This study has revealed a rich and robust array of student
difficulties with understanding the direction of the magnetic
force on a charged particle in a magnetic field. Trends in
student answers were consistent between the various studies
and independent of course offering and instructor. The traditional
university level introductory E&M physics courses we
studied were somewhat successful in teaching students the
correct direction of the magnetic force on a charged particle
moving through a magnetic field but were significantly less
successful for isomorphic questions presenting charges near
magnetic poles. In addition, there is some indication that the
charged pole responses that are prevalent prior to instruction
reappear several months after instruction.
By analyzing student response patterns in more detail,
which was not possible in earlier studies such as in Ref. 7
due to limited answer choices, we also found an interesting
and persistent signal of the sign error in 15%–30% of responses
postinstruction. This number is somewhat misleading:
When given a series of four similar questions, over 40%
of the students made a sign error at least once, illuminating a
significant student difficulty that warrants attention during
instruction.
Our results suggest at least three causes for the sign error.
About 15% of students thought that the magnetic field points
from the south pole to the north. A substantial fraction of
these students correctly applied the right-hand rule to the
incorrect field direction, resulting in a systematic sign error
in their magnetic force responses.
The overwhelming majority of students did not make sign
errors systematically. This lack of systematic rendering of
sign errors led to uncovering two other causes for sign errors.
One is a confusion in choice and execution of the several
right-hand rules available. Although most students recognized
that the magnetic force is in the direction perpendicular
to the velocity and field after instruction, inconsistent execution
of the proper right-hand rule led to an unsystematic sign
error.
The last identified cause of the sign error arises from the
fact that even after instruction, about one-quarter of the students
did not recognize that reversing the order of the vectors
in the cross product reverses the direction of the resultant
vector. This reversal can lead to a nonsystematic sign error.
These students are more likely to make a sign error than
those who understood the noncommutative nature of the
cross product.
Besides the issue of sign errors, we can speculate on possible
reasons for the differences in student responses between
the pole and field line representations. Solving the magnetic
pole representation requires the additional steps of identifying
both the presence and direction of the magnetic field.
Increasing the number of steps increases the overall probability
of making an error. For example, failure to recognize the
presence of a field may have resulted in lack of cuing of the
Lorentz equation F =qv B, or an error in deducing the direction
of the field from the poles can result in an incorrect
answer. Also, students might have been more familiar with
the magnetic field line representation because questions with
magnetic field are likely more practiced and emphasized than
questions with magnetic poles during instruction. In the
course textbook29 field lines are used to represent the presence
of a magnetic field much more frequently than magnetic
poles. Reports from the lecturers also support this claim. In
addition, students have had concrete experience with the attraction
and repulsion of magnets, which have similarities
with the attraction and repulsion of static charges. Thus the
magnetic pole representation might have caused more confusion
of magnetism with electricity than abstract representations
of fields, with which student have had less experience.
Although the design of this study did not explicitly examine
how specific forms of instruction might affect students’
understanding of magnetic force, we point out two implications
for instruction. First, instructors cannot assume that
magnetic field and pole representations are equivalent and
interchangeable from the student’s perspective. To help students
understand the nature of these two representations and
the physics of the relation between them, instructors might,
for example, ask students to explicitly compare given situations
in which only field lines are present with isomorphic
situations in which only magnetic field sources are present
for example, permanent magnets and current-carrying
wires. Because a slight decrease back to the original misconception
was observed after instruction, more so in the
magnetic pole representation, instructors should look for opportunities
to include magnetic poles and magnetic forces in
subsequent instruction by either using magnetism as a context
for