounts of power. Which one has the larger resistance, and how much larger?
A) The 50 W has twice the resistance
B) The 50 W has four times the resistance
C) The 100 W has twice the resistance
D) The 100 W has four times the resistance
P
 2
I R

R
2
P

R
2

1
R
•The potential difference is the same across them both
•The lower resistance one has more power
•The one with twice the power has half the resistance
R

1
P
P
= 100 W
P
= 50 W

V = 120 V
R
100

1
2
R
50

Concept Question
•First, pick a direction for every Kirchoff’s Second Law: loop The total voltage change
•I always pick clockwise around a loop is always zero
•Start anywhere, and set 0 equal to sum of potential change from each piece:
•For batteries: 
V =
E
•It is an increase if you go from – to +
•It is a decrease if you go from + to –
•For resistors: 
V = IR
•It is a decrease if you go with the current
•It is an increase if you go against the current
What is Kirchoff’s Second Law for the purple loop?
A) 0 = +5
C) 0 = –5 I
I
2
2
– 6 – 4 I
3
– 6 – 4 I
3
B) 0 = +5
D) 0 = –5 I
I
2
2
+ 6 – 4
+ 6 – 4 I
I
3
3
0
 
5 I
12 V
2
–
4
I
5
2



6
I
1
I
3
–
6 V

4 I
3

Concept Question
•An ammeter is a device that measures the current (amps) anywhere in a circuit
•To use it, you must route the current through it
•A perfect ammeter should have zero resistance
A
•A voltmeter is a device that measures the potential difference (volts)
V between any two points in a circuit
•To use it, you can simply connect to any two points
•A perfect voltmeter has infinite resistance
+
–
V
A
V
A
Which meter is installed incorrectly?
A) Left voltmeter B) Right voltmeter
C) Left ammeter
E) All are correct
D) Right ammeter

Concept Questions
What is the direction of the force for each of the situations sketched?
A) B) C) D)
E) None of the above
F
 q v B F
 q vB sin
 v p proton
B
Ca +2 ion
4. If q is negative, change the sign electron e
B v Ca B v

Concept Question
Two particles with the same mass are moving in the same magnetic field, but particle X is circling in less time than particle Y. What can account for this?
A) Particle X is moving faster (only)
B) Particle Y is moving faster (only)
C) Particle X has more charge (only)
D) Particle Y has more charge (only)
E) A and C could both account for this mv
 qRB
T

2
 m qB
X
B
Y
•Moving faster doesn’t help
•Higher speed means bigger radius
•Higher charge does help
•You turn corners faster

Concept Question
What could be done to increase the torque on the loop?
A) Increase the size of the loop (only)
B) Increase the current in the loop (only)
C) Make the wire that forms the loop go around several times (only)
D) A and B work, but not C
E) A, B, and C all work
τ 
NI
A B
τ 
I
A B
A
I
R
A
 
B
Edge-on view of Loop

Concept Question
Two wires have the same current I flowing through them. If we want the magnetic field between them to be large and up, we should have the current in the upper one flow _____ and the lower one ______
A) Left, Right
C) Right, Left
B) Left, Left
D) Right, Right

Concept Question

B
 d s
 
0
I
There are currents going in and out of the screen as sketched at right. What is the integral of the magnetic field around the path sketched in purple?
A)

D)

0
(11 A) B)

0
(-11 A) C)

0
(3 A)
0
(-3 A) E) None of the above
•Right hand rule causes thumb to point down
•Downward currents count as +, upwards as –

B
 d s
 
0
 
4 A
1 A
7 A
2 A
5 A

Concept Question
 
B

B n dA

0
A regular tetrahedron (four sides, all congruent) has a cylindrical magnet placed in the middle of the bottom face. There is a total of 0.012 T
 m 2 of magnetic flux entering the bottom face. What is the total flux from one of the three top faces?
A) 0.006 T
 m 2 B) 0.004 T
 m 2 C) 0.003 T
 m 2
D) 0.012 T
 m 2 E) None of the above
•Flux in bottom must equal total flux out other three sides
•Other three sides must have equal flux, by symmetry
     
1 2 3
3
 
1
2
2