# Knight26CT ```Ch26-1.
The solid line has length A and makes an angle  with the negative
y-axis. What is the length of the dotted line?
y
A: A cos
B: A sin
C: A tan
D: sin/A
E: cos/A
x
A

Ch26-2.
A charge Q is on the y-axis at y = d. What is the magnitude of the
E-field at position x, on the y-axis?
kQ
A)
d  x 
B)
kQ
x2  d2
C)
x
2
y
Q
d
kQ
2
 d2 
2
x
D)
kQ
x2  d2
E) None of these
E
Ch26-3.
z



The integral E  dE means E 
z
x dE x
z
 y dE y
z
 z dE x .
Often, from symmetry, one can see that one or more of the three
component integrals vanishes.
An infinite line of charge with linear charge density  is along the
x-axis and extends to  . At the pointA shown, what can you
can about the x- and y- components of E ?
y
A
++++++++++++++++++++++++++
x
A: Ex = 0, Ey &lt; 0
C: Ex = 0, Ey =+
E: Ex &gt; 0, Ey &gt; 0
B: Ex &lt; 0, Ey = 0
D: Ex = 0, Ey &gt; 0
Ch26-4. A circular ring of radius R, uniformly
charged with total charge +Q, is in the xy plane
centered on the origin. The electric field dE at
position z = h on the z-axis, due to a small piece
of the ring with charge dQ, is shown. What is
the magnitude of the field dE?
kQ
A: 2
h
z
dE

h
k dQ
B:
h2
R
k dQ
C: 2
R  h2
D:
k dQ
x
y
dQ
R2  h2
E: None of these.
What is dEz, the z-component of dE ?
A: dE sin
B: dE cos
C: dE tan
What is cos?
h
A: 2
R  h2

h
1 

D: cos 
2
2
R

h


B:
h
R2  h2
E: None of these!
D: None of these.
C:
h
R
Ch26-5.
A circular ring uniformly charged with
positive charge Q is in the xy plane centered
on
the origin as shown. On the z-axis,

E  E z z . Which graph accurately represents
the electric field Ez on the z-axis?
Ez
Ez
z
A)
y
x
z
B)
Ez
Ez
z
C)
z
D)
E: None of these is an accurate representation of Ez
z
Ch26-6.
ˆ ?
What the magnitude of the vector (iˆ  ˆj)  (xˆ  y)
A) 1
B) 2
C) 0
D) Some other number.
ˆ is not a vector.
E) No answer, because (xˆ  y)
Ch26-7.
There are no charges near inside the regions shown. Which of the
following are possible electric field line configurations?
(b)
(a)
(c)
A: (a) only
B: (b) only
C: (c) only
D: None are possible
(all, a and b only, etc.)
Ch26-8.
Consider the four electric field line patterns shown. Assume that
that are no charges in the regions shown. Which, if any, of the
patterns represent possible electrostatic fields.
I
II
III
IV
A: All are possible.
D: None are possible.
B: II only
C: II and III only
E: None of the above.
Ch26-9.
From the figure, what can you say
about the magnitude of the charge
on the bar Q bar , compared to the
magnitude of the charge Q of the
positive point charge?
A: Q bar  Q
B: Q bar  Q
C: Q bar  Q
+
Q
Qbar
From the figure, what can you say
about the net charge on the bar?
A: Qbar = 0
B: Qbar &gt; 0 (that is, the bar has a net positive charge)
C: Qbar &lt; 0 (the bar has a net negative charge)
D: Not enough information in the figure to answer the question.
Ch26-10. Two infinite planes are uniformly charged with the same
charge per area . If one plane only were present, the field due to
the one plane would be E.
B
++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++
A
The field in region B has magnitude…
A: zero
B: E
C: 2E
D: depends on exact position.
C
The field in region A has magnitude…
A: zero
B: E
C: 2E
D: depends on exact position.
Ch26-11. A dipole is placed in an external field as shown. In
which situation(s) is the net force on the dipole zero?
(1)
(3)
A:
B:
C:
D:
E:
(1)
(2)
(1) and (2)
(3) and (4)
(2) and (4)
(2)
(4)
Ch26-12.
velocity
V(t)
+4m/s
-4m/s
time t
0
5
10
15
For 1D motion, if velocity v is constant, then the displacement in
time t is d = v t. If v is not constant, then the displacement is
d 
z
tf
v( t ) dt
ti
. The graph above shows v(t).
What is the displacement after 10 seconds?
A: 3m
D: 18m
B: 8m
C: 10m
E: None of these.
Ch26-13.
Areal mass density  of a tile is mass m per area A:  
m
A
If a sheet has area A and uniform density , then its mass is m = A. If a
sheet has a non-uniform density    x, y , where (x,y) is the postion on
the sheet, then the total mass of the sheet is
bg
m 
z
 da 
z
 d2r 
zzb g
 x, y dx dy
where da = d2r represents an infinitesimal area element.
Consider a square sheet in the x-y plane with edge length L
and density   b  kx , where b and k are constants.
Which expression below correctly represent the integral for
the mass?
L
Green : L   b  kx  dx
L
L2   b  kx  dx
Yellow :
0
0
L
Pink :
 kx dx
L
Blue :
0
Purple : None of these/ don't know.
y
L
  b  kx  dx
0
da’s
L
x
```