3B SCIENTIFIC PHYSICS U8431411 Mode D'emploi page 9

4. Sample experiments
4.1 Generation of a non-periodic wave
Firstly, adjust both exciters so as to produce the
same phase movement.
•
Introduce the absorber frame at the end of the
I-shaped part of the wave channel.
•
Switch on the motor for approx. 1 s.
A short wave train develops which moves through
the wave channel (fig. 2).
4.2 Generation of a periodic wave
•
Switch on the motor for a longer time.
A progressive periodic wave develops at the exciter
and travels to the end of the I-channel.
4.3 Proving that waves transport energy, but
not material
•
Attach the two plastic balls in the middle part
of the I-shaped channel by their threads to dif-
ferent places on the channel wall.
•
Switch on the motor briefly
When the balls are met by the wave train, they
move rhythmically over and back like the water
particles. After the wave train moves through, the
balls are still in the same position.
4.4 Determining the phase velocity of a wave
•
Measure the time which a wave peak needs to
travel from the entrance of the I-shaped chan-
nel to the absorber with motor running.
The speed is calculated as a quotient of distance
and time.
4.5 Relationship between frequency and wave-
length
•
First operate the motor with a low voltage.
•
Measure the wavelength.
•
Then increase the frequency of the motor and
again determine the wavelength.
•
Repeat the experiment with a still greater
number of revolutions of the motor.
The greater the frequency of the wave, the smaller
is the wavelength.
4.6 Reflection of the water wave
•
Remove the absorber frame from the end of
the I-channel.
•
Switch on the motor for approx. 1 s.
A short wave train develops, which moves up to the
end of the I-channel. There it is reflected and trav-
els back towards the wave exciter.
4.7 Phase velocity and group velocity
•
Switch on the motor for approx. 2 s.
It is clearly visible that the wave peaks move with
greater speed to end of the I-channel and after the
reflection, from there towards the wave exciter
than the entire group of waves.
4.8 Standing waves
•
Switch on the motor.
The wave is reflected at the end of the I-channel.
The reflected wave overlaps with the arriving wave.
A standing wave develops. A convincing image of a
standing wave can be achieved with a slight ad-
justment of motor speed.
4.9 Same-phase overlapping of waves
•
Introduce the wave absorber again at the end
of the I-channel.
•
Switch on the motor.
•
First block the exit of the partial channels with
the cylindrical body.
•
Determine the amplitude of the wave after it
enters the I-channel (fig. 3).
•
Open the second partial channel again and
determine the amplitude again at the same lo-
cation.
It is now greater than in the first instance by a
factor of √2. (fig. 4).
4.10 Overlap of waves with a phase-shift of 1/2
•
Rotate the sleeve on the exciter paddle in such
a way that the exciters move in contra-motion.
•
Introduce the separator plate into the area
between the V-shaped section and the I-shaped
section.
•
Switch on the motor.
Where the separator is situated, the out of phase
situation of the two partial waves is clearly visible.
In the I-shaped part of the channel which is not
separated by the plate, the two partial waves meet
and cancel each other out (fig. 1).
The fact that standing waves are formed in the area
of the channel with the separator
plate is to be due to the reflection of the partial
waves behind the separating plate. If the exciter is
only switched on briefly, then it is noticed that the
two partial waves move up to the overlapping posi-
tion. There they are then reflected back into both
channels.
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