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Physics for Scientists and Engineers, 6e
Chapter 34 – Electromagnetic Waves
What is the phase difference between the sinusoidal
oscillations of the electric and magnetic fields in this
figure?
1.
1
impossible to
determine
2.
180°
3.
90°
4.
0
2
3
4
5
25% 25% 25% 25%
1
2
3
4
Figure 34.3b shows that the B and E vectors reach
their maximum and minimum values at the same
time.
An electromagnetic wave propagates in the –y
direction. The electric field at a point in space is
momentarily oriented in the +x direction. The
magnetic field at that point is momentarily oriented
in the
1
1.
–x direction
2.
+y direction
3.
+z direction
4.
–z direction
2
3
4
5
25% 25% 25% 25%
1
2
3
4
The B field must be in the +z direction in order that
the Poynting vector be directed along the –y
direction.
Which of the following is constant for a plane
electromagnetic wave?
1.
1
magnitude of the Poynting
vector
2.
energy density uE
3.
energy density uB
4.
wave intensity
2
3
4
5
25% 25% 25% 25%
1
2
3
4
The first three choices are instantaneous values and
vary in time. The wave intensity is an average over a
full cycle.
To maximize the radiation pressure on the
sails of a spacecraft using solar sailing, the
sheets must be
1.
2.
1
2
very black to absorb as
much sunlight as possible
50%
50%
very shiny, to reflect as
much sunlight as possible
3
4
5
1
2
To maximize the pressure on the sails, they should
be perfectly reflective, so that the pressure is given
by Equation 34.27.
If the antenna in the figure represents the source of a distant
radio station, rank the following points in terms of the intensity
of the radiation, from greatest to least:
(1) a distance d to the right of the antenna
(2) a distance 2d to the left of the antenna
(3) a distance 2d in front of the antenna (out of the page)
(4) a distance d above the antenna (toward the top of the
page).
25% 25% 25% 25%
1
1.
1, 2, 3
2.
2, 4, 1
3.
3, 4, 1
4.
4, 3, 1
2
3
4
5
1
2
3
4
Specifically, (1), (2) = (3), (4). The closest point
along the x axis in Figure 34.11 (choice 1) will
represent the highest intensity. Choices (2) and
(3) correspond to points equidistant in different
directions. Choice (4) is along the axis of the
antenna and the intensity is zero.
If the antenna in this figure represents the source
of a distant radio station, what would be the best
orientation for your portable radio antenna, located
to the right of the figure?
1
1.
up–down along the page
2.
left–right along the page
3.
perpendicular to the page
2
3
4
5
33%
1
33%
2
33%
3
The best orientation is parallel to the transmitting
antenna because that is the orientation of the
electric field. The electric field moves electrons in
the receiving antenna, thus inducing a current that
is detected and amplified.
In many kitchens, a microwave oven is used to cook
food. The frequency of the microwaves is on the
order of 1010 Hz. The wavelengths of these
microwaves are on the order of
1
1.
kilometers
2.
meters
3.
centimeters
4.
micrometers
2
3
4
5
25% 25% 25% 25%
1
2
3
4
Either Equation 34.13 or Figure 34.12 can be used to
find the order of magnitude of the wavelengths.
A radio wave of frequency on the order of 105 Hz is
used to carry a sound wave with a frequency on the
order of 103 Hz. The wavelength of this radio wave
is on the order of
1
1.
kilometers
2.
meters
3.
centimeters
4.
micrometers
2
3
4
5
25% 25% 25% 25%
1
2
3
4
Either Equation 34.13 or Figure 34.12 can be used
to find the order of magnitude of the wavelength.