Information in Radio Waves
RET 2013
Unit 3: Information in Broadcast Transmission
Activity : PHET simulation of radio emission**
Background:
The formation of electromagnetic waves occurs when charged particles (i.e. electrons or protons) are
accelerated. This type of motion produces a wave in the electric and magnetic field surrounding that
charged particle. Just like a ripple on calm water when you toss a stone into it, the EM wave travels
outward from the accelerating electron only at much higher speed (the speed of light). In order to gain
some intuition about how these types of EM waves are produced you will interact with a simulation and
make some observations about the waves that are produced.
Procedures:
Navigate to the site
http://phet.colorado.edu/simulations/sims.php?sim=Rad
io_Waves_and_Electromagnetic_Fields
Click on the Run Now button and follow procedures for
running the Java Applet.
Once the applet is running you should see a window that
looks like the image to the right.
Inquiry:
Part 1: understanding the relationships between accelerated motion for an electron and the field
strength and direction.
Start with the settings in the following screenshot.
Transmitter movement: Manual
Field Display Type:
Curve with vectors
Field Sense: Force on electron
Field Displayed: Radiant field
1. Move the transmitter electron and observe what happens
a. What does the red line represent?
b. What do the arrows represent?
2. Try to move the electron at a constant speed.
a. Does this produce any EM waves?
b. Is this difficult to do? Why or why not?
3. Check the box electron positions and switch the mode to oscillate
a. The curve represents the changing voltage that is moving that electron in the antennae
i. How does the motion of the receiver electron compare to the motion of the
transmitter
4. Change the display field to static display.
a. How do the red lines differ from the radiated field.
b. What is being transmitted in the radiated field?
Part 2: Amplitude and Frequency
1. Reset all the parameters to the default. Check the box to show electron positions. Slide the
frequency bar to the left to create a low frequency EM wave, keeping amplitude the same.
a. Describe the graph of electron positions. How are they the same, how are they
different?
2. Select a high frequency by sliding the frequency bar to the right.
a. Now move the amplitude bar down and describe what happens to the position of the
electrons as you decrease the amplitude?
b. Does the frequency change?
Part 3: Simulate frequency modulated waves
Creating small jitters in frequency is like encoding sound wave on to a carrier radio wave. The carrier
wave can be filtered out at the receiver leaving only the oscillations from the original sound wave to be
sent through the speaker wire.
References:
Adams, Wendy, Michael Dubson, Ron LeMaster, Kathy Perkins, and Carl Weiman. Radio Waves &
Electromagnetic Fields. Computer software. PHET Interactive SImulations. Vers. 1.09. University
of Colorado at Boulder, 10 May 2011. Web. 31 July 2014. <http://phet.colorado.edu/sims/radiowaves/radio-waves_en.jnlp>.