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What makes sound and heat distinct ?

The answer is said to lie in the fact that the former consists of vibration in an ordered fashion while the latter is not. But why would ordered vibration not be heat, when heat is the just the 'jingling' of molecules (ordered or disordered) ?

Shouldn't it be in this way : 'All sounds are heat while all heat is not sound' ?

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  • $\begingroup$ "when heat is the just the 'jingling' of molecules (ordered or disordered)" where did you hear that? Generally I only see it in reference to the disordered motion. $\endgroup$ – JMac May 21 at 14:34
  • $\begingroup$ Here is a counter example to your hypothesis that all sounds are heat. In superfluid helium 4, heat and sound (pressure wave) travel at different velocities. It's actually remarkable that heat travels as a wave in a superfluid and is known as the second sound. $\endgroup$ – wcc May 21 at 15:09
  • $\begingroup$ I have edited the end of my answer to show that, although sound and heat are distinctly different, sound can cause heating. Hope this will help. $\endgroup$ – Bob D May 21 at 21:25
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What makes sound and heat distinct ?

What makes them distinct is that sound is the transport of mechanical energy from one place to another in the form of mechanical longitudinal waves, whereas heat is defined as the transfer of energy from one substance to another due to temperature difference. Further explanation follows.

Sound is a mechanical wave associated with the vibration of some medium (some form of matter (solid, liquid or gas)). It is a longitudinal wave in which the displacement of the medium is parallel to the propagation of the wave. Google up longitudinal waves and check out the explanation of their association with sound on the Hyperphysics website.

Heat is energy transfer between substances due solely to a temperature difference between the substances. It is not the "jiggling of molecules" of a substance. That jiggling is the translational, rotational, and/or vibrational kinetic energy of the molecules of the substance and is considered to be part of the internal energy of the substance (The kinetic energy component. There is also a potential energy component).

The confusion regarding heat is that although it is not the jiggling of the molecules themselves, it is the mechanism for causing the kinetic energy of the jiggling of the molecules of a higher temperature body (where the jiggling is faster) to transfer to a lower temperature body (where the jiggling is slower). This transfer can occur by conduction, convection, and/or radiation. The first two require a medium (solid, liquid or gas) where the molecules of the hot and cold bodies interact with one another so that the higher kinetic energy molecules gives up some of their kinetic energy to the lower kinetic energy molecules. Heat transfer by radiation does not because it is in the form of electromagnetic radiation (transverse waves) capable of traveling through a vacuum with no physical interaction between the hot and cold bodies.

Shouldn't it be in this way : 'All sounds are heat while all heat is not sound' ?

As already stated, sound and heat are distinctly different, so neither is a form or subset of the other. Sound is a form of mechanical energy. Heat is the transfer of thermal energy due to temperature difference. But there is a connection between heat and sound.

Since sound involves molecular vibrations, to the extent that the molecules “rub” against one another there is some friction involved. Friction causes the local temperature of the medium to rise above its surroundings. That can cause heat transfer from regions of higher temperature to regions of lower temperature. Such friction also occurs when materials absorb sound. So in that sense, sound can produce heat. However the energy levels of sound are so small, the increase in temperature and any heat transfer would, I believe, be very small.

I am unaware of any mechanism by which heat transfer can produce sound. Perhaps another contributor is aware of such a mechanism and would like to comment.

Hope this helps.

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  • $\begingroup$ But when we say that something has more heat, we mean that the 'jiggling' is more, right ? $\endgroup$ – Zam May 25 at 4:24
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    $\begingroup$ @Zam No, we say more jiggling is greater internal energy, typically meaning its temperature is higher. Heat is the transfer of energy, not the energy itself. It’s the transfer of jiggling from one thing to another. $\endgroup$ – Bob D May 25 at 5:16

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