Date: Tue Mar 31 09:34:10 2009 Back to Contents ------------------------------------------------------------------------

Author: Bernard Cleyet

Subject: Re: LED photoelectric effect apparatus


What method do you use to find the "stopping" potential, i.e. the
"original" variable bucking potential, or the modern space charge
limited method? With the modern method it's easier to use the most
energetic electrons to determine the "stopping" potential.

Either method your problem could be, IIRC, some of the cathode
deposited on the anode. Do you ensure (I don't know the tube's
geometry) that no light is incident on the anode?

bc remembers the tubes kept in a refrigerator when not in use at UCSC.

p.s. I thought the Pasco System used photo-diodes instead of vacuum

On 2009, Mar 30, , at 14:34, Wallin, Stephen R wrote:

> Thermalization can occur because electrons can interact with each
> other so quickly. The result is a thermal distribution of energy,
> rather than a sharp cut-off. Also electron may loose some of their
> energy before leaving the photo-metal. I'm not so sure about the
> solid state LED version. Does anyone know, who makes a good bunch
> of LEDs for photoelectric purposes?
> Stephen
> -----Original Message-----
> From: [mailto:tap-l-
>] On Behalf Of John Welch
> Sent: Monday, March 30, 2009 2:46 PM
> To: tapl
> 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
> cause this?
> Has anyone tried a different kind of phototube or solid state
> photodiode?
> thanks
> -John
> --
> ******************************
> John Welch
> Cabrillo College Physics Dept.

From Tue Mar 31 09:34:10 2009