vanced physics training. Of far greater concern is teachers’ pedagogical content knowledge. As the above discussion suggests, the knowledge and tools are now available to support the latter, including the research base concerning students’ conceptual understanding, assessment tools such as the Force Concept Inventory, and alternative forms of instruction, such as the emphasis on modeling described earlier. It is uncertain what proportion of the 19,000 physics teachers in the United States engage in instructional practices that are aligned with what is known about learning and instruction in physics, or how many make use of research-based curricular materials, assessments, and approaches. Equally uncertain is the source of their knowledge, that is, whether generalized from their own experiences as physics students, acquired during pre-service teacher education, or developed as a result of professional development programs.

In any given year, the number of physics majors pursuing a secondary education teaching credential is relatively small, and in most institutions of higher education only a few of these students may be simultaneously pursuing a certification program. Variation in program content, student learning experiences, and supervision can be substantial. This is especially problematic with regard to the specifics of how prospective physics teachers acquire knowledge about important characteristics of student learning and the teaching of physics.

In contrast to pre-service teacher education, the professional development of in-service physics teachers is often better organized, especially with regard to regional, state, and national workshops. Many of these opportunities have been supported by federal funding such as the Eisenhower math-science programs, the National Science Foundation’s teacher enhancement projects. Professional societies have played a role as well. The physics teaching resource agent (PTRA) program, run by the American Association of Physics Teachers, is one model of a sustained teacher enhancement project. First funded in 1985, the PTRA program develops workshop materials, prepares exemplary high school teachers to serve as resource agents, and provides support to those agents to offer workshops in their own regions (Nelson and Bader, 2001). Agents continue to receive education over successive summers to expand their repertoire of workshops.

Approximately 500 outreach teachers have been educated, and more than 300 remain active. From 1985 to 1995, the out-

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