# The Kelvin scale->

1. Dec 21, 2003

### deda

Why the Kelvin termo scale has no negative values?
I'm just asking if some one knows and is it ok and why.

2. Dec 21, 2003

### Integral

Staff Emeritus
0 Kelvin is "absolute zero" the point at which all atomic motion ceases. This means all electrons are in the lowest possible energy state and there is no measurable kinetic energy. Since Temperature is a measure of atomic or molecular kinetic energy it is not possible to go negative on this scale.

3. Dec 21, 2003

### Jimmy

http://www.sciencedaily.com/encyclopedia/Temperature

At the bottom of the page is logarithmic scale of temperatures ranging from 1 picoKelvin (that's in the range of the coldest known temperature ever produced) to 10^30 Kelvin (Planck temperature). Each temperature range is hyper-linked to information about what exists at that particular temperature or what processes happen.

4. Dec 21, 2003

### nautica

B/c if there was we would have to reverse the molecular motion.

Nautica

5. Dec 21, 2003

### Andthensome

reversing the molecular motion could only be possible by moving it backwards in time.

6. Dec 21, 2003

### krab

Talking about reversing the motion misses the point. If you take a given system at temperature T, and reverse all the motions, it would have exactly the same temperature T; not -T.

7. Dec 21, 2003

### Andthensome

And you missed my point krab, what I meant was that to get -T you would have to move it backwards in time, not reverse the normal spin of the atoms or whatever the motion was in our normal forward notion of time.

Since all motion forwards in time produces energy which gives heat, reversing time would mean it would be taking away energy if we could view it in our normal time somehow; going into the -T from our perspective, but from someone who lives in a world where their time is normal to them, but backwards to us, it would have T temperature.

8. Dec 22, 2003

### Thallium

I'm confused.

What are the particles like at a lower minus-temperature on the Celsius scale then, if they are half stationary at the zero point on Kelvin?

This is going overboard!

9. Dec 22, 2003

### HallsofIvy

Staff Emeritus
Who said "half stationary"? The concept of 0 Kelvin ("absolute zero") is that ALL particles are completely stopped. Since heat is the random motion of particles, there would be no heat and no temperature. (Actually, due to quantum effects, you can't even get to 0K much less "negative" K.)

Particles at "lower minus-temperature"- i.e. Kelvin temperature close to 0 would "almost motionless". Of course, 0 Celcius, unlike 0 Kelvin, is essentially arbitrary.

10. Dec 22, 2003

### deda

The temperature is kinetic energy dependent.
Kinetic energy depends on mass and square of velocity.
$$E_k=\frac{mV^2}{2}$$
But what if we invert the mass into negative and could it be the case?

11. Dec 22, 2003

### nautica

My statement of reversing molecular motion was only sarcasm to point out the fact that zero Kelvin constitutes no motion (at least in theory). And yes if motoin was moving at the same speed in the opposite direction it would have the same KE.

Nautica

12. Dec 22, 2003

### Achy47

To have a temperature at a value of 0ºK would mean that thermal energy of the system of particles would be zero and therefore the momentum of its constituent parts would also be zero (with positions also known since there is no movement of particles). If the position and momentum of a particle is known to be zero this would be a violation of Heisenberg's Uncertainty Principle.

I hope this helps.
Maybe someone more senior can elaborate (correct) on this.

13. Dec 22, 2003

### Integral

Staff Emeritus
Yes due to HUP, it is impossible ever to actually reach Absolute zero. That was not the question the question was what is the definition of absolute zero.

On a side note, I am not particularly happy with nonsense and "sarcastic" posts in this froum. If you do not know what you are talking about, pleas read and do not post. If you wish to post chic chat go to AOL.

14. Dec 22, 2003

### Achy47

I was under the impression that deda was asking why the Kelvin scale was limited to rational numbers greater than or equal to zero, with the negative values being physically undefined and 0º K being defined but not physically attainable.

Sorry for the misunderstanding. I thought that HUP might have provided some insight for deda in the sense that it defines a numerical floor for physically attainable temperatures.

Edit - Spelling

Last edited: Dec 23, 2003
15. Dec 22, 2003

### krab

If one is uncomfortable with a temperature with a lower limit, then one could take the logarithm of the absolute temp. and call that the degrees M (for My temp.). Of course one would then be out of step with the mainstream. So a relevant question is How historically did the mainstream come to be? I guess it's because of the fact that expansion is linear with our definition of T, not linear with log(T), and early thermometers were usually based on the expansion of something.

16. Dec 22, 2003

### pallidin

The Kelvin scale is, in essence, a measure of the thermal energy within a defined system. 0 degree's Kelvin is the total and complete absense of thermal activity with that defined system.
With that, one can reason that it is impossible to go below 0 degree's Kelvin, as otherwise it would suggest that there is more thermal energy left, which contradicts the the aspect of 0 degree's Kelvin. Understand?

17. Dec 26, 2003

### Mk

You can't slow down anymore if you're not moving.

18. Dec 26, 2003

### NateTG

IIRC the Kelvin scale is related to the ideal gas law. At 0 Kelvin, and ideal gas has 0 volume. In order to get a negative temperature Kelvin, the gas would have to have negative value.

What happened is that researchers discovered that the temperature of a gas and it's volume at a particular pressure have a linear relationship. So the volume would be
V=kT+b
where T is the temperature, k is some constant, b is an offset, and V is the volume.

The Kelvin scale has zero set so that the volume of a gas at a particular pressue is proportional to the temperature that means that the 'b' in the formula above dissapears, so
V=kT

This is usefull for keeping the ideal gas law simple:
PV=nRT
would otherwise be a bit more complicated.

Since temperature is related to many things, there are alternative definitions of temperature which are equivalent for normal materials. However, for some definitions of temperature, there are exotic materials - Bose-Einstein Condensates - that have a negative temperature Kelvin.

19. Dec 27, 2003

### deda

Unlike me, I think you are all reviewing only positive mass.
What if the mass is negative?
Will it expand your K-Scale with negative values?