heat/cold ratios [Was: times lower than] (UNCLASSIFIED)

[Mullins, Bill AMRDEC]

Classification:  UNCLASSIFIED
Caveats: NONE


<engineer hat on>

Anytime you are trying to convey real information (and by real, I mean measureable -- that is, with numbers that mean something), "twice as cold" is meaningless.  All temperature scales measure the amount of heat, not the amount of cold. Cold is simply the absence of heat.  From context, this may be a relative absence, or an absolute absence.   "Twice as cold" is scientifically meaningless.  "Half as warm" would probably mean the same thing to most laypeople, and at least is accurate.

The statements below about degrees Kelvin having the ratio property are true.  But it is only in the context of heat that this means anything.  40 Kelvins is twice as warm as 20 Kelvins.  (Rankine is to Kelvin as Fahrenheit is to Celsius/centigrade; 40 deg Rankine is twice as warm as 20 deg Rankine).   But, there is no temperature "X" about whice it makes sense to say that Y is twice as cold as X, because "cold" isn't generally accepted within the scientific/engineering communities as measureable in the same way that heat is (and there is no need to do so -- everything that is measureable and specifiable about the temperature or other thermodynamic properties of something can be done by discussions and measurements of how much heat it has).

</engineer hat off>

Having said all that, if someone says to me that today is twice as cold as yesterday, and all they really mean is that it is much colder now than it was then, fine.

But if yesterday's temperature is 32 deg Fahrenheit, and "today is twice as cold as yesterday was", there is no way from the statements and data given to figure out what today's temperature is.


>
> At 9/4/2007 11:24 PM, sagehen wrote:
> >I don't think the arbitrary application of a scale to a
> quality makes
> >it a quantifiable substance.  The degrees of the scale are
> quantities,
> >but cold is not.  You (probably) wouldn't say  20š Kelvin is
> twice as
> >cold as 40šK, would you?  For one thing, you'd have to have
> some upper
> >point at which "cold" begins, where "warm" ends.
> >AM
>
> That seems like claiming that in order to say about integers
> that one is "twice as large" or "twice as small" as another,
> one would have to have some point at which "small" ends and
> "large" begins.
>
> I really do believe the Kelvin scale has the ratio property;
> that is, numbers on the Kelvin scale can be multiplied and
> divided.  From Wikipedia, under "Kelvin", "The Kelvin scale
> is a thermodynamic (absolute) temperature scale where
> absolute zero - the coldest possible temperature - is zero
> kelvin (0 K)."  Under "Thermodynamic temperature",
> "Thermodynamic temperature is the absolute measure of
> temperature and is one of the principal parameters of thermodynamics.
> Thermodynamic temperature is an "absolute" scale because it
> is the measure of the fundamental property underlying
> temperature: its null or zero point, absolute zero, is the
> temperature at which the particle constituents of matter have
> minimal motion and can be no colder."
>
> Thus the Kelvin scale measures the motion of particles, their
> kinetic energy -- and one collection of particles can have
> twice the kinetic energy of another.  What I am not sure of,
> given the way the unit of the Kelvin scale is defined in
> relation to the Celsius scale (according to the article
> "Thermodynamic temperature"), is whether, for example, 40
> degrees K or some other temperature is twice as warm as 20 degrees K.
>
> By the way, if I said earlier that on the Kelvin scale one
> could say that -4 is twice as cold as -2, that was a blunder.
>  The Kelvin scale starts at zero and has no negative temperatures.
>
> Joel
>
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>
Classification:  UNCLASSIFIED
Caveats: NONE

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