Physics 104
Exam IIa
11 March 2011
Name _________________________________ [zero or 1 point]
Select and encircle the best answer for each of the following questions: [zero or one point
each]
1.
The SI unit of capacitance is the _________.
a) Farad
b) Volt
c) Joule
d) Franc
2.
If the separation distance between the plates of a parallel-plate capacitor is increased, the
capacitance will _____________.
a) stay the same
b) increase
c) decrease
d) increase to a maximum, and then decrease
3.
The longer a copper wire is, the ____________ we expect its resistance to be.
a) greater
b) smaller
c) neither a nor b
d) neither b nor a
4.
The SI unit of electrical current is the __________________..
a) Henry
b) Ampere
c) Farad
d) Coulomb
5.
In a metal, like copper, the “charge carriers” that can move in response to an electric field
are _____________.
a) neutrons
b) protons
c) sodium ions
d) electrons
Select and encircle the best answer for each of the following questions. Show your work.
[zero, one, or two points each]
6.
Two conducting spheres are 15 m apart. One sphere has an excess charge of 5 C, the other
has an excess charge of -5 C. The voltage difference between the spheres is 87 V.
Calculate the capacitance of the two spheres.
a) 0.0575 F
b) 0.0575 J
c) -0.0575 F
d) 0.115 F
1
7.
At 3.00 V, the current in a resistor is 13 mA. What would be the current if a voltage of
7.00 V were applied to the same resistor instead of 3V?
a) 30 mA
b) 1.62 mA
c) 0.273 mA
d) 0.033 mA
8.
A parallel-plate capacitor is fully charged by a 1.5 V battery. The separation between the
plates is 1.00 mm and the area of each plate is 1.00 cm2. Compute the potential energy
stored in the capacitor.
a) 9.96  10 6 J
b) 6.64  10 13 J
c) 9.96  10 13 J
d) 1.99  10 12 J
2
9.
In a 0.10 m long segment of wire made of some conducting material (   1.0  10 8   m ),
the density of charge carriers is n  1.0  10 28 / m 2 . The charge on each carrier is known to
be –e. Calculate the drift velocity of the charge carriers when a voltage of 1.0 V is applied
to the wire.
m
m
m
m
a) 1.00  10  20
b) 0.0624
c) 3.00  10 8
d) 0.624
s
s
s
s
10.
A fully charged capacitor, of unknown capacitance C, discharges through a resistor,
R  1000 . The half-life of the discharge is 3.61  10 3 sec . How long would it take
for the fully charged capacitor to discharge to 10% of its full charge?
a) 8.32 msec
b) 12 msec
c) 17.3 msec
d) 3.61 msec
3
Work the following problem(s). Show all your work, including diagrams. [10 points each]
11.
a) A 14.0 V battery is connected to a load resistance of R  1000 . (14.0 V is the EMF of
the battery.) The current through R is measured to be I  0.0139 A . What is the power
dissipated in the resistance, R?
b) What are the internal resistance of the battery and the terminal voltage of the battery?
4
12. In the circuit shown in the diagram, use Kirchhoff’s “Rules” to evaluate the unknown EMF,
E, and the unknown resistance, R.
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  x o 
W  K
Wconservative  U
v2
ac 
r

 F  ma
Wnonconservative  E
conservative K f  U f  K i  U i
Constants & miscellaneous formulae
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
area of circle A  R 2
area of rectangle A  ab
 o  4  10 7
N o  6.02  10 23
Tm
A
particles
mole
4
volume of sphere V  R 3
3
area of sphere A  4R 2
Electric Force, Field, and Flux
F k

 F
E
qo
q1 q 2
2
12
r
Einfinite line 
E ring  k
x

2  o x
Qx
2
 a2

3
Ek
Einfinte sheet 
q
r
2

2 o
Flux :
2

q
E  k 2 rˆ
r

Q
Length

dq
dE  k 2 rˆ
r

Q
Area
 
 E     E  dA

E 
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
Q
C
V
1
1
series

C eq
Ci
parallel C eq   Ci
parallel plate capacitor C   o
A
d

t
RC
dischargin g capacitor q  Qo e
Q2 1
1
U
 CV 2  QV
2C 2
2
u
1
oE2
2
Current and Circuits
series R Req   Ri
current I 
Q
t
resistivit y R 
parallel R
Ohm
L
A

E
J
1
1

Req
Ri
V  RI
current density


J  nq v d
Power P  RI 2  VI 
7
V2
R
 I
J 
A