Tutorial details

Students begin by comparing voltage drops across circuit elements in a circuit used to demonstrate the photoelectric effect. They identify the effect of a variable resistor on the potential drop across branches in parallel with it. The work function of the electrodes and the frequency of the incident light are then given. Students come to recognize that when the voltage drop across the evacuated tube is set to zero, the ammeter reading is positive. They then consider how the ejected electrons are effected if the potential difference across the electrodes is gradually increased, or gradually decreased. Students use this to construct an I vs. V graph for the experiment. At this point, students are asked to check their graph with a tutorial instructor. Students often have graphs reversed from left to right, or with the incorrect sign for the stopping potential. They are frequently able to find the errors in their graphs once they are asked to explain its features. Students then obtain a handout of a typical I-V graph from a tutorial instructor.

In section II of the tutorial, students consider the effect on the I-V graph of three separate changes to the experiment: an increase in the intensity of the incident light; an increase in the frequency of the light; and an increase in the work function of the electrodes. In each case, students are asked to check their graph with a tutorial instructor.

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Students should already have been introduced to the idea of photon energy and the work function of materials. Students are assumed to have an understanding of electric potential difference and simple electric circuits, including knowledge of the resistance of ammeters and voltmeters.

Equipment

List

• white board or large sheet of paper with markers

Discussion