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Photoelectric Effect Difficulties

July 17, 2012

An undergraduate student has been working on a research project on student understanding of the photoelectric effect. One of the challenges has been helping the undergraduate student to develop enough of a deep and meaningful understanding in order to navigate through an analysis of written data and interview data. It’s hard to understand others’ confusion when you are still working out your own. I am actually leaning toward making the student right up a section in his thesis about himself as a research subject, in which he discusses and analyzes some of the major misunderstandings that he’s had to work through.

Anyway, here’s quick list of some observed difficulties that physics majors have had in making sense of photoelectric effect phenomena.

#1 Associating speed of ejected photoelectrons with the measured electric current flowing in circuit; difficulty understanding how the current can only depend on the rate of photoelectrons emission (and not their rate of travel).

  • Predicting that decreasing the wavelength will result in an increase in the current, because the photoelectrons will be more energetic, thus faster, thus produce a greater current
  • Believing that driving the stopping voltage in the other direction will result in an increased current, because the emitted electrons will become more energetic, and move faster, producing more current

#2 Incorrectly associating intensity exclusively with the number of incident photons or energy of individual photons (not both); not having a clearly defined classical interpretation of intensity to fall back on, and relate to quantum energy associated with a single photon:

  • Struggling to understand in thought experiments what it means to “hold” the intensity constant or “hold” the number of photons constant
  • Believing that an increase in intensity alone can produce photoelectrons

#3 Incorrectly associating the “work function” with a function that describes how the binding energy of electrons varies with depth

  • Associating the surface levels electrons with zero work function
  • Struggling to understand why the the standard equation describes the maximum kinetic energy of electrons.

#4 Struggling with the apparent similarities between the frequency dependence with Bohr model (i.e., absorption spectral lines) with frequency dependence of photoelectric effect

  • Believing that there is a resonant frequency that will result in maximum photoelectric effect, because the metal *prefers* particular energy transitions

More posts to follow for sure once the thesis takes some more definite shape, but just wanted to get this down before I forget.

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2 Comments leave one →
  1. Andy "SuperFly" Rundquist permalink
    July 18, 2012 1:59 am

    A big misunderstanding I see with students is to mistake the work function for actual depth in the metal. I know that I’m sloppy with my vocabulary there, especially when drawing things with the y-axis representing energy.

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