"Glen Herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message news:<DqK7b.408564$o%2.186361@sccrnsc02>...
> "Luiz Carlos" <oen_br@yahoo.com.br> wrote in message
> news:8471ba54.0309100917.3f0cfd23@posting.google.com...
> > Hi Jerry,
> >
> > The concepts of active and reactive power are important, very
> > important.
> > But the unit of power is Watt, it doesn't matter if it is real,
> > imaginary or complex. If you want to say the power is imaginary, say
> > Pimag = x Watts, not P= x VAr.
> > Or maybe, when we go to market, we should buy 5 kgrice (kgrice is the
> > mass of a kg of rice), 2 kgbean, 3 kgsugar etc. You know what I mean.
>
> But they are measuring two very different things. If you want to know how
> much power a device takes, how much it costs to run and how much heat it
> will make, then you want power as measured in watts. But for many AC
> circuits, power is not volts times amps. You also need to know how much
> current it will use, how big to make the wires, how big the circuit breakers
> and switches should be, and especially the transformers. With
> transformers, you can have different voltages and currents, but the volt
> amps are, more or less, conserved.
>
> Well, reactive power I might agree is a little strange. If you have real
> power and volt amps that should be enough.
>
> As for the rice/beans/sugar question, all can be measured on the same scale
> as units of mass.
Consider the units used for them, and for other grains, on the
commodities markets in the United States. Pounds are the mass units
used for sugar and some kind of beans. Other beans such as soybeans
and rice and other grains such as wheat and barley are measured in
mass units called "bushels." The commodity being bought and sold
needs to be identified for these units to be well defined, of course,
because while a bushel of soybeans is 60 lb (27.22 kg), a bushel of
flax is 56 lb (25.40 kg), a bushel of barley is 48 lb (21.77 kg), and
a bushel of oats is 32 lb (14.51 kg) in the United States or 34 lb
(15.42 kg) in Canada.
>But consider that in the US most recipes measure such in
> cups, a unit of volume. The mass of rice, beans, and sugar in one cup is
> different. Consider the problems of someone converting a recipe between
> cups and kg. When they are not in cups, they use weight units (pounds),
> not mass units (kg).
Wrong. http://w0rli.home.att.net/youare.swf
Actually, 1 lb = 0.45359237 kg, exactly, by definition.
U.K.: Weights and Measures Act of 1963
Canada: Weights and Measures Act of 1953
Australia: Weights and Measures Regulations
U.S.A.: Federal Register Notice of 1 July 1959. Read the current
U.S. law, a discussion of the definition as a different exact fraction
of a kilogram for the 66 years before then, and the 1959 international
agreement on the value given above (which was already in use in Canada
before then) at one of these sites (same file both places)
http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf
http://gssp.wva.net/html.common/refine.pdf
Those pounds are "weight" units, of course. But that does not imply
"not mass units." The word "weight" is an ambigous word, with several
different meanings. Consider, for example, the troy "units of
weight." Unlike their avoirdupois cousins, and unlike grams and
kilograms, they have never spawned a unit of force of the same name.
They are always units of mass.
>(This is a distinction made by physicists and not
> always by others. There is also a pounds mass unit around.)
Those pounds defined as 453.59237 grams are, of course, the pounds
used in commerce. Pounds force are not legal units for this purpose.
That's what pounds were originally, and still are today in the
contexts you are discussing--units of mass. Pounds force are a recent
bastardization, something that was never well defined before the 20th
century, and which even today don't have an official definition.
American Society for Testing and Materials, Standard for Metric
Practice, E 380-79, ASTM 1979.
3.4.1.4 The use of the same name for units of force
and mass causes confusion. When the non-SI units
are used, a distinction should be made between force
and mass, for example, lbf to denote force in
gravimetric engineering units and lb for mass.
> Consider a
> recipe that might be used on the moon. What units would you use? Or a
> recipe to be used by someone in a space station?
Pounds. Grams. Makes no difference as far as their usefulness,
either of these units used on earth would work as well there. On the
moon, you could even get away with pounds force if all your
measurements were in units of force (no eggs by count or water by
volume or anything like that), but you might want to adjust everything
downward proportionately from what you'd use on Earth.
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/t_jeff.htm
But if it be thought that, either now, or at any future time, the
citizens of the United States may be induced to undertake a thorough
reformation of their whole system of measures, weights and coins,
reducing every branch to the same decimal ratio already established
in their coins, and thus bringing the calculation of the principal
affairs of life within the arithmetic of every man who can multiply
and divide plain numbers, greater changes will be necessary.
U.S. Secretary of State Thomas Jefferson, 1790