[vox-tech] [OT] Electrical Engineering Question

[Peter Jay Salzman]

yah, but for some reason i thought i remembered lightbulbs going in the
opposite direction.  they behaved in the opposite way you'd expect.
i'm probably just remembering wrong (which is why i didn't say
anything).  :*)

pete


begin Mark K. Kim <markslist@cbreak.org> 
> In general, resistance increases as heat increases.  Hence super
> conductors are kept cold.  (Or another way to think of it is, for a single
> electron, it is easier for it to travel through a crowd of other
> electrons that aren't bouncing around than it is to go through a crowd of
> other electrons that are... like a real-life traffic.)
> 
> -Mark
> 
> 
> On Sat, 1 Feb 2003, Peter Jay Salzman wrote:
> 
> > ach, one more thing.  i can't believe i forgot this (but the truth is,
> > my department knows i hate experimental physics, and has given me
> > lectures and discussions for the past few years instead of running
> > labs).
> >
> > light bulbs are highly non-ohmic (linear) devices.  the power they
> > consume is highly dependent on the temperature of the filament.  in
> > other words, the power they consume depends on how long they've been
> > left on.   for the life of me, i can't remember whether they consume
> > more or less power as they heat up.  for your sake, i hope it's less.
> > :-)
> >
> > 250W is some kind of average; i don't know how they get that number, but
> > i'm sure they have some kind of procedure like "leave the lightbulb on
> > for an hour and divide by 3600" or something like that.
> >
> > from what i remember about the physics 9C labs, 25% sounds pretty
> > reasonable to me.
> >
> > pete
> >
> >
> >
> >
> > begin Jeff Newmiller <jdnewmil@dcn.davis.ca.us>
> > > On Fri, 31 Jan 2003, Rod Roark wrote:
> > >
> > > > I got my first electric bill at the new house; looks too
> > > > high.  So I decided to do an experiment.
> > > >
> > > > Outside the house is an electric meter.  It reads KWH
> > > > accumulated on 5 dials, and has a horizontal platter that
> > > > appears to spin about 100 revolutions per KWH (anyone know
> > > > if this is exactly true for a standard meter?).
> > > >
> > > > So I figure that means 10 watt-hours per rev, or 36,000
> > > > watt-seconds per rev.
> > > >
> > > > I timed one revolution with most things in the house turned
> > > > off.  45 seconds.  Then I turned on a 250W light bulb and
> > > > timed it again.  32 seconds.  So:
> > > >
> > > >   36,000 watt-secs / 45 secs =  800 watts
> > > >   36,000 watt-secs / 32 secs = 1125 watts
> > > >
> > > >   1125 - 800 = 325 watts -- for a 250W bulb.
> > > >
> > > > How come?  Should I complain to PG&E, or is there some
> > > > gotcha that I'm missing?
> > >
> > > Go look on your meter face for a number labelled Kh.  A typical value is
> > > 7.2.  Units are Watt-hours per revolution.
> > >
> > > Count the number of seconds (S) it takes for the disk to revolve once. (I
> > > usually time two or three turns and average. Total measurement intervals
> > > between 40 and 90 seconds will usually give good results (though if it
> > > spins three times you will need to remember to divide the total interval
> > > by three before using the answer).
> > >
> > > Power = Kh * 3600 / S
> > >
> > > The only difference between your equation and mine is your use of Kh=10.
> > >
> > > Assuming you have a regular meter like mine, the calculation should yield
> > > 810W - 576W = 234W, which could be reasonable. I don't know what kind of
> > > light bulb you used... torchiers output is often adjustable. Beware of
> > > "hidden loads" too... a refrigerator kicking off or on toward the end of
> > > your measurement interval can reduce or increase respectively your
> > > apparent load.
> > >
> > > For those looking to check these answers: the energy meter is almost
> > > certainly more accurate than whatever test equipment the average computer
> > > geek is likely to have laying around for measuring energy consumption
> > > (energy meters have a tested accuracy of <0.3%).  P=V*I always for
> > > simultaneous/instantaneous measurements, but the root-mean-square readings
> > > typically reported by DVMs ONLY hold for _sinusiodal waveforms that are in
> > > phase_.  Those conditions are often not valid in the real world, so beware
> > > of getting out your DVM to double-check your energy meter. :)

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