The actual relationship between temperature and kinetic energy is not as straightforward as is implied by this description. There is no one to one correlation between temperature and average kinetic energy for all materials. While the statements may be true for an ideal monatomic gas, they are not accurate for other materials.
WRT temperatures lower than absolute zero, I must admit I'm a little dubious. I'm no physicist, but from the description I read from the link it doesn't seem like ordinary matter can have "negative temperature" as a whole. Could somebody who actually understands the physics please comment? --
Robert Merkel
I added the stuff about negative temperatures. I don't really understand much about them, I just remembered hearing once they existed, and when I saw the article saying they didn't, I went looking for some information. Here are some more refs if they are any help:
[1],
[2]. Judging by what is said at
[3] it appears that negative temperatures can be attained in quantum systems such as lasers operating at very low temperatures. -- SJK
"Temperature is an intensive property of a system." I am likely to have a rough understanding of what "temperature" is long before I understand what one might possibly mean by "intensive property of a system." This is an example of a sort of definition that decidedly
should not begin an encyclopedia article--it should be rather far down in the article, once the technical apparatus is on the table, clearly explained, so that the people who
need to read this article can actually understand the technical definition. I think the purpose of an encyclopedia is not just to sum up knowledge in technical terms, but involves the more difficult task of making that knowledge as clear as possible to people who don't yet have it. Of course, some scientific concepts simply can't be explained in every encyclopedia article in which they're used, and one
shouldn't try to give backround for those concepts--one should just offer pointers to the more basic articles--in many cases, perhaps just links. But in this particular case (
temperature), here we have an opportunity to make a relatively simple physical concept clear, both on an intuitive, very basic high school physics level and also on more advanced levels. I'm not trying to tell ya'll to do work you don't want to do--I'm just trying to spell out my vision of what we should expect out of scientific articles about ordinary concepts. --
LMS
- I've tried to address your concern, as well as significantly expand the theoretical basis for temperature, address the concept of negative temperatures, and mention temperature measurement. Let me know what you think. Its probably still a bit on the technical side. I think that's OK deep in the body, but we probably need to include more elementary information in the introduction. I welcome any suggestions about what could be added to this page. --Matt Stoker
There's also a first law basis for temperature. For a monatomic ideal gas:
U = (3/2)kT
For more complicated systems (diatomics gases, even solids):
U = .5kT(times the number of quadratic degrees of freedom)
i.e. (5/2)kT for diatoms (not 6/2 because the smallest axis of rotation requires too much energy to access the first h/2π angular momentum)
where the quadratic degrees of freedom are basically the number of ways that particles in the system can store energy (i.e. translational, rotational, vibrational, etc.). What this relies on is the idea of equipartition. Essentially, in order for the system to be in truely random motion, it has to store energy in every accessible resevior equally.
Perhaps it's worth mentioning that absolute zero was originally extrapolated from examining the x-intercept of the pressure vs. temperature line of a gas at constant volume.
It should also be mentioned in the bit on absolue zero that it can not be achieved at all. Heisenberg's uncertainty principle dicates:
ΔEΔt > h/2π
Δr·Δp > h/2π
In order to drop the energy of microscopic particles to zero, implying that its momentum is zero, too, would require that it be that way for an infinite amount of time and that it takes up an infinite amount of space. The logical contradiction is that a particle at absolute zero would also have a definite position at a definition time, something that the uncertainty principle allows no particles to have. What this means is that the particles will continue to wiggle, no matter what, just enough to satisfy the above.--BlackGriffen
- From thermodynamics it can be shown that absolute zero cannot be achieved. However, the Heisenberg's uncertainty principle doesn't exclude this possibility. My understanding is that absolute zero is defined as that point where everything is in the ground state (the lowest energy level). From the Particle in a box problem, we see that for particles enclosed in a "box" the ground state possesses some amount of energy above zero (the zero point energy). Thus even at absolute zero, the system contains some energy, so an infinite amount of time and space are not required to achieve absolute zero. You are correct that for the system to have an energy of zero, it must occupy an infinite space. However, the system temperature can be reduced to absolute zero without reducing the energy to zero.
- Note that the zero point energy arises because of the uncertainty principle. Since the "box" constrains the particle within a certain range of positions, the momentum and thus the energy cannot be zero. Also, note that in the ground state the wavefunction occupies the entire box, so in effect the particle size expands to fill the box.--Matt Stoker
I was just playing with the controls on my monitor, and it has a control to adjust "
color temperature", taking on values from 5000K up to 9300K. Changing the temperature changes the colour of what is being displayed on the screen. What is color temperature? --
SJK
- Read this - [4]. It probably merits inclusion in the article, but I'll leave that to someone who properly understands it. - MMGB
That's the "color temperature" you're looking at there--5000K to 9300K--the frequency of the light being emitted affects the color that the objects actually appear. Color temperature is something that people don't often notice except through electrical equipment (one exception--at dusk, when white objects typically appear light purple). You can see experimentation with color temperature a lot in Kubrick films, for instance in _Eyes Wide Shut_ the light coming in from a window was almost always conspicuously blue, whereas the light from lamps on end tables was more orange. Arc-sodium lights give off an orange hue. Video cameras can typically adjust for color temperature by zooming into a white object and setting the white balance (telling the camera "this object is white"); the camera then adjusts the colors to show true white as white. White-balancing is necessary especially indoors under fluorescent lighting. Cinematographers can also white-balance to objects which aren't white, resulting in the color of that object being downplayed in the image. For instance, you can bring more warmth into a picture by white-balancing off something light blue, like faded blue jeans; in this way white-balancing can serve in place of a filter when a filter isn't available.
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