3D30.50-Dancing Flames – Physics Department

Description: This impressive demonstration can show standing waves, beats, and wave addition both visually by using a series of flames and audibly by using speakers.The hollow tube is filled with gas which escapes though holes evenly spaced at the top (seen as the flames). A loudspeaker at one end creates standing pressure waves inside the tube, resulting in taller flames at the anti-nodes and shorter flames at the nodes. Effectively, the flames act as a very imprecise but exciting oscilloscope!

(The explanation for why this happens is actually not as simple as it appears: it is probably NOT due to the higher pressure at the anti-nodes forcing more gas out the tube, as is often stated, because the pressure difference between the nodes and anti-nodes isn’t nearly large enough to have this effect. One likely explanation is that the increased vibration of molecules at the anti-nodes mixes the propane and O₂ molecules more effectively, causing more oxidation at those sites. It may also be that the increased vibration at the anti-nodes draws in more O₂ from outside the tube (through the holes) and increases combustion at these sites).

Equipment:

Dancing Flames apparatus
Long gas pipe
Matches
(Optional) Oscilloscope

Setup Procedure:

Plug the apparatus in and position it for easy access. Turn on the power strip.
Connect the Long gas pipe speaker to the apparatus using the long zip cord cable.
Zero all inputs on the mixer and set the main mix out to 0. Set both function generators amplitude to 1/2.
Connect the gas hose to the gas valve.
Make sure the professor knows how to use the equipment.
If the professor requests the oscilloscope, connect it to the mixer’s non primary output or directly from the function generators.

Demonstration Procedure:

Main turn on

Turn the amp on (right knob). Set the amp’s speaker volume to 1/2 (left knob). Set the flame speaker to the appropriate volume (about 1/2 volume)(middle knob).
Double check that the Main Mix volume is set to 0.
Turn on the gas valve to full.
Light the tube with a match. It is quickest to light each individual gas port on the tube, one by one.
When the whole tube is lit, turn the gas down to low to get a flame of about 2-3cm.

Using the Microphone

Turn on the mic.
Turn the mixer Input 1 Level knob (white) to the desired volume. A level of 0 works good.
Start talking into the mic.
When finished, turn Input 1 Level down to – and shut off the mic.

Using the Function Generators

Check to make sure each generator’s amplitude is set to 1/2.
Turn on the function generator stack (left box in the stack).
Select the waveform you wish to use. A sine wave works best.
To play a single wave, set mixer Input 3/4 level knob to 0. This plays generator #1 (on the right).
Adjust the frequency dial to get the desired frequency to illustrate a standing wave. To obtain 20-1000Hz, set the generator multiplier to x100. To obtain 1000-10,000Hz, set the generator multiplier to x1K.
To play two waves at once, turn mixer Input 7/8 level knob to 0. This adds in generator #2 (left generator).
Adjust the frequency dial to get the desired frequency.
To demonstrate beats, set both generators to about the same frequency.
To show what happens when you add two waves with different amplitudes, you can reduce the volume of one of the generators (either mixer input 3/4 or 7/8).
When finished, turn mixer Input 3/4 and 7/8 level knob to –.

Using the CD player

Insert a CD into the CD player.
Press the play button on the CD player.
Turn mixer Input 9/10 Level knob to desired volume (depends on CD being used). About 0 to +15 works good.
When finished, press the stop button on the CD player and turn mixer Input 9/10 level knob to –.

Open Flame
Keep away from flammable materials.

Note: If you are not familiar with how the equipment works, it is highly recommended that you print out these instructions and follow them step by step.
Turning the wrong knob or pushing the wrong button can cause the whole apparatus to work incorrectly.