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Why we should teach the Bohr model and how to teach it effectively

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Why we should teach the Bohr model and how to teach it effectively

Sam McKagan,

Kathy Perkins, Carl Wieman

University of Colorado

July 30, 2007

Faculty:

Michael Dubson

Noah Finkelstein

Valerie Otero

Kathy Perkins

Steven Pollock

Carl Wieman

Joe Redish (visiting)

Programmers:

Chris Malley

Sam Reid

Ron Lemaster

John deGoes

Postdocs:

Sam McKagan

Linda Koch

Grad Students:

Wendy Adams

Jack Barbera

Chris Keller

Pat Kohl

Noah Podolefsky

Chandra Turpen

Danielle Harlow

Kara Gray

Charlie Baily*

Content: Understanding atoms crucial for understanding basis of modern technology.

Beliefs: Students can appreciate the beauty of the universe by understanding the fundamental building blocks of nature.

Skills: Good example of model building – can be used to teach scientific reasoning skills.

Relevant at all levels of education

– grade school to grad school

- Researchers* claim that learning the Bohr model is an obstacle to learning true quantum nature of atoms. Should avoid teaching Bohr model entirely.
- Many in PER take this idea seriously – incorporate into curriculum design.
- Area of active debate among high school teachers.

*Fischler and Lichtfeld, “Learning quantum mechanics,” in Research in Physics Learning: Theoretical Issues and Empirical Studies (1992), pp. 240-258.

Fischler and Lichtfeld, “Modern physics and students’ conceptions,” Int. J. Sci. Educ. 14, 181-190 (1992)

- Research poorly conducted.
- Assumption that Schrodinger is only goal.
- Practicing scientists use the Bohr model!
- Bohr model taught in high school.
- Research shows that avoiding discussions of misconceptions doesn’t work. Must explicitly address them!

Design a study to answer this question:

- Is teaching the Bohr model an obstacle to learning Schrodinger model of atom?
Method:

- Can’t do a controlled study because no one will teach course without Bohr model.
- Can we develop a curriculum focused on model-building, including treatment of Bohr model, that leads students to develop understanding of Schrodinger model of atom?

Real World Examples:PMTs, discharge lamps, fluorescent lights, lasers, alpha decay, STMs, LEDs, CCDs, MRIs, BEC

Model Building:Why do we believe this stuff?

Reasoning Development:How do we make inferences from observations?

Memorization

Physics 2130

Modern Physics for Engineering Majors

~200 students

Interactive simulations

Peer Instruction

Solving complex 3D

Schrodinger problems

Special Relativity

Collaborative Homework sessions

- Lectures: focus on model-building, reasons for each model.
- Homework: compare and contrast models, discuss the advantages and limitations of each.

- Democritus – Billiard Ball (450 BC)
- Why? Smallest indivisible chunk of matter
- Problem: Atoms can be ionized.

- Thomson – Plum Pudding (1904)
- Why? Known that negative charges can be removed.
- Problem: Rutherford showed positive charge was small core.

- Rutherford – Solar System (1911)
- Why? Scattering showed small core.
- Problem: electrons should spiral into nucleus in ~10-15 sec.

- Bohr – fixed energy levels (1913)
- Why? Explains spectral lines.
- Problem: No reason for fixed energy levels

- deBroglie – electron standing waves (1924)
- Why? Explains fixed energy levels
- Problem: still only works for Hydrogen.

- Schrodinger – quantum wave functions (1926)
- Why? Explains everything!
- Problem: hard to understand

- Essay question on final exam: “Use words, graphs, and diagrams to describe the structure of a Hydrogen atom in its lowest energy state .”
- Question does not mention models. Which model(s) do students use?
- Analyze responses from random sample of 50 students from each class.

Multiple Models

Models used to describe Hydrogen atom

One Model

Summary:

- More students are using Schrodinger model than Bohr model, BUT…
- About 1/4 still use Bohr model only.
- Only about 2/3 use Schrodinger model at all!
We want to do better!

100%

90%

80%

70%

60%

Percentage of Students

50%

40%

30%

20%

10%

0%

Bohr

deBroglie

Schrodinger

Bohr

deBroglie

Schrodinger

Fall 05 Spring 06

Topics

- Models of the atom through deBroglie
- Electron waves
- Schrodinger Equation
- Other applications of Schrodinger Equation
- Schrodinger model of atom

Not enough explicit connections!

~ 5 weeks

Redesign Curriculum: Add focus on connections between Schrodinger model and other models, model-building exercise.

Fall 06:

- Add focus on connections between Schrodinger model and other models.
- Add general model building exercise.
Spring 07:

- Add interactive computer simulations:
- Models of the Hydrogen Atom*
- Rutherford Scattering*

- In homework, students work through sims, describe reasons for adopting each new model.

*http://phet.colorado.edu

http://phet.colorado.edu

Multiple Models

Models used to describe Hydrogen atom

One Model

Summary:

- ~80% of students use Schrodinger model.
- Only ~10% use Bohr model alone.
Teaching Bohr model does not prevent students from learning Schrodinger model.

100%

New Curriculum

Old Curriculum

90%

+sim

80%

70%

60%

Percentage of Students

50%

40%

30%

20%

10%

0%

Bohr

deBroglie

Schrodinger

Bohr

deBroglie

Schrodinger

Bohr

deBroglie

Schrodinger

Bohr

deBroglie

Schrodinger

Fall 05 Spring 06 Fall 06 Spring 07

See our article at:

http://arxiv.org/abs/0707.1541

Download simulations (free!) from PhET website:

http://phet.colorado.edu

Course materials (lecture notes, homework, exams) available at:

http://per.colorado.edu/modern