Transcript Inquiry-Based Instruction for Elementary Physics: Hi

Learning Difficulties and Teaching
Strategies Related to Electric Circuits
David E. Meltzer
Department of Physics and Astronomy
Iowa State University
Ames, Iowa
Research on Learning of
Electric Circuit Concepts
• Pre-college students
– Shipstone (1984)
• Early work with college students
– Fredette and Lochhead (1980)
– Cohen, Eylon and Ganiel (1982)
• Extended investigations with college students
– Shaffer and McDermott (1992)
– Harrington (1995)
Why do students have such difficulty
learning about circuits?
• Lack of practical experience with circuits
• Widespread linguistic imprecision in dealing
with subtle physical concepts confounding of
“current,” “voltage,” “energy,” and “power”
• Abstract and counterintuitive nature of closely
related concepts e.g., charge, field, force, potential, etc.
General Problems with Circuits
• Students retain confusion with “building
block” concepts such as charge, field, potential, etc.
• Students struggle with common
representations of circuits e.g., relating circuit
diagrams to drawings, and drawings to actual equipment
• Students do not have separate concepts
attached to the words “current,” “power,”
“energy,” and “voltage.” (it’s all “electricity”)
This exacerbates tendency toward confusion
• “batteries have constant current” [instead of voltage]
• “current gets used up in circuit elements” [instead of
energy]
Example: Exercise given in an elementary physics course:
(1) Draw a circuit diagram for the physical layout shown below (assume
bulbs are identical)
(2) Draw a physical layout that corresponds to the circuit diagram shown
below.
Results: Many students drew physical layout for #2
to be same as shown in first diagram!
The two most universal conceptual
difficulties regarding circuits:
• Most students believe strongly that electric
current gets used up as it moves through
circuit elements.
• The overwhelming majority of students are
certain that a battery will always produce the
same amount of current regardless of the
circuit to which it is attached.
It is EXTREMELY DIFFICULT to persuade
students that these ideas are not correct!
Pitfalls of All Instructional Methods
• Students are very good at devising theoretical
justifications (for observations) that conform to their
original beliefs.
• Students will frequently “observe” nonexistent
phenomena (e.g., differences in bulb brightness) that
they believe should be present.
• Students may greatly exaggerate the significance of
minor observational discrepancies (to match their
theoretical preconceptions).
[It is possible that some physicists may occasionally
behave in a similar manner . . .]
Observation: Bulbs in two-bulb circuit
are dimmer than bulb in one-bulb circuit
Students’ Theorizing to Explain
Observations
Student: Same current goes through battery in
each case; battery always has to produce the
same amount of current.
Instructor: But since these bulbs are dimmer,
doesn’t that mean that less current goes
through these bulbs than through the bulb in
the one-bulb circuit?
Student: Yes. The same current flows out of
battery, but it is shared between the two
bulbs, so each gets only half.
Effect of “successful” intervention by
instructor on student understanding:
• After discussion with instructor, students agree that
less current goes through battery in case of two bulbs
in series. (Concept of “current conservation” is
reviewed.)
• Five minutes later, students make observations of
two-bulbs in parallel: this time, both bulbs are same
brightness as one bulb alone.
• Question to students: How does amount of current
through the battery now compare to single-bulb
case?
• Answer [given by 90-100% of students]: Current is
the same!
What about using ammeters . . .
• Randal Harrington asked physics majors
(who had completed E&M + lab course) to
compare ammeter readings in these circuits:
A
A
Result: 20% said readings would be different.
Difficulties with “Voltage”
• “Potential Energy” is a difficult concept in itself;
“Electric Potential” is still more confusing.
• Students are very slow to understand that difference
in electric potential between two points is what is
proportional to current flow.
• Common usage of “voltage” and “V” in Ohm’s law
seriously aggravates this problem.
• After study of loop rule, students will frequently
confuse “voltage drops” with current drops (i.e., reemergence of idea that “current gets used up” )
Instructional Strategy Developed at
University of Washington
Physics Education Group led by L. C. McDermott
“Physics By Inquiry” and “Tutorials in Introductory Physics”:
Extended hands-on investigations using batteries and bulbs.
1) Introduce concept of complete circuit: try to light bulb with wire
and battery
2) Introduce concept of current: current not “used up”; current
through a battery depends on circuit configuration.
3) Introduce concepts of resistance and equivalent
resistance: “indicator” bulb with various configurations.
4) Introduce ammeters, voltmeters, and concept of potential
difference.
5) Finally, introduce concepts of energy and power.
Advantages of “Current First” Strategy
• Avoids need for immediate grappling with
“potential” concept.
• Notion of “flow” relatively easy for students to
accept.
• Averts probable early confusion of “energy
loss” with current “non-conservation.”
• Allows much deduction and model-building
based on qualitative observation.
Another Strategy: Emphasis on
Potential
“Workbook for Introductory Physics” by Meltzer and
Manivannan; for in-class use without relying on lab.
• Extended development of electric forces and
fields, electric potential energy, and electric
potential;
• Intensive study of current, “voltage” [potential
difference], and Ohm’s law before discussion of
circuits;
• Step-by-step analysis of very simple circuits.
Balance Sheet of “Workbook” Strategy
• Advantages:
– close contact between circuit theory and
preceding development of field, force and energy
concepts
– provides an option when lab work is not required
or not available
• Disadvantages:
– confusion between current and potential is
aggravated
– only very simple circuit configurations are dealt
with
– lack of “hands-on” a potentially fatal constraint on
understanding
Summary
• If universally held misconceptions regarding
circuits are not explicitly addressed, most
students will never give them up.
• Methodical attention to subtle distinctions
among key circuit concepts is essential.
• Whatever strategy is adopted, common
conceptual difficulties will make their
presence evident in numerous guises.