Designing Animated Simulations and Web-Based Assessments to Improve Electrical Engineering Education - Web-Based Engineering Education: Critical Design and Effective Tools - page 78

Student Interviews current source rather than a source of invariant

voltage (Engelhart & Beichner, 2004). Students
may also fail to differentiate between current and
voltage, and power and energy (McDermott &
van Zhee, 1984). Previous research has primarily
been concerned with simple direct current (DC)
circuit problems, and this may inadvertently guide
one towards instructional decisions that reinforce
misconceptions and difficulties students have
when learning in other contexts. As part of an
Office of Naval Research (ONR) funded project
at Vanderbilt University, we extended research
of student understanding of electric circuits into
the domain of alternating current (AC) circuits.
We were motivated by questions such as, to what
extent do students exhibit the same misconceptions
that they exhibit for DC circuits? How do students
interpret time-varying phenomena?

In interviews with students working on electrical
circuit problems, we found that students had much
greater difficulty understanding time-varying phe-
nomena in circuits. We also found that students
focused on manipulating formulas and performing
numerical calculations during problem solving,
and not applying the underlying principles or
invariants , such as Kirchoff's or Ohm's laws,
that govern circuit behavior. Analyzing com-
mon student difficulties that we identified, and
by studying expert problem solving behavior,
we developed a web-based tool (Inductor) for
assessing and guiding students' learning of DC
and AC circuits. Using Inductor we explored an
additional research question: What are the effects
of automated, invariants-based feedback on self-
assessment and learning of electric circuit behav-
ior? We found that by using this feedback students
improved their problem solving performance in

Table 1. List of misconceptions

Misconceptions Related to AC Circuits
1. Spatial AC misconception. The sinusoidal AC voltage and current waveforms are not a representation of variation of these variables
at a point in time. Rather they depict a variation of their magnitudes along the length of the wire in which the current is flowing. For
example, students said that a string of identical light bulbs in series when connected to an AC source would light up in sequence, and
some of the light bulbs may be on when others are off. At the same instant of time, the brightness of the bulbs would vary depending on
their position in the circuit.
2. Negative part of AC cycle is just a mathematical artifact. No current flowing in circuit or power delivered during negative part of
AC cycle. For example, a number of students said that a light bulb only lights up during the positive part of the sinusoidal cycle. Others
said that there could be “no such thing as negative current. That is just a mathematical artifact. If current reverses, the electrons would
reverse direction too. They would then run into each other, stopping flow, which implies there could be no current.”
3. Alternate form of this misconception. The negative current “cancels” out the positive current. So bulb will never light up when you
connect to true AC source.
4. Empty pipe misconception. During AC cycle electrons stop, turn around, and go the other way. In some cases when you have very
long wires, they may never reach the light bulb connected to the end of the wire. Students thought that you would need two fuses to
provide protection in an AC circuit, where you could do with one in a DC circuit.
5. Incorrectly importing DC models to explain AC.
• Students often surmised that the alternating current going through a resistor was constant in time.
• Students often hypothesized that a capacitor behaved the same in AC and DC circuits.
6. Difficulties understanding circuit behavior when AC and DC signals are combined. Students had difficulty “separating” or recog-
nizing the AC and DC components of a signal in problems in which the midpoint of a sinusoidal voltage was not zero.
7. More generally, difficulty thinking of circuit behavior when multiple waveforms, frequencies are combined. Even advanced stu-
dents stated that the number of channels you can got from cable TV was a function of the number of wires in the cable, or the thickness
of the cable.