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Re: Ohm's Law



Yes, flicker frequency. I believe N. America is the only place where you
have 60 Hz mains rather than 50 Hz - I'm in Italy.

This discussion has given me a good idea for a student who is looking for
an IB extended essay research topic. I believe that I've seen the
asymmetrical variation you refer to, without really observing it - the
mental picture I have is of an asymmetrical curve.

Mark

At 16:58 07/03/03 -0800, Bernard Cleyet wrote:
Yes the temp. must be high enuff to disassociate the tungsten halogen gas
on the
filament. At lower temps. the reverse reaction occurs. It's intended to
remove
the W deposited on the envelope.

You high be able to predict the diameter of the filament as the function
given by
Levi is:

M* = 0.0069 * D^1.06, where D is the filament diameter in mm and M* is the
fractional change in brightness (Lumens). this is for gas filled lamps
and 60 Hz
mains.

Tho Levi discusses the tungsten iodine cycle, I doubt the data and other I've
given was obtained from such lamps. I wouldn't be surprised if it's
approximately
correct for them tho.

BTW I've head of 50 Hz mains but not 100! Also did you find the variation
was, as
I predict, asymmetrical?

bc who thinks maybe you're referring to the "flicker" frequency with 50Hz
mains
-- obviously UK speak (Australia too?)


Mark Sylvester wrote:

> The variation is certainly noticeable when one uses the Pasco "light
> sensor" with a datalogging interface. We were using a halogen lamp on 12V
> ac as the source in a polarisation experiment some time back. The
> measurements drifted up and down in a regular way that turned out to be a
> slow beat between the 100 Hz of the mains and the sampling frequency, not
> quite 20 Hz. We learnt that we had to use well-smoothed DC. We also learnt,
> btw, that halogen lamps die very quickly when run at below their rated
voltage.
>
> Mark
>
> At 14:28 07/03/03 -0800, Bernard Cleyet wrote:
> >"There is some hysteresis caused by the time lag of the temperature
> >response to
> >the heating/cooling cycles, but the difference in resistance due to these
> >temperature excursions isn't a great fraction of the overall average
> >resistance
> >as a function of time."
> >
> >A 71/2 W lamp works well as a strobe for setting the speed of a disk
> >turntable. Knowing this prompted me to check in my favo. reference book,
> >Levi's Applied Optics. Sure nuff, lotsa data.
> >
> >It gives heating ( brightness 0=>90% and cooling 100=>10%) times and
total
> >variation in brightness for various lamps powered by 50 and 60 Hz.
> >
> >They also have extensive tables on W props from which one may
determine the
> >temperature and resistance. More easily, one (bc hopes to do) may use
a dual
> >beam (dual trace) o'scope directly. Or more hi-tech, digitize and
plot the
> >product.
> >
> >Here's some, 115V 60 Hz, extracted data:
> >
> >gas filled first:
> >
> >W M*% t(h) (msec) t(c) (M* % variation
from mean,
> >total;
> >40 27 65 26
> >100 13 125 59
> >500 4.5 380 190
> >
> >vacuum
> >6W 74! 39 12
> >40 14 128 58
> >
> >Another, rather interesting, but conforms to intuition, is a graph of
> >brightness variation (M*) with wavelength for a 120 V lamp (wattage
not given
> >-- is this like voltage?). The variation is 10% to 4%, 0.4 micron to 1.2
> >(respectively)
> >
> >The large values of t(h) and t(c) paradoxically belie the large values
of M*.
> >The paradox is resolved by examining the graphs of heating and
cooling. The
> >curves are approximately exponential, so, for example, a 40 W (gas
filled)
> >lamp's brightness (lumens) drops from 100% to less than 35% in only ten
> >milliseconds. the eye's is a log detector, so not so
obvious. Despite W lamp
> >resistance is ~ linear WRT the applied voltage (quasi DC), I suspect its
> >variation should be quite noticeable.
> >
> >bc
> >
> >
> >
> >David Bowman wrote:
> >
> > > Regarding Mark's observation:
> > >
> > > >Interesting to see the evolution of consensus on this topic. In its
> > > >previous incarnation (inverbation?) on this list (when I brought it
> > up) the
> > > >view was that the filament lamp is a non-ohmic *device* even if the
> > > >tungsten wire is an ohmic conductor.
> > > >
> > > >Mark
> > >
> > > I suspect that this phenomenon may be a function of the set of just
> > > which list members happened to respond in these two cases. Maybe
> > > last time more of the non-ohmites answered the call, and this time
> > > the ohm-ites responded.
> > >
> > > The thing is that an incandescent lamp plugged in to the local
> > > electric utility will behave, to a pretty decent approximation,
> > > 'ohmically' because the filament doesn't change its absolute
> > > temperature by a very great fraction over the time between heating &
> > > cooling cycles (1/100 sec in Europe & 1/120 sec in North America) of
> > > the applied AC waveform. To a semi-decent approximation, when the
> > > lamp is operating the instantaneous voltage and current wave forms
> > > obey V(t) = R*I(t) throughout all phases of the applied AC wave
> > > where R is almost time independent. There is some hysteresis
> > > caused by the time lag of the temperature response to the heating/
> > > cooling cycles, but the difference in resistance due to these
> > > temperature excursions isn't a great fraction of the overall average
> > > resistance as a function of time.
> > >
> > > Of course if the lamp was operated with a much higher frequency AC
> > > power source, then even these tiny (so-called non-ohmic) hysteretic
> > > effects would vanish. We just must not operate the lamp at *such* a
> > > high frequency that the filament's inductive reactance becomes a
> > > significant fraction of its resistance (otherwise it would be an
> > > inductive load rather than a resistive one). But there is a wide
> > > range of frequencies where this is not a problem and still the
> > > lamp acts fully "ohmically".
> > >
> > > David Bowman
>
> Mark Sylvester
> UWCAd
> Duino Trieste Italy

Mark Sylvester
UWCAd
Duino Trieste Italy