Re: brightness vrs. power

[Leigh Palmer <palmer@SFU.CA>]

> PHYS-L@lists.nau.edu writes:
> > > Radiant energy depends on temperature of a filament. For instance when
> > two
> > > similar bulbs are connected in series the net current is (approximately)
> > > halved. The power dissipated in both bulbs is reduced by factor 1/4.
> >
> > I don't know how you calculated this, but it is incorrect. Light bulbs
> > do not obey Ohm's law. They are extremely nonlinear.
>
> This may be true, but I saw a demonstration about three summers ago in
> which the instructor broke the glass on a household bulb and immersed the
> filament in a bath of water with an electric stirrer to keep the
> temperature as uniform as possible.  He varied voltage and measured
> current -- I have to say the relationship looked awfully linear to me.

To paraphrase George Bush, read my words! I said light bulbs, not
tungsten filaments immersed in water. Good grief!

This is getting surreal. Physics is supposed to be, above all else,
a description of the real world.

This afternoon I will set up a demonstration that I have never done
before. I will hook four sixty-watt bulbs in series-parallel. I will
compare the light output visually with that of a single sixty-watt
bulb by shining both alternately on the same white wall. I expect:

1. That the series-parallel combination will draw more power from
   the line than the single sixty-watt bulb. (Ohm's law says that it
   should draw the same power.)

2. That the illumination of the wall will be different in the two
   cases. (Naive interpretation which confuses luminous with radiant
   quantities will predict only that the illumination provided by
   the series-parallel combination will be greater, given that the
   dissipated power is greater. The least physical interpretation,
   assuming Ohm's law and confusing the quantities, would have the
   two illuminants being the same.) Note that "different" here means
   differing in either illuminence, or color, or both.

I would also do the nine-lamp experiment except that we do not have
enough apparatus or (and this is most important) matched sixty-watt
bulbs to do it. If this grows into a good demo we'll build it up on
a board and acquire what we need.

Leigh