  We discuss an investigation exploring students' difficulties with equations
involving resistance, capacitance and inductance. We find that introductory
physics students have great difficulty understanding, e.g., how the resistance
of an ohmic resistor can be written in terms of the potential difference across
it and the current through it, but it does not change when the potential
difference across the resistor is varied. Similar confusions arose in problems
relating to capacitors and inductors. We discuss these difficulties with
equations in the context of introductory physics students' performance on
questions about circuit elements both in the free-response and multiple-choice
formats.
