Instructors_Guide_Ch28 - 28 Current and Conductivity...

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28 Current and Conductivity Recommended class days: 2 Background Information Chapters 25–27 contained a high density of new ideas and new mathematical techniques. This chapter, by contrast, is more relaxed. The intent is to allow students to practice using the charge model and field model for an important practical application—understanding current—before introducing the very abstract idea of the electric potential. Students all know the word “current,” but most don’t really know what a current is. • Many students think that current is “used up” when passing through devices such as light bulbs. In this instance, they are confusing current with energy transfer. • Students know that current has something to do with electrons, but most students i. Think that electrons are the charge carriers for any and all kinds of currents. ii. Think it is somehow “obvious” that electrons flow through wires. iii. Don’t distinguish the conventional current I from the electron flow rate. • Other students do not think of current as a “flow” of charged particles. • Most students do not understand the conditions under which a current exists. Consequently, much of this chapter is focused on developing a concrete model of current flow. Student Learning Objectives • To use the charge model and the field model to develop a concrete model of current in a conductor. • To examine the evidence by which we know that current in a metal is due to the motion of electrons. • To develop a micro/macro connection between the motion of charge carriers and the conventional macroscopic current. • To introduce conductivity and resistivity as important parameters describing the electrical properties of materials. Pedagogical Approach Most textbooks introduce the electric potential before studying current. Potential is a very abstract idea. Students find the electric potential to be one of the most, perhaps the most, difficult topics of the entire course. I’ve found that students respond better if the charge and field models are pushed as far as possible before introducing the potential. This has two pedagogical advantages. First, it keeps the analysis of situations tied more closely to the tangible concepts of charges and forces. Second, it gives students more practice using, and thus more confidence with, electric fields 28-1
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28-2 Instructor’s Guide prior to learning about the electric potential. Consequently, this chapter focuses on developing a model of current in terms of charges, forces, and fields. An important issue in Chapter 25 was the evidence by which we know about charges and their properties. The emphasis on evidence continues here with the questions: • How do we know that something “flows” when we say that there is a current? • How do we know
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Instructors_Guide_Ch28 - 28 Current and Conductivity...

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