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Re: [Phys-l] Hewitt's three answers



I must admit I had the same doubt as Ludwik and figured it depended on
what one means by "flash". After all, if I take a bulb and "wave" it
back and forth, I don't see flashes like those I see with an LED light.
However, I searched for a slow motion video of an incandescent bulb and
found a couple that showed a noticeable flicker, if not a flash. Does
it depend on the type of incandescent bulb?

Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Ludwik
Kowalski
Sent: Wednesday, March 28, 2012 6:30 PM

Interesting; thanks for sharing. Your observations show that I was
wrong, thinking that variations of intensity would not be noticeable.

On Mar 28, 2012, at 6:22 PM, Tom Bross wrote:

I recently took a slow motion video of a drop forming and falling from
a spigot at 410 frames per second. I illuminated the area with a 100 W
incandescent light bulb and on playback I noticed the variation in
brightness of the white wall in the background, no doubt due to the
small but noticeable cooling and warming of the bulb's filament.

On Mar 28, 2012, at 4:44 PM, Ludwik Kowalski
<kowalskil@mail.montclair.edu> wrote:

Look at Hewitt's "Flash Frequency" question (page 202 in the April
2012, issue ofThe Physics Teacher). In my opinion all three answers, in
this multiple choice test are incorrect, in my opinion. My answer would
be 0 flashes.

Yes, there are 120 current maxima in each second. But the light
intensity, in the case of an incandescent light bulb, is practically
constant due to "thermal inertia." Do you agree?