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chem_nerd09
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Just for fun, I was thinking of trying to design one. How could we design a thermometer that could accurately measure negative temperature of a spin system?
chem_nerd09 said:Just for fun, I was thinking of trying to design one. How could we design a thermometer that could accurately measure negative temperature of a spin system?
stewartcs said:What's a spin system?
Also, there is no such thing as negative absolute temperature.
CS
xepma said:Yes there is.
(One over) Temperature is defined as the change of entropy with respect to the change of energy, i.e. roughly
[itex]\frac{1}{T} = \frac{\partial S}{\partial E}[/itex]
This means that the temperature is negative whenever the entropy decreases with increasing energy. Usually, the larger the energy of the system is, the larger its entropy will be. For free gases, for instance, the temperature is linear with respect to the mean kinetic energy of the atoms.
xepma said:I never claimed that a system can reach 0 Kelvin.
xepma said:On the contrary, in the spin system a negative temperature (or any other for that matter) actually corresponds to a hotter system than a system with a positive temperature. The meaning behind this is that when the systems are in thermal contact there will be a net energy flow from the hotter system to the colder one - just like you would expect. (Please note that most systems are not capable of obtaining a negative temperature)
stewartcs said:Sure you did, in post #4 when you said "[y]es there is". My quote was with reference to absolute temperature scales. Kelvin is an absolute scale, so by saying "yes there is" you are implying that negative Kelvin exists. Now, whether or not you meant that, I do not know. But that's what you wrote. Hence my reply.
True, but that's why I mentioned that it can only be accomplished by performing some amount of work. The spin system is then no longer isolated, so it entropy can decrease. The complete system (including whatever device you use to perform the work) has ofcourse an increasing entropy.The entropy of an isolated system during a process always increases, or remains constant if the process is reversible. Since no system is reversible, it always increases, not decreases.
Yea, that's indeed the ref I used. But you can also check wikipedia. It's pretty worthwile to study these concepts, cause it shows the subtleties associated with thermodynamic properties. Also, the exercise of the spin system is pretty funny to work on (I suggest the book by Schroeder: Thermal Physics - his treatment is pretty clear).Spin systems are definitely not covered in introductory thermodynamic textbooks. Nor is the concept of negative absolute temperatures. However, this is beyond the normal macro view of thermodynamics and more in line with condensed matter physics. So, I admit there may be some theoretical description that allows for a "negative temperature"...but that's beyond my area of expertise and definitely beyond the macro view of thermodynamics!
I did find this article that seems to support what you are saying to some degree (however I didn't check the works cited), and appears to be your source for the given references.
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/neg_temperature.html
CS
A thermometer that can measure negative temperature is a special type of thermometer that uses a different scale than the traditional Celsius or Fahrenheit scale. It measures temperatures below absolute zero, which is the lowest possible temperature in the traditional scales.
A thermometer that can measure negative temperature uses a different scale called the Kelvin scale. It is based on the absolute zero point, which is -273.15 degrees Celsius or 0 Kelvin. This scale allows for temperatures below absolute zero to be measured.
Measuring negative temperature can help scientists study and understand extreme conditions such as in outer space or in certain chemical reactions. It can also provide more accurate measurements in certain fields, such as cryogenics or quantum mechanics.
One application is in studying superfluids and superconductors, which exhibit properties at extremely low temperatures. It can also be used in the development of new materials and technologies that require extreme temperatures. Additionally, it can help in understanding the behavior of particles at the quantum level.
Yes, there are limitations to this type of thermometer. It can only measure temperatures below absolute zero, and it cannot accurately measure temperatures above absolute zero. Additionally, it requires precise calibration and specialized equipment, making it more difficult to use compared to traditional thermometers.