Download Exam 3 - UNC Physics

Phys25-002, Spring 2005, Exam 3
Student Name: ___________________________ Seat Number: ________
Sequence Number: 101
Honor Pledge and signature:
I have neither given nor received unauthorized aid on this examination. _____________________
Instructions:
This exam is closed book, closed notes. However, you may use a calculator.
Mark your answers to the multiple-choice questions on a Scantron answer sheet.
Show all of your work on these test papers for full credit (no other scratch paper is allowed).
Questions 1-13 are worth 3 points each, while questions 14-22 are 4 points each.
Equations and conversion factors that may be useful:

 F
q
q
V
U
kq
E
Ek 2
E
  EA cos   encl
V 
V 
qo
s
o
qo
r
r
F  qE
I
q1 q 2
r2
F k
Q
t
Q
V
L
R
A
C
V  IR
Req  R1  R2  R3  
F

t
  N
x rms 
1
2
u E  12  o E 2
d
P  IV
uB 
  NIAB sin 
N o I
o I
2r
Bwire 
Bloop 
2R
U  12 LI 2
q (t )  Q(1  e t /  )
  RC
q (t )  Qe t / 
 L R
  BA cos 
Bsolenoid   o nI
L   o n 2 Al
I

 rod  Bvl sin    NBA  sin t I  (1  e t /  )
t
R
V
1
I rms  rms Z  R 2  ( X L  X C ) 2 X C 
X L  L
Z
C
1
o 
 2f Pav  I rmsVrms cos 
LC
  L
x max
I S VP N P


I P VS
NS
f  f 1  u / c 
c  f
c  3.00  10 8 m/s
B2
B2
2
u  uE  uB   o E 
 oE 
2 μo
μo
1
2
I  12 I o
U  12 CV 2
B2
2 μo
1 C eq  1 C1  1 C 2  
F  ILB sin 
 o I1 I 2
L
2d
 o A
C eq  C1  C 2  C 3  
1 Req  1 R1  1 R2  
F  qvB sin 
C
n1 sin 1  n 2 sin  2
sin  c 
Telescope : M total 
f obj.
f eye.
N
f
tan  B 
M 
N
f
L  f obj.  f eye.
 o  8.85  10 12 C 2 /(N  m 2 )
k
n2
n1
1
4o
n2
n1
E  cB
p U /c
1
1
1


do di
f
f mirror   12 R
Magnifying glass : M np  1 
e  1.60  10 19 C
I  uc
2
I  I o cos 2 
c  1/ ε o μ o
Pav 
m-
f  number 
di
do
f
D
Microscope : M total 
d sin   m
n
c
v
r. p. 
1
f
 di N
f obj. f eye.
sin   1.22
 o  4  10 7 T  m/A
 8.99  10 9 N  m 2 /C 2
I av
c
n water  1.33

D
1. Which one of the following is NOT a correct statement about electromagnetic waves?
1. The magnitude of the electric and magnetic fields are equal.
2. The energy densities of the electric and magnetic fields are equal.
3. The electric and magnetic fields are always at right angles to each other.
4. The direction of propagation is found from the Poynting vector: S = E x B/μo
2. If you are given a Polaroid, which of the following procedures would be best for accurately determining
the transmission axis of this polarizer?
1. Align the Polaroid with another polarizer and view light passing through. When the light appears
brightest, the transmission axes are aligned.
2. Shine polarized light through the unmarked polarizer and rotate until light no longer passes through.
3. Look through the polarizer at an LCD display. When the display image disappears as you rotate the
polarizer, the transmission axis will be horizontal.
4. Look through the polarizer at light reflected off a horizontal surface. When the light appears
brightest as you rotate the polarizer, the transmission axis will be horizontal.
3. If the electric field of an EM wave doubles, what happens to the magnitude of the magnetic field and the
intensity of the wave?
1. Both the magnetic field and intensity double.
2. The magnetic field is reduced by 50%, and the intensity remains the same.
3. The magnetic field doubles, and the intensity quadruples.
4. The answer depends on the frequency of the wave.
4. An electromagnetic wave traveling in the +y direction is known to have its electric field pointing in the
+x direction. In what direction is the magnetic field pointing?
1. -x
2. -y
3. +z
4. -z
5. When a ray of light passes through a glass window into air, what happens to its speed?
1. It slows down.
2. It speeds up.
3. Its (reduced) speed remains the same.
6. Describe the image you see if you look at your reflection from the back of a spoon held at arms length.
1. The image is virtual, upright, and reduced.
2. The image is virtual, upright, and enlarged.
3. The image is virtual, inverted, and reduced.
4. The image is real, upright, and enlarged.
7. If you open your eyes underwater, everything looks blurry because the focal lengths of your eyes change
when they are in contact with water. When this happens, where is the light focusing in your eye?
1. on the retina
2. in front of the retina
3. behind the retina
8. A friend wears glasses that make her eyes look bigger than they really are. Is your friend nearsighted or
farsighted?
1. Nearsighted
2. Farsighted
2
9. A lens is needed to project the image of an object on a screen. What kind of lens must be used, and how
far must it be from the object?
1. Only a converging lens may be used, where do > f.
2. A diverging lens may be used, where do < f.
3. Either a converging or diverging lens may be used as long as do > f.
10. A lens is used to project an image on a screen. If the top half of this lens is covered, what will happen
to the image?
1. The top half of the image will disappear.
2. The bottom half of the image will disappear.
3. The entire image will still be visible, but the magnification will be reduced.
4. The entire image will still be visible, but the brightness of the image will be reduced.
11. A thin film floating on water is observed to be dark near the edges where it is thinnest. What can be
concluded about the index of refraction of this film?
1. n < 1.33
2. n > 1.33
3. n = 1.33
12. Light passing through a single slit is observed to create a diffraction pattern. As the slit is narrowed,
the fringes move outward until no dark fringes are visible. What is the width of the slit when this first
occurs?
1. W = 0
2. W = λ
3. W = λ /2
4. W = 2λ
13. What color of light provides the best resolution?
1. red
2. yellow
3. green
4. blue
14. A radio station broadcasts at a frequency of 80 MHz. How far will this signal travel in 0.67 ms?
1. 200 km
2. 400 km
3. 54 km
4. 5.4 x 107 m
15. The maximum value of the electric field in an electromagnetic wave is 1.5 V/m. What is the rms value
of the magnetic field associated with this wave?
1. 1.1 nT
2. 3.5 nT
3. 5.0 nT
4. 7.0 nT
3
16. An object 4.0 cm in height is placed 8.0 cm in front of a concave mirror with a focal length of 10.0
cm. What is the position of its image in relation to the mirror and what are its characteristics?
1. 4.4 cm in front of the mirror, real, 2.2 cm high
2. 4.4 cm behind the mirror, virtual, 2.2 cm high
3. 40 cm in front of the mirror, real, 20 cm high
4. 40 cm behind the mirror, virtual, 20 cm high
17. What approximate angle from vertical must a fish underwater look to see a fisherman on the shore?
1. 41º
2. 48º
3. 90º
4. Impossible – Total internal reflection does not allow the fish to see the fisherman.
18. The closest a person can read a newspaper comfortably is at a distance of 75 cm from his eyes. What
is the refractive power of the contact lenses that he needs to use in order to read the newspaper at a
normal near point of 25 cm?
1. +5.33 D
2. -5.33 D
3. +2.67 D
4. -2.67 D
19. What is the maximum possible magnification of a converging lens with f = 5 cm?
1. 3
2. 5
3. 6
4. 10
20. In a two-slit experiment, a third-order bright fringe is observed at a displacement angle of 7.40º. If the
wavelength of light used is 605 nm, what is the separation between the slits?
1. 4.7 μm
2. 5.6 μm
3. 9.4 μm
4. 14.1 μm
21. What thickness of a soap bubble (n = 1.38) in air results in constructive interference when light with
wavelength 610 nm reflects off this film?
1. 153 nm
2. 221 nm
3. 305 nm
4. 332 nm
22. The angular resolution for an eye is 0.0085º. What is the minimum separation between two point
sources of light that this eye can distinguish if the light sources are at a distance of 40 m?
1. 3.0 mm
2. 5.9 mm
3. 1.2 cm
4. 34 cm
Answers: 1, 4, 3, 4, 2, 1, 3, 2, 1, 4, 2, 2, 4, 1, 2, 4, 2, 3, 3, 4, 4, 2
4