Interference between electric and magnetic concepts in introductory physics

We investigate student confusion of concepts of electric and magnetic force. At various times during a traditional university-level course, we administered a series of simple questions about the direction of force on a charged particle moving through either an electric or a magnetic field. We find that after electric force instruction but before magnetic force instruction most students answer electric force questions correctly, and we replicate well-known results that many students incorrectly answer that magnetic forces are in the same direction as the magnetic field. After magnetic force instruction, most students answer magnetic force questions correctly, but surprisingly many students incorrectly answer that electric forces are perpendicular to electric fields, as would happen if a student confused electric forces with magnetic forces. As a further indication of interference between electric and magnetic concepts, we also find that students’ responses depend on whether electric or magnetic force questions are posed first, and this effect depends on whether electric or magnetic force was most recently taught.

Figure 1

Example showing question text and illustrations received by students who were questioned with the field line representation. Each student answered eight questions in which the direction (up, right, into, and out of) and type (electric and magnetic) of field were varied.

Figure 2

Example showing question text and illustrations received by students who were questioned with the pole representation. Each student answered four questions in which the direction (up and right) and type (electric and magnetic) of field were varied.

Figure 3

Student responses to electric and magnetic force questions previous to instruction in both electricity and magnetism.

Figure 4

Student responses to an electric and magnetic question following instruction in both electricity and magnetism. The fields were represented with electric and magnetic field lines, respectively.

Figure 5

Results showing how students’ answers to electric and magnetic force questions changed from before to after instruction in magnetism. Fields were depicted using field lines.

Figure 6

Results showing how students’ answers to electric and magnetic force questions changed from before to after instruction in magnetism. Fields were depicted using poles.

Figure 7

Student responses showing the effect of question order (or cognitive priming) on the field line representation.

Figure 8

Student responses showing the effect of question order (or cognitive priming) on the pole representation.