How Things Work II
(Lecture #25)
Instructor: Gordon D. Cates
Office: Physics 106a, Phone: (434) 924-4792
email: cates@virginia.edu
Course web site available through COD and Toolkit
or at http://people.virginia.edu/~gdc4k/phys106/spring07
March 30, 2007
Announcements
• I will hold office hours this coming Tuesday, April 3rd,
at 10:30AM for people who have trouble with the
other times. With no homework due, it is a good time
to come to shore up your understanding of homework,
quiz, or midterm questions that you found confusing.
• Lectures 21-24 have been posted on the web.
updates are to follow shortly.
Other
Radio waves are generated when charges
accelerate
A radio transmitter
works by pushing
charges back and
forth in an antenna.
The anatomy of electromagnetic waves
Remember that the thing at
left is a PLOT electric and
magnetic field as a function
of z. It is not just a funny
shaped thing that moves
through space!
•
•
•
The wave is perpetuated because of a cycle:
-
The changing magnetic field produces an electric field ...
- and the changing electric field produces a magnetic field.
At a snapshot in time, an electromagnetic wave has
- a sinusoidally varying electric field in one direction,
- and a corresponding sinusoidally varying magnetic field in the other direction.
The whole thing propagates along at the speed of light.
Transmitters, tank circuits, and antennas
•
The transmitter puts out a voltage that alternates back and forth at the
right frequency.
•
A “tank circuit” is used to greatly enhance the degree to which charge
sloshes back and forth.
•
Charge moves back and forth on the antenna creating electromagnetic waves.
Tank circuits are central to being able to
broadcast and receive on a single
frequency
•
On the transmit end, the tank circuit allows you to broadcast much more
power with less driving voltage.
•
On the receive end, the tank circuit allows you to select out one frequency
and one frequency only.
How a tank circuit works
•
•
The cycle begins with a charged capacitor, and thus a voltage, and no current.
•
•
At peak current the capacitor is uncharged and change in current with time is briefly zero.
•
Finally the capacitor is charged oppositely to the beginning, and the process repeats in reverse.
The voltage on the capacitor starts current flowing, but the inductor resists a change in the current,
and develops an electric field (and hence voltage) that slows the increase in current.
As the capacitor starts to charge the other way, and hence develop a voltage, the current starts to
decrease, but the inductor again resists a change in the current, and develops an electric field, and
voltage, tending to slow the decrease in current.
The tank circuit as just one example of a
resonant system
There is a direct analogy between a tank circuit and a
mass oscillating on a spring.
Current
Motion
Capacitor
Spring
Stores energy that
can cause current.
Stores energy that
can cause motion.
Inductor
Mass
Resists change in
current
Resists change in
motion.
Encoding sound on a radio wave:
Amplitude Modulation (AM) Radio
In AM radio, the signal representing sound is used to
modulate the amplitude of the broadcast radio wave.
Encoding sound on a radio wave:
Frequency Modulation (FM) Radio
In FM radio, the signal representing sound is used to
modulate the frequency of the broadcast radio wave.