On 11 May 2010
at 21:52, Tony Duell wrote:
Since they are in series, they are both passing
the same current.
Therefore the power disipated is porportional to the resistance. A 40W
bulb will have a higher resistance than a 100W bulb, so, making
reasonable assumptions about the bulbs, the 40W one will be brighter.
Yes, I was
sort of hinting at a possible solution to the original
Sure. i never actually saw the original post... It would be interesting
to measure the votlages across the various bulbs in the OP's circuit.
problem--in fact, the resistance difference
becomes much larger as
the 40W lamp begins to glow. If you put the series-connected lamps
on a variac and slowly inch the voltage up, it's surprising how the
positive temperature coefficient of resistance starts kicking in.
That is something I am going to have to try (having never done it).
But most people will, without thinking, say
"Oh, a 100W bulb is
brighter than a 40W, so the answer is obvious." And wrong.
These are the same people that think you _have_ to draw 13A from a 13A
mains socket, i guess ;-)
[A buyl disipating 100W is likely to be righter than one disipating 40W.
But in this circuit, the buls are not disipating their rated powers]
But then we learn more when something doesn't
work the way it's
"supposed to", than when it does, don't we? That's one issue I have
Of course. I can't rememebr who said it, but there's a quote something
like "The words that come before the greatest progresses in science are
not "Eureka" but "Hey, that's curious"'
with a lot of secondary-school laboratory
courses. You do the
experiment, can pretty much guess from the course material what's
going to happen and it does. Time for lunch.
One thing that I learnt very early on (from my parents, not at school, I
hasten to add) is that if something does not do wht you expect, you stop
and investigate. Most of the time it will be because you've done
something silly. Just occasionally it's because there's something
interesting going on.
OK, you folks are going to just force me to go put some bulbs
in series
and play around with them. :-) Multiple stable states would indeed be
most interesting!
You also remind me that school can occur anywhere, any time, under any
circumstances. During college I worked part time repairing TV's etc.
Many TV's back then (early 1960's) had one or more "oil-filled" or
"oil-impregnated" (can't recall which) paper caps, often somewhere in
the horizontal oscillator circuitry or thereabouts IIRC. They would go
bad and have to be replaced, but what was really interesting were the
bad ones. Something had happened inside to turn them into what I
perceive to be a battery with a rather large series resistance. I think
I first discovered this when I discharged a bad one, then measured it
with my ohm meter and it proceeded to peg the meter. I shorted it out
again for a bit, then measured it with a voltmeter. Sure enough it was
still generating somewhere around 1 volt. I was determined, so I
shorted it out again and left it shorted all night. The next day after
removing the short the voltmeter said the battery was still alive and
well - perhaps a bit diminished (it's been 50 years after all). I kept
one of those shorted out on my workbench for months and measured it
periodically. It never did run down completely. I wish now I'd had the
presence of mind to measure the effective series resistance of the
"capattery", but I did not think of that at the time. I think I read an
explanation of the chemistry/process that causes this a few years ago,
but I've since forgotten the explanation. Though I did not understand
the phenomenon at the time (still don't) I think it was a very good
practical lesson in learning not to assume too much, as well as learning
to sometimes expect the unexpected.
Later,
Charlie C.
Back when I was taking a course in numerical
methods, I had a teacher
who'd assign seemingly easy programs. You had a week to code it up;
just about any text would have the method documented. He'd post the
data set the day before the assignment was due.
Of course, after about the first two assignments, you learned that
his data were going to be pathological and destined to break any "out
of the book" method. In a previous life, he'd worked for NASA and I
suspect, learned things the hard way.
That's the sort of teacher I would like...
-tony