  Throughout introductory physics, students create and interpret free body
diagrams in which multiple forces act on an object, typically at a single
location (the object's center of mass). The situation increases in difficulty
when multiple objects are involved, and further when electric and magnetic
fields are present. In the latter, sources of the fields are often identified
as a set of electric charges or current-carrying wires, and students are asked
to determine the electric or magnetic field at a separate location defined as
the observation location. Previous research suggests students struggle with
accounting for how a measurement or calculation depends on the observation
location. We present preliminary results from a studio-style, algebra-based,
introductory electricity and magnetism course showing the prevalence of correct
and incorrect responses to questions about observation location by analyzing
student written work involving vector addition of fields.
