Re: can someone explain the candela a little?

in article 86pwtriga7i.fsf@xxxxxxxxxxxxxxxxxx, Esa A E Peuha at
esa.peuha@xxxxxxxxxxx wrote on 04/05/2005 05:44:

> robert bristow-johnson <rbj@xxxxxxxxxxxxxxxxxxxx> writes:
>
>> as i read the SI definition of the unit of luminous intensity, the
>> "candela", it seems to me to be equivalent to power:
>
> Not exactly. You are missing the fact that luminous intensity is
> defined in terms of the sensitivity of the human eye.

since i first posted this, i finally came across a web page that finally did
explain it (the NIST site did not). it's at:

http://www.electro-optical.com/whitepapers/candela.htm

the fact that the candela was given such prominence as a sorta "fundamental
unit" in SI and retains it as such, is silly.

i'm in audio and acoustics and it's like the SI system standardizing
perceived sound pressure level into a physical unit by including the
Fletcher-Munson curves into a *physical* definition of that unit of
intensity. those curves are important perceptual data, but they are not
physics, per se. they are pschoacoustics. i can squirt some sound at an
acoustic intensity of 10 watts/m^2 toward a tree and the tree could give a
rat's ass about the Fletcher-Munson curves.

those luminousity curves do not belong in a definition of a physical unit.

in article 1guio97.cw0qmq1khbrp6N@xxxxxxxxxxxxxxxxx, J. J. Lodder at
nospam@xxxxxxxxxxxxxxxx wrote on 04/05/2005 05:45:

> robert bristow-johnson <rbj@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> this is more of a question regarding what might be considered fundamental
>> dimensions of physical quantity.
>
> There is no such thing as the 'fundamental dimension'
> of a physical quantity, unless you just invented it
> without telling the world about it.

no, *I* am not the only one. we, in our normal experience of physical
reality, define some "basic" units (pretty much anthropocentrically or
arbitrarily) of ubiquitous physical quantities that appear to us, in our
normal experience of reality, to not have any direct derivation from the
others. commonly time, length, and mass. most of the others are derived
units from these basis units.

for instance the dimension of force is mass x length / time^2 because we
don't look at the concept of force as anything other than the
time-derivative of momentum. We don't say force is proportional to the time
derivative of momentum (and toss in some constant of proportionality), we
say force is the derivative of momentum w.r.t. time. i s'pose we could have
declared force to be a completely different and new physical quantity
undefinable by any other physical quantity, independently defined a unit of
force (call it a "farg"), observed in experiment that it's proportional to
the time derivative of momentum, then said something like F = alpha dp/dt
and alpha would have dimensions of fargs x time^2 / (mass x length). but
there is no need to do that. it isn't like that the concept of force
existed independently in nature without time or length or mass, so we could
and did define force solely and naturally in terms of time, length, and
mass.

here's another for instance (going back to Boltzmann), at one time people
didn't understand heat to be "merely" the random motion of molecules in
solids, liquid, and gas. heat was a totally different concept of physical
quantity existing independently in nature outside of quantities of time or
length or mass (or momentum, force, mechanical energy or power derived from
the big three: time, length, mass). so we define this temperature unit
based on how much the mercury moves in a thermometer. we thought it was
this separate kind of stuff.

but now we know different. heat is not some new and separate physical
quantity but is about the kinetic energy of the particles of matter. we
could define temperature in terms of energy (in Joules) per particle per
degree of freedom, instead of Kelvin (and all we would do is multiply T in K
by half of the Boltzmann constant, and that's what we would have). so if
you let the dimension of temperature of objects be the same as energy, then
the Boltzmann constant is dimensionless. but if you define temperature as
this "other stuff", not simply energy per particle, and you in dependently
define a unit of temperature, then the dimension of the Boltzmann constant
is energy/(degree of temperature). in MKS it's Joule/Kelvin.

in the CGS electrostatic units they define the Coulomb Force Constant to be
the dimensionless 1 ( epsilon_0 = 1/(4 pi) in cgs). that results in the
dimension of electric charge being length / time x sqrt(length x mass) or
velocity x (length x mass)^(1/2). now if you like to think that is what
this stuff is that lives on (or is a property of) electrons and protons,
that it's velocity x sqrt(length x mass), then i guess charge is not a
"fundamental dimension" of physical quantity and the dimension of tthe
permittivity, epsilon_0, is dimensionless.

but some of us continue to have trouble accepting that. if you understand
electromagnetic charge to be this totally different concept of physical
quantity than length, time, and mass, then the Coulomb Force Constant has to
have dimension force x length^2 / charge^2 or (mass x length^3 / (time^2 x
charge^2)).

>> assuming we can stay away from the issues
>> of whether or not the speed of light is dimensionless (since it is defined)
>> and the equivalent question of whether energy and mass are the same
>> dimension of quantity (let's say they're not, for this case).
>
> Once again: physical quantities do not -have- dimensions.
> We can -assign- dimensions to them, in any way we please,
> provided we are consistent about it.
> All debate on what a dimension 'really' is is pointless.

perhaps you're correct because we'll end up just repeating stuff we went
over months or years ago, and i certain do not impune your expertise and
authority in physics but there *does* remain some need for us mortal
engineers and other lesser forms of life to comprehend some of this.

i'm still down to four: time, length, mass, and electromagnetic charge.
every other measure of physical quantity comes from those four. perhaps
there is some other "stuff" at a subatomic level not included. perhaps time
and length are exactly the same thing (despite our common experience) or
stated differently, perhaps mass and energy are the same thing in every
situation.

i guarantee you that even if theoretical physicists have transcended these
distinctions of physical quantity, us mortal cannot lest some Mars lander
crashes into the planet's surface at high velocity. i don't remember who's
quote this is "Mathematicians routinely ignore units, engineers do so at
their peril." i would have expected physicists to be more like the
engineers than the mathematicians in this case.

sorry, Jan, you're the expert but we gonna have to continue to disagree on
this. i don't think mass is the same kinda stuff as time.

> The candela is best forgotten.
> It is a left-over from the days
> when quantitative measurement of intensities
> was difficult or even impossible.
> Nowadays there is no good reason not to use just watts.

i think here we agree totally agree. like the Mole and Kelvin, it really
does not describe any separate physical quantity.

--

r b-j rbj@xxxxxxxxxxxxxxxxxxxx

"Imagination is more important than knowledge."


.

Relevant Pages

  • Re: Sci Am proposes SR wrong and Aether exists
    ... > Mass is not, length and time are. ... This is another problem with modern physics, ... reviewed literature that mass *is* a dimension in physics. ... The same goes for charge as dimension and charge as measurement. ...
    (sci.physics.relativity)
  • another pass at the "dimensions of physical quantity" argument again (was: can someone explain the c
    ... >>> There is no such thing as the 'fundamental dimension' ... >>> of a physical quantity, ... commonly time, length, and mass. ... so that we may compare (and know if such a comparison can possibly have ...
    (sci.physics.research)
  • Re: can someone explain the candela a little?
    ... >>> dimensions of physical quantity. ... >> There is no such thing as the 'fundamental dimension' ... commonly time, length, and mass. ... > i'm still down to four: time, length, mass, and electromagnetic charge. ...
    (sci.physics.research)
  • Re: Sci Am proposes SR wrong and Aether exists
    ... >>This is another problem with modern physics, ... mass in the same sentence and not recognizing your error. ... It is the kilogram that is measureable, ... The dimension of mass is merely the quality ...
    (sci.physics.relativity)
  • Re: Nobel Prize for David Thomson?!
    ... >> energy in a given mass. ... There is no energy in a given mass! ... mass is merely a dimension. ... mass in physics is not a dimension. ...
    (sci.physics.particle)
Loading