Re: Alternating Current (II)

[brian whatcott <inet@INTELLISYS.NET>]

At 17:08 1/29/99 -0500, Hugh Haskell wrote:
> I have done this experiment before and I think what you are getting is the
> "AC-variation" on the basic light output of the bulb. Once the bulb heats
> up to its average temperature, then as the ac-current varies, the
> temperature varies around that average by a small amount. I would guess
> that wht you saw was a variation of only a few percent (less than 5%) in
> the amount of light emitted over time, ....

I was more than comfortable with Hugh's exposition to this point.

> ...and since the light output is
> proportional to the power consumed in the bulb, which is proportional to
> the square of the current, what you are seeing is not |sin wt|, but (sin
> wt)^2....
> Hugh

...but the suggestion that light output is proportional to the square of
the current is in error  ..er.. discrepant in several small ways:
 we expect physicists to rail about the non-Ohm's law behavior of metals
when heated to incandescence: so we grudgingly accept that energy input is
not proportional to I^2.R but rather I^2.R^x
 or to put it in plain English: if resistance increases with temperature
then power input varies with I^n where n is a little more than 2.

 There is however another effect which it is particularly apposite to
mention in this closing year of the century: the ultra-violet catastrophe,
no less!
 A hundred years ago,  one was exercised to assign an analytical expression
to the spectral energy distribution of a hot body.

   It was found that the mode of this distribution moved to higher
frequency at increased power, and not to make too much of a performance on
the issue,
Stefan and Boltzman had 25 years or so earlier found a fourth power
relation between energy and temperature, Wien found a relation for the
shift in wavelength with temperature, paving the way for Planck to exhibit
his splendid analytical formula which depended on the indivisibility of
energy...

 How on Earth did I get to this point? Ah, yes: it's the question of LIGHT
being proportional to energy in a lamp: if we understand light to mean
visible light, or visible and near IR light, then light cannot be
proportional to energy consumed. It is the aforementioned luminaries of the
closing 19th century who made this plain. (Most energy from an incandescent
is in the IR: hence the amazing gains for the brighter halogen bulbs
running clear through the visible and into the UV!

Here endeth...

Brian
p.s. I haven't even mentioned an analytical model for the thermal mass of
the filament...
brian whatcott <inet@intellisys.net>
Altus OK