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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
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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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 Spring '10
 kant

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