Instructors_Guide_Ch29

# Instructors_Guide_Ch29 - 29 The Electric Potential...

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29 The Electric Potential Recommended class days: 2 Background Information Of all the concepts in introductory physics, electric potential is the hardest to grasp for most students. It is a very abstract idea, far removed from most phenomena with which they are familiar. There’s good evidence that most students leave introductory physics with little knowledge of the electric potential or how it is used. Students do recognize the term voltage because of its common use. However, they associate voltage with vague and imprecise notions about the “flow of electricity.” Most students are unfamiliar with the terms potential and potential difference , and even those who recognize the words have essentially no knowledge of the concepts to which they refer. Students have a strong tendency to confuse the electric potential with either the electric field or the electric potential energy, and they are liable to use the three interchangeably. After stressing the importance of vectors for electric field calculations, and giving many practice examples using vector superposition, I’ve been surprised to find students attempting to find the “components” of the electric potential. The fact that the expressions for the field and the potential of a point charge look similar likely contributes to students’ misunderstanding. The primary investigation of student understanding of the basic concept of electric potential (apart from the applications of the concept to circuits) is unpublished work at the University of Washington. Students there received a conventional introduction to the electric potential. They also had a laboratory in which they used a voltmeter to measure electric potentials and find electric fields on weakly conducting carbon paper with painted-on conducting electrodes. Student under- standing was evaluated during the laboratory, a week after the laboratory and lectures, and on the final exam. There was strong evidence that most students did not understand the principles of the laboratory and gained little or no knowledge of electric potential from the experience. The majority of students didn’t know what the voltmeter measured. When questioned, they stated that it measured “the flow of charge” or “the difference of charge” between two points. When questioned about the electric field, nearly all students answered by referring to the electrodes on the carbon paper and to the positive and negative terminals of the power supplies, rather than to any measurements they were making with the voltmeter. A week after the lab, only 10% of students could describe how to use a voltmeter to find the electric field at a point. On the final exam, students were shown equipotential drawings, such as the one in the figure, and asked to compare the electric field strengths at points A and B and at C and D. They were also asked about the field direction at different points. Only about 25% of students were generally successful with these questions. Roughly two-thirds of

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