Solutions to Homework Questions 7

Physics 112 Homework 7 (solutions) (2004 Fall)
Chapt21, Conceptual-7: In radio transmission, a radio wave serves as a carrier wave and the sound wave is
superimposed on the carrier wave. In amplitude-modulation (AM) radio, the amplitude of the carrier wave varies according to
the sound wave. The Navy sometimes uses flashing lights to send Morse code between neighboring ships, a process that has
similarities to radio broadcasting. Is this AM or FM What is the carrier frequency What is the ignal frequency What is the
broadcasting antenna What is the receiving antenna
Solution:
The flashing of the light according to Morse code is a drastic amplitude modulation – the amplitude is
changing from a maximum to zero. In this sense, it is similar to the on-and-off binary code used in
computers and compact discs. The carrier frequency is that of the light, on the order of 10 14 Hz. The
signal frequency depends on the skill of the signal operator, but it is of the order of 1 Hz, as the light
flashes on and off. The broadcasting antenna for this modulated signal is the filament of the lightbulb in
the signal source. The receiving antenna is the eye.
Chapt21, Conceptual-8: When light (or other electromagnetic radiation) travels across a given region, what
is it that moves
Solution:
As an electromagnetic wave travels through space, the things that move are fluctuations in electric and
magnetic fields oriented at right angles to one another. These fluctuations in the fields transport energy
as they travel along.
Chapt21, Conceptual-10: How can the average valuue of an alternating current be zero, yet the square root
of the average squared value not be zero
Solution:
The average value of an alternating current is zero because its direction is positive as often as it is
negative, and its time average is zero. The average value of the square of the current is not zero, however,
since the square of positive and negative values are always positive and cannot cancel.
Chapt21, Conceptual-13: Radio stations often advertise “instant news”. If whatt they mean is that you
hear the news at the instant they speak it, is their claim true About how long would it take for a message to travel across
this country by radio wave, assuming that these waves could travel this great distance and still be detected
Solution:
Radio waves move at the speed of light. They can travel around the curved surface of the Earth, bouncing
between the ground and the ionosphere, which has an altitude that iss small compared to the radius of the
Earth. The distance across the lower 48 states is approximately 5000 km, requiring a travel time of
(5x10 6 m)/(3x10 8 m/s) ~10 -2 s. Likewise, radio waves take only 0.07s to travel halfway around the Earth. In
other wordws, a speech can be heard on the other side of the world (in the form of radio waves) before it
is heard at the back of the room (in the form of sound waves).
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