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Name: Leslie
Status: student
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I understand that the definition oftemperature is the average kinetic energy. My question is why is this the case? Why is it that atoms, and atomic bonds, tend to vibrate when heated. Why isn't the temperature increase taken up by another mechanism?


If we treat this question as a mechanistic (rather than a philosophical) question . . . I would hazard a guess (and maybe the physicists in the team can check me on this) that since heat and translational motion are both "non-quantized" (as opposed to, say, electron excitation) then we can expect that all levels of heat can be "matched-up" to translational motion.

This is not true for rotational and vibrational motion which have energy barriers or quanta of energy that are specific to particular motion types - particular IR (heat) can cause rotational, bending, wagging, stretching, etc. because that required energy is available.

So, maybe that is the place to start for this discussion.

Greg (Roberto Gregorius)


There are two interpretations of the question "Why is [insert phenomenon] like that?": it can mean
(1) What is the mechanism that allows for such a phenomenon to follow from the observed cause? or
(2) Why is nature designed like that?

The first question can be answered through a scientific process. The second question is more philosophical and cannot be answered in science.

I think your student's question is more in the nature of #2. It is a good idea to have your students learn the distinction so that they can be more aware of the nature of science and philosophy.

Greg (Roberto Gregorius)

Hi Les,

Sounds like there is a basic misunderstanding of temperature here.

Temperature can be thought of as both a "microscopic" and "macroscopic" concept. (I do not mean literally 'visible in a microscope'; I mean 'imagine yourself the same size/scale as atoms')

Microscopically, temperature is the aggregation of many kinds of vibrations between atoms. These can manifest themselves in vibrating bonds or in movement of molecules. Thinking of temperature as purely kinetic energy is OK for an ideal monoatomic gas (e.g. well-spaced hard spheres), but it is not the best general description.

Macroscopically, temperature guides heat flow. Heat flows from hotter objects to colder ones (e.g. vibrations are transferred from the "more vibrating" to the "less vibrating". Humans can perceive these vibrations as "warmth", and we can generally tell the difference between warmer and colder objects (although we can be fooled).

Your question seems to imply that there is something more fundamental to temperature than vibrations -- you say that we heat something and "then" the vibrations occur. But temperature is not anything more than these vibrations. By "heating" something, all you are doing is increasing the various types of vibrations. The vibrations do not "correlate" with temperature -- they are the "temperature". The way we sense these vibrations is as 'heat', but that is only how we sense it.

In summary, "heat" is not "something" that causes vibrations -- it is the vibrations.

Hope this helps,

Burr Zimmerman

Hhmm! And I am not being condescending. If you understand a non-circular definition of temperature in terms of average kinetic energy, then, please provide a non-circular definition of energy. No number of examples suffice. The question is not "HOW ENERGY BEHAVES" that is "EASY" but what is "ENERGY" This is just a suggestion, but are "TEMPERATURE" and/or "ENERGY" just what we decide them to be??

Vince Calder

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