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Re: SI and nothing else.



I've watched several parts of the thread on "doing away with calories"
over the past two weeks. The most recent, regarding the equivalence (or
not) of the Kelvin and Celsius degrees, caused me to verify that which I
hoped I had properly remembered. Actually, I'm on sabbatical this term
and it's Saturday --- I just needed a break here in the middle of the day
from my research.

The SI unit of temperature seems to still be defined the same way as it
was in my youth. The SI unit is the Kelvin and it is defined to be
equivalent to the Celsius degree. The Celsius (or Centigrade) degree is
defined to be 1/100th of the temperature difference between the freezing
point (defined to be zero) and the boiling point (defined to be 100) of
pure water at 1 atm. The thing that varies (but not much over the last 20
years) is the "position" of the zero-point of the Kelvin system (which
currently seems to be -273.1475C) relative to the Celsius system. I have
looked in over twenty current documents and I find no exceptions other
than updates on the zero-point of the Celsius system relative to zero in
the Kelvin system (or as it is most commonly quoted, as the zero-point of
the Kelvin system relative to zero in the Celsius system).

The relationship of the thermodynamic or "small" calorie (cal) and the
nutritional or "large" calorie (Kcal) to the Joule has also not changed.
There has been a "continuing confusion" between the thermodynamic and
nutritional calories principally (if not solely) due to them both being
regularly referred to as "calories" in the popular press. The small or
thermodynamic calorie still seems to be defined as the heat required to
raise the temperature of 1 gram of pure water (at 1 atm.) 1 degree Celsius
when averaged over the range from the freezing point to the boiling point.
(I'm told by our friends in Chemistry that the specific heat at constant
pressure varies less than 0.3% over the above range.) The nutritional
calorie (Kcal) is usually defined as "that amount of food having the
energy-producing value of a 'calorie'". Oops! This "calorie" is really
the Kcal or 1000 thermodynamic calories. And may I say, we do no one a
service when 4 out of the 5 current generation of calculus based
introductory physics books in which I looked use "cal" for the small
calorie and "Cal" for the large calorie. How many of our students really
notice that one unit is all lower case and the other starts with a
capital? For that matter, do all of our faculty notice the difference?

From over 30 years ago, I still remember the embarrassment (almost pain)
of listening while a favorite professor laughed as he read the first draft
of an article I had written in which I had the "two calories" confused.
(Actually, confused is not strictly correct --- I was simply unaware that
there was a difference.) My proposed article for the state Academy of
Science magazine discussed the ease with which a person should be able to
lose weight by simply wearing cooler clothes in the winter. Ha, ha!

If you're off by three orders of magnitude, it's not at all difficult to
show that one can easily trim off excess pounds by wearing cooler clothes
"... given the specific heat of the water that comprises most of our
bodies, the average metabolic rate of a typical adult, the the infra-red
emissivity of typical skin, the temperature difference between the human
body and the average cold day," etc., etc. Well, you get the picture.
And, it turned out that I was not alone. None of the under-graduate
students and only one graduate student were aware of the difference.
Also, three of the faculty had forgotten that the "two calories" were not
equal. The most common views expressed at the time were
"... while they're not exactly equal, but they're sensibly the same"
"It's sort of like using 10 instead of 9.8 for g. It's close enough
for simple calculations"
and more which I'll not repeat.

If you think this confusion is not really as prevalent where you are as it
was at the small college/university where I went to school (Emory
University in Atlanta), or if you think that time and the modern
information age has reduced the problem to insignificance, then ask some
of your students and or fellow faculty about the difference between the
"two calories." One bit of caution and a popular phrase from at least
three generations of my family regarding questions in general:
DON'T ASK UNLESS YOU'RE WILLING TO HEAR THE ANSWER!


ERTEL SENDS. _____________________
/ Prof. John P. Ertel \
/ jpe@nadn.navy.mil \
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