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Sound Waves
Set the signal
generator to produce a sinusoidal signal in the range
300-800 hertz that you can hear with the attached earpiece.
(Make sure the sine wave shape is depressed, not square
or triangle wave). Use the microphone, connected to
Din 1 of the ULI to measure the frequency of the sound.
Download
the file microphone.MBL from the course web page by
going to “class activities” and scrolling to the bottom
– be sure to use Internet Explorer not Netscape
to open this LoggerPro file. Save the file on your
computer, then double-click on it to start LoggerPro
with the appropriate settings.
1.
Be sure that the graph axes are labeled “Microphone
Voltage” and “Time”. If they are labeled simply “Voltage”
and “Time”, you have not opened the correct file and
should re-read the comments above. Place the speaker
right up against (touching) the microphone. Collect
data with LoggerPro.
·
What is the frequency
of the sound you are hearing (as measured from the graph)?
Be sure to indicate how you measured this.
·
What is the period?
Again, show how you arrived at this number.
·
Sketch
a graph of the wave, including axes and labeled scales.
2.
How would you determine
the wavelength of this sound? What is the wavelength?
(Hint: The speed of sound in air is 343 m/sec)
3.
About
how long does it take this sound wave to travel the
length of the room? (Hint: You need to estimate the
length of the room in meters.)
4.
Determine the entire range of frequencies that
you can detect with your ears. Change the range on
the signal generator if necessary by pushing a different
button at the top of the signal generator. You should
not adjust the amplitude/output level knob as you scan
through the frequencies unless necessary for ear preservation.
5.
As you scanned through the frequencies, did
the sound volume you heard appear to change? (Circle
the correct answer.) Is this due to a change
in wave amplitude, or to something else? Explain.
Electromagnetic
Waves
1. The figure shown below is a representation of an electromagnetic wave that is
propagating along the x-direction. The electric
field is alternating in the y direction and the
magnetic field is alternating in the z direction.
Use the cross product, E × B to determine the
direction in which the wave travels. The direction in
which it travels is:
2. The mathematical expression for
the wave is
. Use the signs of the x and t terms in
the expressions to determine the direction in which
the wave travels. Does the wave travel in the positive
x or negative x direction?
3. Are your answers to questions
#1 and #2 above consistent with one another? If not,
should they be?
4. Sketch a wave that is moving in the positive y
direction (you will need to sketch in 3 dimensions).
5. State the mathematical expression
for the wave in question #4,using the expressions in
question #2 as a guide. Using the fact that the direction
of the wave is given by the direction of the cross product,
E × B, verify that the sketch you drew in question
#4 is actually moving in the positive y direction.
6. Write the complete mathematical
expression for an electromagnetic wave in which the
electric field is oscillating in the x direction,
the magnetic field is oscillating in the y direction
and the wave is traveling in the in the negative z
direction. Hint: Watch your minus signs!
7. Below is a copy of the the sketch
of the electromagnetic wave shown in question #1. If
this wave is blue light, sketch a second wave that might
be red light on the blank axes provided.
8. About how long does it take this
light wave to travel the length of the room? (Hint:
Use the same estimate of the length of the room that
you used in part 3 of the section about sound waves.)
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