Tuesday, February 07, 2012

A Contest

Define heat.

Now that looks silly, but a lot of confusion starts there especially when somebunnies blather about radiation exchange between two bodies at defined temperatures. It is, IEHO, a lot trickier than it looks.

Eli has been piddling, or at least trying to get the confused to think about this, over at Science of Doom.

While you are at it take a look over at the Weasel's, and then at this talk by Leif Svalgaard, interesting implications for all the solar cyclists, little ice age and medieval hot pot fans out there.


DeWitt said...


You might want to check out John Denker's take on thermodynamics

"The term "heat" is a confusing chimera. It is partly energy, partly entropy, partly temperature, and partly who-knows-what. It shows up in a variety of idiomatic expressions, as discussed in section 16.2.

By itself, the word "heat" has at least five sensible and widely-used but mutually-inconsistent technical meanings (not to mention innumerable nontechnical and metaphorical meanings). It is not worth arguing about the relative merits of these meanings, except to say that each has some merit. I observe that a typical thoughtful expert will use each of these meanings, depending on context. It would be nice to have a single, universally-accepted meaning, but I doubt that will happen anytime soon."

EliRabett said...

DeWitt, exactly what Eli was getting at. Still the Bunny believes that one can reach an inclusive, but always correct definition that covers all the heat sinks, which is what this is about (see cartoon below the comment sidebar)

Hank Roberts said...

It's, um, quantum.

Thad said...

What is heat? -Is there any such thing as an igneous fluid? -Is there any thing that can with propriety be called caloric?
From whence came the heat which was given off in this manner, in the forgoing experiments?
It is hardly necessary to add, that any thing which any insulated body, or system of bodies, can continue to furnish without limitation, cannot possibly be a material substance: and it appears to me to be extremely difficult, in not quite impossible, to form any distinct idea of any thing, capable of being excited, and communicated, in the manner the heat was excited and communicated in these experiments, except it be MOTION.

I am very far from pretending to know how, or by what means, or mechanical contrivance, that particular kind of motion in bodies, which has been supposed to constitute heat, is excited, continued, and propagated, and I shall not presume to trouble the Society with mere conjectures; particularly on a subject which, during so many thousand years, the most enlightened philosophers have endeavoured,. but in vain, to understand.

An Inquiry concerning the Source of Heat Which is Excited by Friction. By Benjamin Count of Rumford, F. R. S. M. R. I. A.

Phil. Trans. R. Soc. Lond. 1798 88, 80-102

John said...

Heat is not the same as "Thermal Energy:"
Thermal energy is energy that a body has by virtue of being at a temperature T > 0K.
Heat is thermal energy that is in motion from one body to another.
So (for example) if a body is at a temperature of 500 C, and the metal is surrounded by a layer of perfect insulation, then the body has thermal energy, but no heat. Because the thermal energy isn't flowing from one body to another.

EliRabett said...

John, while that is fairly standard, as John Donne Rabett said,

No body is an island
No energy stands alone
Each body can be subdivided
Into more bodies
In which internally heat is flown

That definition gets you into the issue of where is the system and where is the surroundings which s arbitrary. For example take a chunk of metal and heat a small part of it. Heat flows within the chunk.

Moreover, even in an isothermal body, energy is flowing from one part to the other, in balance, but there are fluctuations which can be used for some purposes such as random noise generation.