Cold

[TimbaLanD]

I found my car this morning blanketed by frost. After cleaning it, I started driving and thought if heat is a source of energy, what is “COLD”? is it negative enerty?

[YT2095]

no, it`s the default state, the Absence of heat energy.

[TimbaLanD]

does that mean, cold has energy?

[YT2095]

no, something without energy is cold.

[TimbaLanD]

All matter has energy, right? So, what if that matter is ultra cold? it does not mean it has no energy, does it?

[YT2095]

it means it has less heat energy than something warmer.

 

edit: after reading this thread a few time throught the day, I realise my answers may not be ideal for you as a way of thinking, consider it This way; Other than 0 Kelvin, some -273c (Absoloute Zero, and Unacheivable), there is No such thing as "Cold", only varying degrees of heat.

Colder or hotter simply points to a direction in relation to a comparison as whether it has more or less Heat than this comparator.

 

hope that makes a little more sense to you?

[5614]

The hotter something is the more thermal (heat) energy it has.

 

So something which is cold, like the ice on your car has little energy. I didn't say none, it has some, just not a lot.

 

If you cooled something to 0K (0 Kelvin or -273C) then it would have no thermal energy. However it is physically impossible to cool something to 0K (known as absolute zero), we can get very very close, but never reach it.

 

So one way to look at it is that everything has some thermal energy... even if it is a very small amount.

[Klaynos]

Lets get some terms at least slightly more correct:

 

Hot/Cold = relative terms they have no real physical meaning other than relative genrally relative to our body temperatures

Heat = an energy transpher from one system to another

Temperature = a measure of the average KE of molecules/atoms in a system.

 

So something that is colder than something else has a lower temperature and therefore a lower KE.

 

At zero Kelvin, what is belived to be the lowest possible achiveable temperature the average KE of a system should also be zero (although in reality this cannot be achived due to the uncertainty principle).

 

Nothing can be colder than zero Kelvin, but things can be hotter than it, so hot/cold is relative...

[T-Nemesis]

I always thought that a rise in temperature is caused by vibration of [molecules], and the faster they vibrate the hotter the body becomes. That would also explain why there is a limit to how cold something can be, as the [molecule] can not vibrate any less than if it were stationary. Likewise the limit of how hot something can become should be at the limit of how fast the molecule could vibrate.

[ecoli]

I always thought that a rise in temperature is caused by vibration of [molecules], and the faster they vibrate the hotter the body becomes. That would also explain why there is a limit to how cold something can be, as the [molecule'] can not vibrate any less than if it were stationary. Likewise the limit of how hot something can become should be at the limit of how fast the molecule could vibrate.

 

This is correct. The amount of molecule vibrates determine how much KE energy it has (and thus the tempurate) If a liquid start vibrating more, it breaks intermolecular bonds and becomes a gas.

 

If a molecule stops vibrating then it would violate the Uncertainty principle, which is impossible, AFAIK.

[Lifshiztz]

I don't know much about the uncertainty principle, but doesn't that just say that you can never measure the exact location of a sub-atomic particle, not that a particle can never be in one exact location? If something had absolutely no thermal energy, I'm pretty sure it would stop moving altogether.

[ecoli]

I don't know much about the uncertainty principle, but doesn't that just say that you can never measure the exact location of a sub-atomic particle, not that a particle can never be[/i'] in one exact location? If something had absolutely no thermal energy, I'm pretty sure it would stop moving altogether.

 

So can never measure the exact location AND the exact speed, because the act of measuring one changes the other. If thermal energy goes to zero, then the sub-atomic particle stops moving, enabling you to measure the speed AND location. Since this cannot happen, we can assume that reaching absolute zero is impossible.

[Lifshiztz]

So can never measure the exact location AND the exact speed, because the act of measuring one changes the other. If thermal energy goes to zero, then the sub-atomic particle stops moving, enabling you to measure the speed AND location. Since this cannot happen, we can assume that reaching absolute zero is impossible.

 

I'm not sure if that's actually the case.

 

Let's say you have a particle with zero thermal energy (it is absolutely still, and is not moving). It has an exact speed and location. However, if you were to measure this position and speed, the electrons you use to measure the particle (by reflecting them off of the particle) would displace the position and change the speed of the previously still particle. I believe that is the definition of the uncertainty principle. We can infer that the particle's position is in a particular location and that its speed is 0, but we cannot measure that because if we did, we'd displace the particle we're trying to measure.

[swansont]

Note that a particle that is very slowly moving has a large deBroglie wavelength, which means you can't know its exact location. Not being able to reach absolute zero is a consequence of the third law of thermodynamics. I don't think the HUP tells you anything extra here.

[YT2095]

I don't think the HUP tells you anything extra here.

 

LOL, I don`t think anything "Extra" has been added since my last post in here, it`s all been a rehash of the same.

[JonM]

Would theoretically 0 kelvin be the complete absence of energy?

[ecoli]

Would theoretically 0 kelvin be the complete absence of energy?

 

yes, and it would be theoretical only because we can't acheive this low a temperature, and possibly cannot achieve it.

[starbug1]

Is it a matter of the uncertainty principle that we can never reach absolute zero, or is it a matter of we just haven't figured out how?

[guardian]

AFAIK, it has little to none to do with the uncertainty principle. As mentioned above, 0K would be the absence of energy - so what would we be achieving this 0K for? If you are trying to achieve it for a particle - you've lost the battle since a particle is a form of energy. On another note, it may theoretically be possible but that would manifest itself in whatever you're cooling 'disappearing' as its energy would be absorbed by whatever you're using to cool it but since a particle (non-massless) still has a fair bit of energy even at billionths of K this to me would seem quite impossible.

[JonM]

Well... you can't have no energy right?

 

I mean that would cause some serious problems in lots of theories?

 

It would just be like... nothing

[BigMoosie]

Is there a theoretical upper limit to temperatures? Perhaps the point where if it vibrated any faster it would be exceeding the speed of light on a very small scale?

[JonM]

Is there a theoretical upper limit to temperatures? Perhaps the point where if it vibrated any faster it would be exceeding the speed of light on a very small scale?

 

 

well when you get to really high tempertures (trillions like at the big bang) I think a lot of the laws of physics fall apart...

[swansont]

Is there a theoretical upper limit to temperatures? Perhaps the point where if it vibrated any faster it would be exceeding the speed of light on a very small scale?

 

 

No. E ~ 1/sqrt(1-v²/c²)

 

When you get speeds arbitrarily close to c, energy goes to infinity.

[timo]

If you are trying to achieve it for a particle [T=0 K'] - you've lost the battle since a particle is a form of energy. On another note, it may theoretically be possible but that would manifest itself in whatever you're cooling 'disappearing' as its energy would be absorbed by whatever you're using to cool it but since a particle (non-massless) still has a fair bit of energy even at billionths of K this to me would seem quite impossible.

Temperature is not energy; it´s a parameter describing the energy distribution of a system. 0 K doesn´t mean "no energy" (unless you adjust your E=0 level to this point). The inobtainability of T=0 K is a statistical argument not a "the world will end if we reach T=0"-one or a "that´s forbidden by uncertainty"-two.

[reyam200]

well when you get to really high tempertures (trillions like at the big bang) I think a lot of the laws of physics fall apart...

 

than the laws of physics are not finite. they can be broken.

 

When you get speeds arbitrarily close to c' date=' energy goes to infinity.[/quote']

 

what would it look like? would it BECOME light?