Why heat?

This is a spontaneous posting and perhaps if I think about it for a bit the answer will become obvious, but why do we talk of heat when dealing with entropy and thermodynamic processes? When we speak of a system losing heat energy to its surroundings (through friction, etc) what about longer wavelengths of energy such as microwave and radio? Does a ball rolling down a hill lose energy due to radio emissions? As an extension of that, why can't we harness a heat engine that produces radio waves as an inefficient by-product?

I have a couple of ideas but I want to know if there is a well-defined answer first...

 

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Can't provide you with a precise "textbook" answer but it seems pretty obvious to me. For one thing, "thermodynamic" even contains heat in it's name.

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For another, it would seem to me that expressing the energy involved in any process as heat is a rather convenient way of keeping different things in common units for comparison or computations.

 

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I'm trying to decide if it's "thermodynamic" rather than, say, "radiodynamic" for a reason. Or perhaps all wavelengths > visible spectrum are considered heat by definition in systems involving entropy? If so, why is that the general cutoff of useful energy?

 

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I seriously think it may be just a matter of convention, unspoken or not.

 

Because heat is "an emergent property of motion". A dynamical system is a system where there's internal motion. A hot solid is one where the atoms are essentially vibrating like the clappers. A hot gas is one where the molecules are moving faster than a rifle bullet. Temperature is a measure of average kinetic energy. Think of it as average motion. There's the same motion to a 2kg cannonball doing 1000km/s as in two adjacent 1kg cannonballs doing 1000 m/s. Entropy is a measure of available energy, and doing work averages it out. The motion averages out. When entropy increases, the "sameness" of the system motion increases, until eventiually you can't do work any more.

Probably. But not much. And only because it's rotating, not because it's descending.

I don't know.

But what I do know is that when a system loses heat energy, it loses kinetic energy. It loses internal motion, which is typically radiated away. And there's an important clue about this in Compton scattering and pair production. In Compton scattering some of the photon energy-momentum is converted into the kinetic energy of the electron. You can then use the residual photon to perform another Compton scatter, and another, and another, until you can discern no photon any more. That photon has been converted into the motion of electrons. In pair production however, you can use a photon to create an electron. And a positron. And you need a nucleus to do it. But you catch the drift. In a deep fundamental sense, the electron is made of motion. So are you. And so is everything else.

 

This is good

OK perhaps my question is malformed. Yes heat is motion but why does heat radiate away from a body in the IR spectrum and not longer wavelengths? Or perhaps it does? Again, why heat?

 

This goes back to the fundamentals of what is a thermodynamical process, and what is energy? Now energy is something that is not as apparent as something like acceleration, but it definitely has some kind of magnitude, and it is somehow conserved.

Now entropy has something to do with the odds of a certain process occuring, and how those odds, and the process itself, change with time.


Also, I think a cold blackbody will give off most of its radiation as radiowaves.

 

Agreed, defining energy is difficult but I'm comfortable with the general idea that the more concentrated it is (relative to the general local concentration), the more usable it is. Maybe I'm answering my own question here...perhaps heat becomes unusable on Earth because the Earth has a background temperature in the IR range..? It still leaves questions (such as whether objects emit ultralong wavelength radiation, or whether run-of-the-mill friction causes loss of energy in spectrums beyond IR, etc)

I'm being a bit lazy on my posts here, kind of hoping that someone with some thermodynamic education pops in...

 

It does, RJ. If you were in space in a spacesuit your temperature would drop, and keep on dropping. You aren't losing heat via convection, but through radiation. See Thermal radiation on wikipedia where you can read this:

"Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The mechanism is that bodies with a temperature above absolute zero have atoms or molecules with kinetic energies which are changing, and these changes result in charge-acceleration and/or dipole oscillation of the charges that compose the atoms. This motion of charges produces electromagnetic radiation in the usual way..."

 

Cute

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Perhaps what I'm subconsciously getting at is, is my definition or rather 'concept' of energy valid in all cases? (That concept being energy is more useful as it is more concentrated relative to its environment)

This may just be a trivial rewording of entropy but I've never heard anyone define "useful energy" in such a way.

 

Possible counter-example: electron shells

Anyway I think Farsight helped explain the answer, which is that heat in this case is molecular motion of all forms, not just in the frequency of IR. If anyone has a better answer, join in.

 

Not quite, you can go a bit deeper. We associate energy with motion, but don't forget the self-energy of a gravitational field. A gravitational field has energy, and this energy isn't in motion. Instead it's "stress-energy" described by the stress-energy tensor, see wikipedia. On the right of the article you can see a picture akin to the bowling-ball-in-the-rubber-sheet analogy. There's tension around the bowling ball. If you swiftly removed the bowling ball, you'd see ripples propagating through the rubber sheet. You've now got motion, conveying energy. But if you could freeze-frame one of these ripples and examine it closely, you'd still see tension.

Note that the rubber-sheet analogy is imperfect because it relies on gravity to depict gravity, and it's back-to-front in that tension is negative stress. A better analogy would feature a gin-clear elastic block under pressure. When you insert a planet the surrounding space is stressed, you have created a pressure gradient. If you then rotate the planet you subject the space to shear stress, and if you move the planet you get a "bow wave" due to its momentum.