Transcript Techniques and Tools for Teaching the Photoelectric Effect

Techniques and Tools for
Teaching the Photoelectric Effect
Sam McKagan,
Ward Handley, Kathy Perkins, Carl Wieman
January 9, 2007
AAPT Winter Meeting
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…
Previous Research
• R. N. Steinberg, G. E. Oberem, and L. C.
McDermott, “Development of a computerbased 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.
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.
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, 3rd by another professor.
Photoelectric Effect Simulation
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.
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
Percentage
Example Clicker Question
60
50
40
30
20
10
0
56
31
38
31
13
A
17
8
23 23
16
2
B
0
C
Fa05
Sp06
Fa06
36
D
5
0
E
In Sp06 we asked students to draw graph BEFORE showing options.
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?
Results: Common Exam Question
Would the ammeter read zero current
or a non-zero current if you were to:
a. Double the intensity of the light? Why?
b. Increase battery voltage? Why?
c. Change material of target? Why?
Percentage Correct
a b c
UW w/o PT 40 20 65
N
26
UW w/ PT
CU Fa05
CU Sp06
85 40 75 36
87 85 91 189
88 84 86 182
CU Fa06
78 77 90
94
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
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