1. Techniques and Tools for Teaching the Photoelectric Effect Sam McKagan, Ward Handley, Kathy Perkins, Carl Wieman January 9, 2007 AAPT Winter Meeting

2. Photoelectric Effect One of the most important experiments in showing inadequacy of classical physics. Basis for photon model of light. Typical modern physics course spends 1-2 50 minute lecture on this topic. Many professors think it is very simple and easy to understand: –“Why do you have a simulation on the photoelectric effect? It is the most straightforward concept in all of modern physics.” Research shows that it is not so easy…

3. Previous Research R. N. Steinberg, G. E. Oberem, and L. C. McDermott, “Development of a computer- based tutorial on the photoelectric effect,” Am. J. Phys. 64, 1370 (1996). –Interviews with students to discover difficulties. –Develop computer-based tutorial to improve student understanding. Also, Knight, Five Easy Lessons, describes some informal studies.

4. Previous research After instruction, many students have difficulty: –drawing a qualitatively correct I-V graph. –distinguishing the effects of changing intensity and changing wavelength. –interpreting the circuit diagram (especially variable voltage). –understanding that it is the light, not the voltage, which causes electron ejection. –understanding significance of PE experiment.

5. Our Study PE Tutor was quick add-on to traditional course. Can we do better by developing an entire curriculum designed to teach the photoelectric effect using interactive engagement and a really cool simulation? –Same material used in Modern Physics for Engineering Majors in Fa05, Sp06, Fa06. –First 2 semesters taught by curriculum designers, 3 rd by another professor.

6. Photoelectric Effect Simulation

7. Our Curriculum Interactive Simulation used in: –Three 50 minute interactive lectures –2+ hour homework activity Incorporate Research Findings: –Use interactive engagement techniques. –Address common student difficulties. –Provide visual model of experiment. –Avoid cognitive overload by modeling variable voltage supply as battery with a slider and including view with only most energetic electrons.

8. Our Curriculum Lecture: Use interactive simulation to provide model of experiment. Clicker questions ask students to predict experimental results, changes to graphs, etc. Homework: Predicting results of changes to experiment Calculations Many essay questions asking students to: –explain their reasoning –compare predictions and observations –explain the meaning of specific ideas

9. Example Clicker Question Percentage In Sp06 we asked students to draw graph BEFORE showing options.

10. A surprising consequence of interactive engagement techniques: students ask really hard questions! Wouldn’t there be less current at low voltages because the electrons would fly off in different directions and not hit the other plate? How does the work function relate to where the element is in the periodic table? Why is intensity independent of frequency for light but not sound? Can two photons give energy to a single electron? How does the photon decide where it’s going to hit? Shouldn’t those accelerating electrons be emitting light? Wouldn’t kinetic energy of the electrons eventually level off because they can’t go faster than the speed of light?

11. Results: Common Exam Question Percentage Correct Would the ammeter read zero current or a non-zero current if you were to: abcN UW w/o PT40206526 UW w/ PT85407536 CU Fa05878591189 CU Sp06888486182 CU Fa0678779094 a. Double the intensity of the light? Why? b. Increase battery voltage? Why? c. Change material of target? Why?

12. Resources Download the simulation (free!) from the PhET website: http://phet.colorado.edu Course materials (lecture notes, homework, exams) available at: http://jilawww.colorado.edu/~mckagan/2130archive Username: admin Password: bohr