Sound waves: why do air molecules oscillate?

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Discussion Overview

The discussion centers around the question of why air molecules oscillate and interact to form sound waves when a drum is struck. Participants explore the mechanisms behind sound wave propagation, including analogies and explanations suitable for high school students, while considering the underlying physics of molecular interactions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that when a drum is beaten, air molecules are pushed back and forth by the drum membrane, leading to a chain reaction of oscillation among neighboring molecules.
  • Others question the validity of the marble analogy, noting that air molecules are spaced out and primarily move in random directions, which complicates the idea of direct collisions.
  • A participant shares a personal anecdote about learning that interstellar nebulae also exhibit sound propagation despite their diffuse nature, referencing a previous discussion on the topic.
  • Some argue that the continuum approximation, which simplifies the behavior of gases, is a valid approach to understanding sound waves, while others emphasize the importance of electromagnetic interactions in molecular collisions.
  • There is a debate over whether the repulsion between molecules during collisions is primarily due to electromagnetic interactions or the Pauli exclusion principle.
  • One participant suggests that teaching students about non-contact collisions could enhance their understanding of molecular interactions.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of the marble analogy and the nature of molecular interactions, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

Participants highlight the complexity of explaining molecular interactions, including the role of electromagnetic forces and quantum principles, while acknowledging that simplifications may be necessary for educational purposes.

Who May Find This Useful

This discussion may be of interest to educators, students in physics or chemistry, and anyone exploring the fundamental principles of sound wave propagation and molecular interactions.

  • #31
Nugatory said:
The air molecules aren’t oscillating, the air pressure is oscillating.
The molecules have a huge range of velocities; it's just the average of all the molecules' motions in a local region that's the density (pressure) and motion in the macroscopic sense.
 
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  • #32
sophiecentaur said:
The molecules have a huge range of velocities; it's just the average of all the molecules' motions in a local region that's the density (pressure) and motion in the macroscopic sense.
Yes. We plot the pressure at a point and we get a nice sinusoidal function of time, but that is a macroscopic phenomenon that emerges from averaging the much less orderly movement of the molecules. That orderly sinusoidal pressure oscillation does not imply a similar oscillation of the air molecules; they aren’t swaying back and forth in unison like a line of dancers with locked arms.
 
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  • #33
Nugatory said:
Yes. We plot the pressure at a point and we get a nice sinusoidal function of time, but that is a macroscopic phenomenon that emerges from averaging the much less orderly movement of the molecules.
Electric current is an even stronger analogy. The electrons move in every direction and bounce off the atoms, but the average of all their motions gives the direction and magnitude of current. Even in AC current, the electrons do not wiggle back and forth in unison.
 
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  • #34
Actually, I think the marble analogy is quite a good one. There is this famous model of balls in a container whose bottom vibrates rapidly so that the balls jump around wildly. Would be interesting to see whether you can even simulate a wave when also one of the side walls is made to oscillate albeit more slowly.
 
  • #35
DrDu said:
Actually, I think the marble analogy is quite a good one. There is this famous model of balls in a container whose bottom vibrates rapidly so that the balls jump around wildly. Would be interesting to see whether you can even simulate a wave when also one of the side walls is made to oscillate albeit more slowly.
There are some good school models of kinetic behavior in gases but there are also some really poor ones with motors and rubber diaphragms which clatter a lot but their ‘wave’ patterns are mainly in one’s imagination. I suspect that small ball bearings in a container, excited and kept up by a high power piston / actuator using pink noise, would work well. But it would cost more than a budget level school demo.
 
  • #36
DrDu said:
Actually, I think the marble analogy is quite a good one.
Yes - you can treat gas molecule interactions as perfectly elastic hard-sphere collisions and you arrive at pretty much exactly the correct answer (so long as you appropriately scale the sphere radii in your model).
 

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