Instructors_Guide_Ch38 - 38 Quantization Recommended class...

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Quantization Recommended class days: 3 Background Information From a pedagogical perspective, the photoelectric effect is the critical experiment that points the way toward quanta. Although Planck and the blackbody spectrum have historical precedence, this story cannot be presented in a meaningful way to introductory students. Not only are students unsure how to interpret a continuous spectrum, they don’t have the knowledge or background to understand how this spectrum was a crisis for classical physics. The photoelectric effect experiment, by contrast, provides a straightforward distinction between classical and quantum ideas. Hence it is a focus of this chapter. The photoelectric effect is one of the few aspects of quantum physics on which there has been a reasonable amount of research (Steinberg, Oberem, and McDermott, 1996). In their study of sopho- more students in a modern physics class, the researchers found that most students had significant conceptual difficulties with the photoelectric effect itself and with the photon model used to explain the results. Using informal surveys, I’ve found that only a small fraction of students in a modern physics class understands the significance or the implications of the photoelectric effect. When asked on an exam to explain how the photoelectric effect was inconsistent with classical physics, the majority of students wrote that the mere existence of the photoelectric effect violated classical physics. Only a very small minority could articulate how the photon model succeeds where the classical model fails. Three issues that cause particular trouble are: • Not understanding the experiment itself—how it works or what is measured. • Not understanding the basic ideas of the photon model of light. • Not being able to use either the classical or the photon model to reason about how the experimental results would change if some parameter of the experiment is varied. The usual presentation of the photoelectric effect experiment assumes that the students understand basic circuits and the use of potential difference. As earlier chapters in this guidebook have documented, many students who have completed conventional instruction cannot reason with or about potential difference. For example, most cannot correctly determine the potential difference across a gap in a circuit—which is what the photoelectric tube is. This has serious implications for their ability to understand the photoelectric effect. The Steinberg et al. research found that less than 50% of students, on the final exam, could explain why the current never goes negative or why the current is not zero when V = 0. Most “explanations” were based on Ohm’s law. When asked to draw a current-voltage graph for the phototube, one-third of the students drew the graph for a resistor—linear, passing through the origin, and with both positive and negative current. In addition, a significant fraction of students thought that electron ejection from the cathode was due to the battery. When asked to explain the
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Instructors_Guide_Ch38 - 38 Quantization Recommended class...

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