Gas station without pumps

2012 April 25

Photoeletric effect

Filed under: home school — gasstationwithoutpumps @ 16:30
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Brain Frank has just posted an exploratory exercise on his blog Teach. Brian. Teach.: Photoeletric Effect.  This exercise relies on a simulation from the University of Colorado at Boulder.

The simulation is of a standard phototube experiment.  A phototube is a vacuum tube diode, in which the cathode is illuminated by a light source.  The photons excite electrons in the cathode, raising some of them to high enough energy levels to become unbound from the atoms and leave the cathode.  The electric field accelerates them toward the anode (or repels them, if the diode is biased backward).  The energy of the electrons is basically the energy of the photons minus the energy needed to raise the electrons from the ground state to the unbound state.  (At very high illumination levels, you can have one photon exciting the electron out of the ground state and another raising it to the unbound state, but I don’t think that effect is being simulated.)

At forward voltages, the current is determined by the illumination, independent of the bias—essentially all the released electrons go to the anode. At reverse biases, only the higher energy electrons have enough speed to make it to the anode. The energy of the highest-energy electrons can be estimated from the reverse-bias voltage at which the current drops to zero.

The simulation seems pretty good, but I don’t know exactly what effects they are modeling.  For the zinc target with high forward bias, there is a current peak around 135 nm, but from the spectral lines at NIST, I would have expected a peak around  127 nm.  I don’t know if the problem is a limitation of the simulation or a limitation of my understanding.

I know that my understanding of quantum effects is very limited, and the simplistic view of the photoelectric effect given in Wikipedia does not cover some of the phenomena being simulated here.  But since I don’t know exactly what phenomena are being simulated, I have no way of predicting the behavior.

I find it frustrating to do the sort of discovery experiment that Brian is proposing using a simulation.  If I knew precisely what was being simulated, there would not be much discovery, but trying to reverse engineer a simulation from its behavior seems to me a rather irritating and frustrating exercise. I not only have to guess at what physics is important, I also have to guess at what physics the writer of the simulator thought was worth including, and what simplifying assumptions he made.  (For example, is the simulation including the absorption of the glass or quartz tube holding the vacuum?)

I suppose I could read the source code (PhET provides that) or read the 17 “Teaching ideas” on the web page for the simulation. The teaching ideas look like a wide range of different lesson plans for labs, demos, and homework questions.  I looked at one of the “advanced” ones, but it seemed to only use the Wikipedia-level model, which does not explain a drop in current with shorter wavelengths.

I’d much rather have real experiments than simulated ones—even if the crudeness of my measurement tools limits the quality of the data I can collect.  The value of simulations is more in writing them and seeing that they predict the behavior you observe than in running someone else’s black-box model.

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