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Physics 104
Exam Ia
14 February 2011
Name _________________________________ [0 or 1 point]
Select and encircle the best answer to the following questions [0 or 1 point each]:
1.
The SI unit of electric charge is _________________________.
a) Newton
b) Joule
c) Coulomb
d) Volt
2.
Conventionally, the electric field vector points in the direction of ____________ potential.
a) increasing
b) decreasing
c) unchanging
d) green
3.
The SI unit of electric potential is the _____________.
a) Coulomb
b) Newton
c) Volt
4.
5.
d) Joule

N
An electron, q  e , is accelerated from rest by a uniform electric field, E  14 xˆ .
C
What is the electric force on the point charge?
a) 2.24  10 18 N xˆ
b)  2.46  1012 N xˆ
c) 14N xˆ d)  2.24  10 18 N xˆ
Two large flat plates are parallel and 0.005 m apart. The voltage difference between the
plates is 25 V. What is the magnitude of the E-field between the plates?
V
V
V
a) 0.125
b) 5000
c) 0.125 V
d) 6.25  10  4
m
m
m
Select and encircle the best answer to the following questions [0, 1, or 2 points each]:
Show your work for partial credit.
6.

N
A uniform electric field, E  14 , passes through a flat surface at an angle of 35o with
C
the normal. The surface is a rectangle with sides 5.00 m by 3.4 m. Compute the electric
flux through the surface.
J
N m2
N m2
N m2
a) 195
b) 0.825
c) 238
d) 195
C
C
C
C
1
7.
A repulsive force of 3.00 N is exerted on a 5.00  10 6 C charge by another charge, Q,
which is 0.200 m from the first charge. In Coulombs, Q equals _____________.
a)  3.38  10 6
b) 2.67  10 6
c) 3.75  10 5
d) 1.33  10 5
8.
A point charge (q = 15 C) is located at the origin of coordinates. What is the magnitude of
the electric field due to that charge at the point (2 m, 4 m)?
N
N
N
a) 3.02  1010
b) 6.75  10 9
c) 4.5  10 8
d) 6.75  10 9 N
C
C
C
2
A point charge, q  3.8  10 19 C & m  1.20  10 27 kg , is located 3.35  10 2 m away
C
from an infinite sheet of charge with   2.0  10 5 2 . If the point charge is released from rest,
m
with what speed does the particle hit the sheet?
a) 2.40  1013 m / s
b) 2.88  10 14 m / s
c) 4.90  10 6 m / s
d) 1.70  10 7 m / s
9.
10.
A particle having electric charge q  2  10 9 C and mass m  2  10 27 kg is in a uniform


N
electric field E  3.5 xˆ . If the charged particle is displaced ds  0.8m xˆ  0.8m yˆ , what
C
is its change in voltage?
a) -2.8 V
b) 5  10 9 V
c) -2.8 J
d) 2.8 V
3
Work the following problem(s). Show all your work, including diagrams. [10 points each]
11.
C
. Use Coulomb’s “Law”
m
to obtain the integral(s) necessary to evaluate the electric field vector at a distance
R = 10 m from the line. Be sure to draw a picture.
Consider an infinitely long straight line of charge, with   6
4
12.
Three point charges are shown in the figure below.
Q1 = 5.0 C at (2.0 m, 0.0 m)
Q2 = -2.0 C at (0.0 m, 3.0 m)
Q3 = 3.0 C at (0.0 m, 0.0 m)
Compute the electric potential, V, at the origin due to Q1 and Q2, and then compute the
potential energy change, U , that results from placing charge Q3 at the origin.
5
Mechanics
x  xo  vox t 
1
axt 2
2
E  K U
K
v x  vox  a x t
1 2
mv
2
2
v x2  vox
 2a x x  xo 
W  K
Wconservative  U
 F  ma
Wnonconservative  E
Constants
k
1
 9  10 9
4  o
g  9.8
melectron
N m2
C2
 o  8.85  10 12
C2
N m2
m
m
e  1.6  10 19 C
c  3  10 8
2
s
s
31
 27
 9.11  10 kg
m proton  1.67  10 kg
 o  4  10 7
N o  6.02  10 23
Tm
A
particles
mole
Electric Force, Field, and Flux
F k

F
E
qo
q1 q 2
2
12
r
Einfinite line 

2  o x
 
 E     E  dA
Ek
Einfinte sheet 
E 
q
r
2

2 o

q
E  k 2 rˆ
r

Q
Length

dq
dE  k 2 rˆ
r

Q
Area

Q
Volume
q enclosed
o
Potential Energy, Voltage & Capacitance
U
1
4o

i j
qi q j
rij
b 

Vb  Va   E  ds
V 
U
1

q o 4o
qi
r
i


Uniform E : dV   E  ds
a
6
V 
1
4o

dq
r

V
V
V
E
xˆ 
yˆ 
zˆ
x
y
z