Firstly, assuming constant temperature and pressure. The speed of a sound wave in a medium is constant. For any wave, the speed of wave propagation is wavelength times frequency. If we keep increasing the frequency of sound to a ridiculously high value so that the corresponding wavelength is in the order of micro-meters. Would the resulting wave be considered a sound wave or a heat conduction?
Essentially the resulting wave is just vibrating the molecules mechanically instead of electromagnetically (think IR waves) with a similar wavelength.
If someone could make me picture/visualise the answer that would be great because I tend to be a visual learner. Thanks!
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Actually, yes! Well, at least in solids, for at least some kinds of conduction.
So, in solids, there are two main ways heat can move from one part of an object to another.
The first is heat transport by the movement of free electrons, which mostly happens in metals. You can think of a metal as being filled with a sea of electrons that whiz about in random directions, unattached to any specific atom. Sometimes, one of these electrons can receive a kick from a vibrating atom in a hot part of the metal and fly off to transfer that energy to an atom in a colder area. Incidentally, this is the reason solids that are good conductors of electricity (metals) tend to also be good conductors of heat–both stem from an abundance of free electrons.
The second, which directly relates to your question, is heat transport by phonons. As the name suggests, a *phonon* is to sound what a photon is to light. In the same way that any electromagnetic wave can by broken down into a set of minimal chunks called photons, a vibrational wave in a solid can be broken down into chunks called phonons.
A phonon isn’t a “real” particle in the same sense as a photon, though. It’s what physicists call a “quasiparticle”– a sort of collective behavior of real particles, where the collective behavior itself acts like its own particle. For a rough analogy, you can think of a traffic jam as a quasiparticle made of cars. A traffic jam is a bunching-up of slow moving cars, but it isn’t made out of any specific, permanent set of vehicles. Different cars will enter and leave the jam as it develops. However, the jam as a whole possesses object-like properties such as a position, a velocity independent of the velocities of the individual cars that make it up, a half-life, and so on.
So back to the main topic, heat transport by phonons is when one atom transfers its vibration to an adjacent atom, which transfers its vibration to another atom, and so on in a cascading wave of vibration. This is the exact same mechanism by which sound moves through solids, and the transfer of heat in this way can be described mathematically as the motion of sound particles (i.e. phonons) that transfer energy from hot areas to cold areas. The main differences between this and what we normally think of as sound is 1) thermal phonons tend to be in a random mix of frequencies and directions and 2) the frequencies involved are far too high to be audible to humans.
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