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'[OT] mF'
1999\11\03@123512 by Sean Breheny

face picon face
Hi Nick,

Well, I don't think that it is very convenient to always carry along all
of the base units in a calculation. So, according to this standard,we
should not say "a 5 Volt battery", but instead, "a 5 Joule per Coulomb
battery"?

It seems to me that having at least a standard set of derived units is
very convenient. However, it also makes sense that different sets of
units will be used in different contexts. For example, if you want to do
a lot of electromagnetics stuff, the CGS system is better than the
regular MKS SI units because in CGS, the special constants (such as
permittivity of free space, permeability of free space, etc.) drop out of
Maxwell's Equations.

In fact, if you do astrophysics,I'm told that it is common to transform
everything into a set of units in which the speed of light is 1, planck's
constant is 1, and the gravitational constant is 1,or some such
combination,and derive all other units from those. We even has a problem
about that in my intro physics (mechanics&special relativity) course!

Sean



On Wed, 3 Nov 1999, Nick Taylor wrote:

{Quote hidden}

1999\11\03@130444 by Wagner Lipnharski

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Sean Breheny wrote:
>
> Hi Nick,
>
> Well, I don't think that it is very convenient to always carry along all
> of the base units in a calculation. So, according to this standard,we
> should not say "a 5 Volt battery", but instead, "a 5 Joule per Coulomb
> battery"?

In some way I use to think that whenever you want to transmit a formula,
circuit, diagram, whatsoever, do it in a way that even an
extra-terrestrial could understand if you supply a legend, or a "primer"
that can be used at any drawing or formula.  The primer here is
basically (at least at this planet) defined to be the SI,
internationally accepted... well is it even in States?

If the E.T. could define what is "V" from the "primer" list, probably he
could be able also to understand what is O, H, N and other elements to
recreate your breathing atmosphere, while you would be gasping for air
inside their chip.

If an even an E.T. could do it (with all language communication
problems), I think we could do it too, right?

1999\11\03@140836 by Nick Taylor

picon face
Sean ... it's worse than that ... since neither the joule not the
coulomb are among the seven SI based units, this is how they do it:
  V = (m^2)(kg)(s^-3)(A^-1)

So we "should" say, "a 5(m^2)(kg)(s^-3)(A^-1) battery",

  ohm = (m^2)(kg)(s^-3)(A^-2)

or say, " a 470(m^2)(kg)(s^-3)(A^-2) resistor"!

I'm certainly glad that we have our wonderful government to tell us
how to speak (and think).
- Nick -

Sean Breheny wrote:
{Quote hidden}

1999\11\03@151436 by Barry King

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face
<x-rich><color><param>0000,0000,0000</param>> Sean ... it's worse than that ... since ne
ither the joule not the

> coulomb are among the seven SI based units, this is how they do it:

>    V = (m^2)(kg)(s^-3)(A^-1)

>

> So we "should" say, "a 5(m^2)(kg)(s^-3)(A^-1) battery",


</color>NO NO NO!

NOBODY is saying we shouldn't use the derived units of the SI.

That is NOT what NIST says!


The Volt is an accepted derived unit.  So is a Farad.


You are encouraged not to use the cgs system and some other
traditional sets of derived units in favor of the standard units, either
derived or base units.


For many types of problems, if you need to manipulate quantities
that are in the standard units, the fact that they are defined in terms
of the base units (no multipliers) actually helps.  For example, one
Pascal applied to a 1 m^2 area applies a total force of 1 Newton.


However, the reason that people won't give up all the odd unit
systems is because of what Sean pointed out:  That the units come
out even for certain types of problems.  But is easy for him to take
the answer he gets in cgs, and convert to standard mks, at least
they are metric.  Try that in nautical miles, lbs force, and seconds.
you have to be a rocket scientist to get the conversions right.
Sometimes even THEY don't.  <<sigh>


-Barry.

<nofill>

</x-rich>

1999\11\07@022117 by Robert A. LaBudde

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<x-flowed>At 09:50 AM 11/3/99 -0500, Sean wrote:
>This seems silly. How can you not allow gauss,erg,dyne,torr, and micron?
>These are all in common use. More importantly, I think a bar is a dyne per
>cm^2,so it would be really silly to allow a until but not the base units it
>came from!

A 'bar' is 100,000 Pa, so you don't need dynes or cm.

================================================================
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</x-flowed>

1999\11\07@025101 by Sean Breheny

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Yeah, I was thinking about that after I said it and a dyne/cm^2 seemed
awful small for a bar. I guess they are totally unrelated and I just had
the idea that they were.

Sean

At 02:19 AM 11/7/99 -0500, you wrote:
{Quote hidden}

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
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