Title: Concept Question
1Concept Question
What is the length of the long diagonal of a box
that is 2 cm?? 2 cm ? 1 cm? A) 1.732 cm B) 3
cm C) 5 cmD) 9 cn E) None of the above
2 cm
1 cm
2 cm
2Concept Question
A box of dimensions 2 cm?? 2 cm ? 1 cm has charge
density ? 5.0 ?C/cm3 throughout and linear
charge density ? 3.0 ?C/cm along one long
diagonal. What is the total charge? A) 2 ?C B)
5 ?C C) 11 ?C D) 29 ?C E) None of the above
3.0 ?C/cm
2 cm
5.0 ?C/cm3
1 cm
2 cm
3Concept Question
2.0 ?C
 What is the direction of the force on the purple
charge?  Up B) Down C) Left
 D) Right E) None of the above
5.0 cm
5.0 cm
2.0 ?C
5.0 cm
7.2 N
2.0 ?C
 The separation between the purple charge and each
of the other charges is identical  The magnitude of those forces is identical
7.2 N
 The blue charge creates a repulsive force at 45?
down and left  The green charge creates an attractive force at
45? up and left  The sum of these two vectors points straight left
4Concept Questions
Four charges of equal magnitude but differing
sign are distributed in a square around a central
point as shown at right. Which direction will the
electric field point at the center? A) Left B)
Right C) Up D) Down E) No direction it is
zero
Four charges of equal magnitude but differing
sign are distributed in a square around a central
point as shown at right. Which direction will an
electron accelerate at the center? A) Left B)
Right C) Up D) Down E) No direction it is
zero
5Concept Question
The electric field lines outside of a mystery
region are sketched at right. The net charge in
the mystery region is A) Positive B) Negative
C) Zero D) Insufficient Information
6Concept Question
A point charge q is at the center of a (a)
sphere (b) joined hemispheres (c) cylinder (d)
cube. What is the total electric flux out of the
shape?
What is the answer for the cube? A) lt 4?keq B) gt
4?keq C) 4?keq D) Insufficient information
2a
q
2a
2a
7Concept Question
A cube of side a has an electric field of
constant magnitude E E pointing directly out
on two opposite faces and directly in on the
remaining four faces. What is the total charge
inside the cube? A) 6Ea2?0 B) 6Ea2?0 C)
2Ea2?0D) 2Ea2?0 E) None of the above
a
a
a
8Concept Question
A hollow conducting sphere of outer radius 2 cm
and inner radius 1 cm hasq 80? nC of charge
put on it. What is the surface charge density on
the inner surface? On the outer surface? A) 20
nC/cm2 B) 5 nC/cm2 C) 4 nC/cm2 D) 0 E) None of
the above
80? nC
 The Gaussian surface is entirely contained in the
conductor therefore E 0 and electric flux 0  Therefore, there cant be any charge on the inner
surface
1 cm
2 cm
 From the symmetry of the problem, the charge will
be uniformly spread over the outer surface
cutaway view
 The electric field
 The electric field in the cavity and in the
conductor is zero  The electric field outside the conductor can be
found from Gausss Law
9Concept Question
q
 Four charges q are each arranged symmetrically
around a central point, each a distance a from
that point. What is the potential at that point?  0 B) 2keq/a C) 4keq/a
 D) None of the above
q
q
q
10Concept Question
 If V A(x2 y2  2z2), what is Ez?
 2Ax 2Ay 4Az
 B) 2Ax 2Ay 4Az
 C) 4Az
 D) 4Az
 E) None of the above
11Concept Question
potentials in kV
1
1
 In the graph at right, what type of charge is at
X, and what at Y?  Positive, both places
 B) Positive at X, negative at Y
 C) Negative at X, positive at Y
 D) Negative, both places
0
2
2
3
X
Y
4
3
4
 Positive charges dont want to climb the high
mountain at Y  Must be positive charge repelling them!
 Positive charges want to flow into low valley at
X  Must be negative charge attracting them!
 Electric fields are perpendicular to
equipotential surfaces
12Concept Question
 What is the potential at point X?
 11 V B) 11 V
 C) 10 V D) 10 V
 E) 8 V F) 8 V
0 V
1 V
8 V
13Concept Question
What would happen if the drop started with charge
2e, but it suddenly picked up another
electron? A) It would start moving downwards B)
It would start moving upwards C) Since it gained
BOTH mass and charge, no effect

 For elementary particles, electric forces are
much stronger than gravity  Increased gravity is negligible
 Electron has negative charge
 Force on negative charges is opposite direction
of electric field
14Concept Questions
When we close the switch, how much charge flows
from the battery? A) 36 ?C B) 4 ?C C) 18 ?C D) 8
?C E) 10 ?C
2 V
3 ?F
6 ?F
 The voltage difference across each capacitor will
be 2 V.
When we close the switch, which capacitor gets
more charge Q on it? A) The one with the bigger
capacitance B) The one with the smaller
capacitance C) They get the same amount of
charge D) Insufficient information
Q
Q
Q
Q
15Concept Questions
What is the capacitance of the structure shown at
right? A) 180 ?F B) 120 ?F C) 30 ?F D) 20?F E)
none of the above
60 ?F
60 ?F
60 ?F
 They are connected endtoend, so they are in
series
 The two ends of the middle capacitor are at the
same potential therefore, there is no charge on
this capacitor  You might as well ignore it
16Concept Question
20 V
1 ?F
2 ?F
 Capacitors in parallel have the same voltage
difference ?V  The larger capacitor has more energy
For each of the two circuits, which capacitor
gets more energy in it? A) The 1 ?F capacitor in
each circuit B) The 2 ?F capacitor in each
circuit C) The 1 ?F capacitor in the top circuit,
the 2 ?F capacitor in the bottom circuit D) The 2
?F capacitor in the top circuit, the 1 ?F
capacitor in the bottom circuit E) They are equal
in each circuit
1 ?F
2 ?F
20 V
 Capacitors in series have the same charge Q
 The smaller capacitor has more energy
17Concept Question
The capacitor below already has
charge/potential/energy stored in it. How does
the energy change if we slide a dielectric in? A)
It increases B) It decreases C) It stays the
same D) Insufficient information
20 V
 To help us think about it, add a bit more of the
circuit, like a battery to charge the capacitor,
and a switch to connect/disconnect it  When you add the dielectric, the capacitance
increases, C increases  If the switch is open (disconnected) then the
charge is unchanged  From U Q2/2C, we see that U must decrease
 If the switch is closed (connected) the battery
keeps potential unchanged  From U C(?V)2/2, we see that U must increase
 The outcome depends on the rest of the circuit