Download Chapter 24

Electromagnetic Waves
1. At every instant, the ratio E B associated with an electromagnetic wave is equal to
a.  0 .
b.  0 .
c. H.
d. c.
e. G.
ANS: d
2. The resonance frequency of an LC circuit is
1
a.
.
2 LC
1
b.
.
 00
c.
d.
e.
L2  C 2 .
1
2 L2  C 2
1
2 L2  C 2
.
.
ANS: a
3. The first experiments that provided evidence to support Maxwell’s theory of electromagnetic radiation
were done by
a. Faraday.
b. Hertz.
c. Gauss.
d. Marconi.
e. Clerk.
ANS: b
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Electromagnetic Waves
4. When a charged particle is accelerated it radiates
a. waves that have the particle’s velocity.
b. electromagnetic waves.
c. longitudinal waves.
d. electrons.
e. mechanical waves.
ANS: b
5.
 
E B
0
is called
a. Maxwell’s third equation.
b. the Lorentz force.
c. Faraday’s relation.
d. the Poynting vector.
e. the coil energy density.
ANS: d

6. What is the maximum value of the E field in V/m 3.50 m from an 800 W source?
a. 62.6
b. 31.3
c. 55.4
d. 29.5
e. 71.2
ANS: a

7. What is the maximum value of the B field in T 3.50 m from an 800 W source?
a. 1.30  10 7
b. 6.85  10 7
c. 5.93  10 7
d. 3.12  10 7
e. 2.09  10 7
ANS: e
8. What solar power in W is delivered to a 160 m 2 area when the electromagnetic flux of 1000 W m 2 travels
in a direction perpendicular to the surface?
a. 1.70  10 5
b. 1.71  10 5
c. 1.60  10 5
d. 1.05  10 5
e. 1.06  10 5
ANS: c
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9. What is the radiation pressure in N m 2 on a perfect absorber when the electromagnetic flux of 1000 W m 2
travels in a direction perpendicular to the surface?
a. 2.53  10 6
b. 6.91  10 6
c. 3.29  10 6
d. 6.67  10 6
e. 3.33  10 6
ANS: e
10. What is the force of radiation pressure in N on a 160 m 2 area when the electromagnetic flux of
1000 W m 2 travels in a direction perpendicular to the surface?
a. 2.61  10 4
b. 5.33  10 4
c. 5.12  10 4
d. 4.89  10 4
e. 5.01  10 4
ANS: b
11. What is the Poynting vector for a wire of length  , resistance R and radius a carrying a current I?
IR
a.
e
R 2V
b.
2 a
I 2R
c.
2 a
A 2R
d.
4 a2
e.
V2
RIa
ANS: c
12. Which of the following has the greatest wave length?
a. Ultraviolet light
b. Infrared waves
c. Radio waves
d. Visible light
e. Microwaves
ANS: c
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Electromagnetic Waves
13. Which of these electromagnetic waves can be generated by LC circuits?
a. Ultraviolet light
b. Radio waves
c. Visible light
d. Microwaves
e. Both b and d
ANS: e
14. Which one of the following has the greatest frequency?
a. Gamma rays
b. Visible light
c. Microwaves
d. Radio waves
e. Infrared rays
ANS: a
15. Which of the following will penetrate the most lead?
a. X-rays
b. Ultraviolet light
c. Microwaves
d. Gamma rays
e. Infrared rays
ANS: d
16. The Earth is 1.49  1011 meters from the Sun. If the solar radiation at the top of the Earth’s atmosphere is
1340 W m 2 , what is the total power output of the Sun in W?
a. 7  10 27
b. 2  10 30
c. 6.62  10 26
d. 3.74  10 26
e. 2.98  10 25
ANS: d
17. If the radiant energy from the sun comes in as a plane EM wave of intensity 1340 W m 2 , calculate the
peak values of E and B.  0  4  10 7 N A 2 .
a. 300 V/m, 10 4 T
b. 1000 V/m, 3.35  10 6 T
c. 225 V/m, 1.6  10 3 T
d. 111V/m, 3  10 5 T
e. 710 V/m, 2.3  10 6 T
ANS: b
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18. Peak values for a neodymium-glass laser are 6.0 joules for 1 ns. If the cross-section of the laser beam is
1 cm 2 , what are the maximum values of E and B?  0  4  10 7 N A 2 .
a. 4  10 7 V m , 0.3 T
b. 3  10 8 V m , 1 T
c. 2  10 8 V m , 0.7 T
d. 5  10 6 V m , 0.1 T
e. 2  1010 V m , 71 T
ANS: c
19. If the maximum E component of an electromagnetic wave is 600 V/m, what is the maximum B component
in T?
a. 1.4
b. 1.8  10 5
c. 2  10 6
d. 10 3
e. 1.6  10 10
ANS: c
20. Find the force in N exerted by reflecting sunlight off a reflecting aluminum sheet in space if the area
normal to the sunlight is 10,000 m 2 and the solar intensity is 1350 W m 2 .
a. 0.72
b. 0.09
c. 9
d. 45
e. 0.18
ANS: b
21. What is the average value in W/m2 of the Poynting vector S at a point 1 meter from a 100 watt lightbulb
radiating in all directions?
a. 1
b. 4
c. 2
d. 8
e. 12
ANS: d
22. A 100 kW radio station emits EM waves in all directions from an antenna on top of a mountain. What is
the intensity S in W/m2 of the signal at a distance of 10 km?
a. 8  10 5
b. 8  10 6
c. 3  10 3
d. 0.8
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Electromagnetic Waves
e. 2.5  10 5
ANS: a
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23. Determine the amount of energy in J carried in 1 meter of a 3.5 milliwatt He-Ne laser beam if the crosssectional area of the beam is 5  10 6 m 2 .
a. 1.7  10 11
b. 3.3  10 6
c. 1.2  10 11
d. 2.3  10 6
e. 4.2  10 3
ANS: c
24. How much electromagnetic energy in J is contained per cubic meter near the Earth’s surface if the
intensity of sunlight under clear skies is 1000 W m 2 ?
a. 3.3  10 6
b. 3.3
c. 0.003
d. 10 4
e. 3.0  10 5
ANS: a
25. At a distance of 10 km from a radio transmitter, the amplitude of the E field is 0.20 V/m. What is the total
power emitted by the radio transmitter in kW? c  3.00  10 8 m s ;  0  4  10 7 N A 2
e
j
a. 10
b. 67
c. 140
d. 245
e. 21
ANS: b
e
j
26. What is the maximum radiation pressure in Pa exerted by sunlight in space S  1350 W m 2 on a flat
black surface?
a. 2.25  10 5
b. 0.06
c. 7  10 4
d. 4.5  10 6
e. 9.0  10 6
ANS: d
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Electromagnetic Waves
e
j
27. What is the maximum radiation pressure in Pa exerted by sunlight in space S  1350 W m 2 on a highly
polished surface?
a. 1.4  10 2
b. 0.12
c. 9.0  10 6
d. 4.5  10 5
e. 2.3  10 6
ANS: c
28. What is the wavelength of 100 MHz television EM waves?
a. 0.3 cm
b. 3 m
c. 9 km
d. 10 m
e. 0.3 m
ANS: b
o
29. Find the frequency in Hz of x-rays of wavelength 1 A  1010 m .
a. 3  1018
b. 3  1010
c. 6  10 9
d. 3  10 8
e. 3  10 20
ANS: a
30. Green light has a wavelength of 5.4  10 7 m . What is the frequency in Hz of this EM wave in air?
c  3.00  10 8 m s
e
j
a. 5.6  10
b. 6.0  1011
c. 9.0  10 8
d. 3.0  1010
e. 1.8  1015
14
ANS: a
31. An FM radio station broadcasts at 98.6 MHz. What is the wavelength of the radio waves in m?
a. 60.8
b. 6.08
c. 3.04
d. 0.314
e. 3.3  10 3
ANS: c
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32. What should be the height in m of a 1/4 wavelength dipole antenna if it is to transmit 1200 kHz radio
waves?
a. 11.4
b. 0.60
c. 1.12
d. 62.5
e. 250
ANS: d
33. An FM radio antenna 1.5 meters long is aligned parallel to the E vector of an incoming radio wave. If the
electric field vector E has a maximum value of 7.0  10 4 V m , what is the induced voltage (rms) across the
length of the antenna?
a. 0.10 V
b. 1.5 mV
c. 0.0050 V
d. 0.74 mV
e. 1.1 mV
ANS: d
34. A solar cell has a light gathering area of 10 cm 2 and produces 0.2 A at 0.8 V (DC) when illuminated with
S  1000 W m 2 sunlight. What is the efficiency of the solar cell?
a. 16%
b. 7%
c. 23%
d. 4%
e. 32%
ANS: a
35. Near the surface of the planet, the Earth’s magnetic field is about 0.5  10 4 T . Approximately how much
energy in J is stored in 1 m 3 of the atmosphere because of this field?
a. 10 1
b. 10 2
c. 10 3
d. 10 4
e. 10 5
ANS: c
36. The sun radiates energy at a rate of 3.86  10 26 W . Its radius is 7.0  108 m . If the distance from the Earth to
the Sun is 1.5  1011 m , what is the intensity of solar radiation at the top of the Earth’s atmosphere in W m 2 ?
a. 1370
b. 1020
c. 1410
d. 1270
Chapter 24
e. 1290
ANS: a
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Electromagnetic Waves
37. A possible means of space flight is to place a perfectly reflecting aluminized sheet into Earth orbit and use
the light from the Sun to push this solar sail. If a huge sail of area 6  10 5 m 2 and mass 6000 kg were placed
into orbit and turned toward the Sun, what would be the force in N exerted on the sail? (Assume a solar
intensity of 1380 W m 2 .)
a. 0.176
b. 0.245
c. 0.349
d. 0.438
e. 5.52
ANS: e
38. The light from the Sun takes 8 1/3 minutes to get to the Earth. If the speed of light is 3.00  10 8 m s , how
far is the Earth from the Sun in m?
a. 1.2  1010
b. 1.3  1011
c. 1.5  1011
d. 1.15  1011
e. 1.4  1011
ANS: c
39. When the plane of polarization is rotated by a substance, that substance is
a. a polarizer.
b. an analyzer.
c. optically active.
d. a reflector.
e. an inductor.
ANS: c
40. Polarization of light suggests that light waves are
a. compressional waves.
b. pressure waves.
c. longitudinal waves.
d. transverse waves.
e. absorptive waves.
ANS: d
41. When unpolarized light is passed through two polarizing filters in succession, its intensity is decreased by
87.5%. Determine the angle between the transmission axis of the two filters.
a. 15°
b. 30°
c. 60°
d. 75°
e. 20°
Chapter 24
ANS: c
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Electromagnetic Waves
42. Unpolarized light is passed through three successive Polaroid filters, each with its transmission axis at 45°
to the preceding filter. What percentage of light gets through?
a. 0%
b. 12.5%
c. 25%
d. 50%
e. 33%
ANS: b
43. c  0  0 is equal to
a. 5.
b. 4.
c. 3.
d. 2.
e. 1.
ANS: e
 
44. E dA is equal to
z
a.  0 E0 .
b. c.
c. E Q .
d. Q e0 .
e.  0i .
ANS: d
 
45. B  dA is equal to
z
a. 0.
b. 1.
c. 2.
d. 3.
e. 4.
ANS: a
 
46. E ds is equal to
d B
a.
.
dt
dE
b.
.
dt
d E
c.
.
dt
d B
d. 
.
dt
z
Chapter 24
e. 
d E
.
dt
ANS: d
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Electromagnetic Waves
E
is equal to
Bc
a. 0.
b. 1.
c. 2.
d. 3.
e. 4.
47.
ANS: b
48. The momentum delivered to a perfectly reflecting surface in time t by a wave arriving at normal incidence
is
a. U c.
b. Uc.
c. 2U c .
d. 2Uc.
e. c U .
ANS: c
49. Maxwell’s equations do not require the existence of
a. particles with electric charge.
b. particles with magnetic charge.
c. discrete values of electric charge in units of e .
d. induced electric fields.
e. either (b) or (c).
ANS: e
50. When the electric field vectors at all points along an electromagnetic wave point to the north, and the
magnetic field vectors all point to the east, the wave is traveling
a. south.
b. west.
c. northeast.
d. straight up from the surface of the Earth.
e. straight down toward the surface of the Earth.
ANS: e
51. Electromagnetic waves are emitted by
a. a fully charged capacitor.
b. a closed circuit consisting of a battery, a bulb and connecting wires.
c. an open circuit consisting of a battery, a bulb and connecting wires when the switch is closed.
d. an inductor in an LR circuit in which the switch has been closed for a long time.
e. intersecting rays of light.
ANS: c
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Electromagnetic Waves
52. As opposed to sound waves, light waves
a. do not require the presence of a medium.
b. are not longitudinal waves.
c. can be polarized.
d. are described by options (a), (b) and (c).
e. are described by both (a) and (c) only.
ANS: d
53. The intensity of sunlight reaching the Earth is about 1000 W m 2 . How close do you have to be to a 100 W
point source of light that is 30% efficient for the intensity to be the same as that of solar radiation?
a. 2.4  10 3 m
b. 4.9  10 2 m
c. 8.0  10 3 m
d. 8.9  10 2 m
e. 9.8  10 2 m
ANS: b
54. An observer measures the intensity of radiation she receives from 3 point sources. For source A, 1000 m
away, I A  4 W m 2 . For source B, 500 m away, I B  8 W m 2 . For source C, 2000 m away, IC  0.25 W m 2 .
Rank the sources in order of decreasing power.
a. PA  PB  PC .
b. PB  PC  PA .
c. PA  PC  PB .
d. PB  PA  PC .
e. PA  PB  PC .
ANS: a
55. Light waves cannot
a. be polarized.
b. show a Doppler shift.
c. be trapped between two parallel perfect reflectors.
d. possess mass.
e. exert pressure.
ANS: d
56. Two spaceships otherwise identical use different solar sails. Sail A is a perfect absorber; sail B a perfect
reflector. Compare the momentum gained by ships A and B, p A and p B , after equal time intervals when
each starts at the same distance from the sun and travels in a radial direction.
a. p A  p B  0 .
b. p A  pB .
c. p A  pB .
d. p A  pB .
Chapter 24
e. The momenta cannot be compared if the two ships do not travel the same distance.
ANS: d
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Electromagnetic Waves
57. Two spaceships otherwise identical use different solar sails. Sail A is a perfect absorber; sail B a perfect
reflector. Compare the momentum gained by ships A and B, p A and p B , after the ships travel equal
distances when each starts at the same distance from the Sun and travels in a radial direction.
a. p A  p B  0 .
b. p A  pB .
c. p A  pB .
d. p A  pB .
e. The momenta cannot be compared unless the travel times are equal.
ANS: d
58. Two spaceships use solar sails. Sail A is a perfect absorber; sail B a perfect reflector. Compare the amounts
of energy that could be stored in the ships’ batteries as a result of the interaction of the solar radiation with
the sails.
a. U A  U B  0 .
b. U A  0 ; U B  0 .
c. U A  0 ; U B  0 .
d. U A  U B  0 .
e. U A  U B  0 .
ANS: b
59. You can detect whether or not light reflected from a horizontal surface, such as a swimming pool, is
polarized or partially polarized by passing the light through
a. a pair of polarizing sheets with their axes oriented in the same direction.
b. a pair of polarizing sheets with their axes oriented at right angles to one another.
c. a single polarizing sheet oriented vertically.
d. a single polarizing sheet oriented horizontally.
e. a single polarizing sheet and rotating the sheet while holding its plane perpendicular to the direction of
travel of the light.
ANS: e
60. You can detect whether or not light reflected from a vertical surface, such as a window, is polarized or
partially polarized by passing the light through
a. a pair of polarizing sheets with their axes oriented in the same direction.
b. a pair of polarizing sheets with their axes oriented at right angles to one another.
c. a single polarizing sheet oriented vertically.
d. a single polarizing sheet oriented horizontally.
e. a single polarizing sheet and rotating the sheet while holding its plane perpendicular to the direction of
travel of the light.
ANS: e
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61. At one instant in time, the electric field between two plane parallel capacitor plates, each of area 0.57 m 2 ,
is changing at the rate of 3.2 N C  s . If we ignore fringe fields at the edges of the plates, the displacement
current in A between the plates is
a. 1.4  10 11 .
b. 1.6  10 11 .
c. 2.8  10 11.
d. 5.0  10 11 .
e. 5.6  10 11 .
ANS: b
62. At one instant in time, the electric field between two plane parallel capacitor plates, each of area 0.57 m 2 ,
is changing at the rate of 3.2 N C  s . If we ignore fringe fields at the edges of the plates, the conduction
current in the wires leading into the plates is
a. 1.4  10 11 .
b. 1.6  10 11 .
c. 2.8  10 11.
d. 5.0  10 11 .
e. 5.6  10 11 .
ANS: b
63. The equation that is not one of Maxwell’s equations is
  Q
a. E  dA 
.
0
 
b. B  dA  0 .
  d B
c. E  ds 
.
dt
 
d E
d. B  ds   0 I   0  0
.
dt


 
e. F  qE  qv  B .
z
z
z
z
ANS: e
64. If a plane electromagnetic wave traveling in the x-direction has
B
is equal to
t
a. kBmax sin kx   t .
that
b g
b g
.
sin b
kx   t g
.
sin b
kx   t g
sin b
kx   t g
.
b. cBmax sin kx   t .
c. Bmax
d.  cBmax
Bmax
e.
c
ANS: c
E
  kEmax sin kx   t , we can deduce
x
b
g
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Electromagnetic Waves


65. In an electromagnetic wave, E and B are
a. perpendicular to one another.
b. perpendicular to the direction of wave propagation.
c. parallel to the direction of wave propagation.
d. described by both (a) and (b).
e. described by both (a) and (c).
ANS: d
66. For an electromagnetic wave in a given volume of space, the instantaneous energy density associated with
B2
the magnetic field, u B 
, is related to the energy density associated with the electric field, u E , by
2 0
1
a. u B  u E .
2
b. u B  u E .
c. u B  2u E .
1
d. u B   0u E .
2
e. u B  2 0 u E .
ANS: b
67. In the diagram below, the electric field vector points upwards in the page and the magnetic field vector
points out of the page directly at you. This electromagnetic wave is traveling

E

B
a. upwards.
b. downwards.
c. to the right.
d. to the left.
e. into the page.
ANS: c
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68. In the diagram below, the electric field vector points upwards in the page and the magnetic field vector
points into the page directly away from you. This electromagnetic wave is traveling

E

B
a. upwards.
b. downwards.
c. to the right.
d. to the left.
e. into the page.
ANS: d
69. In the diagram below, the magnetic field vector points upwards in the page and the electric field vector
points out of the page directly at you. This electromagnetic wave is traveling

B

E
a. upwards.
b. downwards.
c. to the right.
d. to the left.
e. into the page.
ANS: d
70. In the diagram below, the magnetic field vector points upwards in the page and the electric field vector
points into the page directly away from you. This electromagnetic wave is traveling

B

E
a. upwards.
b. downwards.
c. to the right.
d. to the left.
e. into the page.
ANS: c