Date: Tue Mar 31 14:01:45 2009
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Author: Zani, Gerald
Subject: Re: LED photoelectric effect apparatus
This may not be the source of your troubles, but there has been for many
years a problem with the experimental methods commonly published to
interpret the data from LEDs to get Plancks constant.
Garver's TPT article does not cite the following important reference:
Answer to Question #53. Measuring Planck's constant by means of an LED
Am. J. Phys. 66, 12 (1998)
To sum up the problem, you cannot drive the LEDs with one value of
The Garver article says to drive the LEDs at "maximum intensity". What
is "Maximum intensity"?
The Morehouse citation points out this problem, and says it is better to
vary the current from zero to max for each LED, plot all the points,
produce the complete diode curve for each LED and only then to determine
the best slope of this curve.
This may give a better experimental value from each LED?
But again, this may not be the source of your particular trouble?
From: firstname.lastname@example.org [mailto:email@example.com] On
Behalf Of John Welch
Sent: Monday, March 30, 2009 4:46 PM
Subject: [tap-l] LED photoelectric effect apparatus
Hey all -
we're making several homemade PE experiments using LED's, based on the
article by Wayne Garver in the May 2006 Physics Teacher.
For our phototubes, we 'borrowed' two1P39 tubes from our old Pasco PE
experiments, and bought a third one from a surplus distributor. With the
Pasco tubes, we get a nice value for Planck's constant. When we use the
surplus tube with everything else the same, we still get a nice straight
line for our stopping voltage vs frequency graph, but the slope is about
25% too low. When we first had this problem, I sent the tube back and
got a different one from the surplus guy, but the new one he sent had
the same problem. I'm trying to figure out what could cause something
like this. I would expect differences in the coatings to show up as
different values for the work function, but not to effect the slope.
Does anyone have experience with this, or have an idea as to what might
Has anyone tried a different kind of phototube or solid state
Cabrillo College Physics Dept.
From firstname.lastname@example.org Tue Mar 31 14:01:45 2009