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ELECTRICITY
I.
INTRODUCTION
A. Overview of the Unit
In this unit students compare their ideas about electricity with the behavior of
simple battery and bulb circuits, which they construct themselves. In addition
they will examine the behavior and interactions of static charge. The purpose of
the observable demonstration is to provide the raw materials from which
students can compose a model that explains electrical phenomena. High-capacity
capacitors are used for part of the work to draw out the nature of transients in
the dc circuits and make possible bridges between the behavior of static charges
and electrical circuit phenomena.
We have developed this unit of instruction to meet the following criteria:
1. The instruction is intended for approximately 20 class and lab hours.
2. No algebra should be employed or required.
3. Equipment requirements should be kept to a minimum.
The first is merely a practical consideration. More than this amount of time is too
much for one topic in a one-semester course. Less than this leaves too little time
for conceptual development. We feel strongly that elementary teachers cannot
make use of a formula-based understanding of the subject with their students.
The understandings that are being developed by the teachers must be
conceptual. While we believe that the course must be more than what would be
offered to elementary students, we only use technical equipment that is
absolutely necessary to support the development of the desired understandings.
These few ground rules result in the selection of a subset of the possible topics
which might be dealt with in such a unit of instruction. We found the notion of
potential differences across individual circuit elements to be extremely difficult to
develop without either the inclusion of voltmeters and ammeters and
mathematical formalism or the development of some other tool such as a
computer simulation to render this aspect of electrical phenomena more real to
the students. The introduction of the meters also would require additional time
for developing an understanding of their function and use. Hence, we do not
deal directly with that particular topic.
On the other hand, much can be ascertained by reasoning about the directly
observable behaviors of circuits: the lighting of bulbs and the response of a
simple magnetic compass when near the wires of a circuit. Hence, we have
found that it is entirely possible to reason successfully about the behavior of
simple electric circuits (relative brightnesses of bulbs and what happens when
changes are made to circuits) in terms of current, resistance, and the voltage
developed by the batteries.
Electricity
Instructor Materials
©2001 American Association of Physics Teachers
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B. Acknowledgments and Origins of Ideas
A number of efforts have focused on the issue of understanding electric circuit
phenomena. We are indebted to the ongoing work of our colleagues. Differing
goals and boundary conditions preclude our use in their entirety of any of these
excellent efforts. We have included a number of elements from several projects
to create a coherent, effective unit. We have knowingly included ideas from
•
The lessons on "Batteries and Bulbs," which have been used in a number of projects,
originally appeared in the Elementary School Science (ESS) Project,
•
The Mel Steinberg's CASTLE Project, most notably using large capacity (0.25 to 0.49 Farad)
capacitors,
•
The AAPT Electrostatics Workshop developed by Robert Morse, Rodney Labrecque, Peter
Shaffer, and Richard Heckathorne,
•
The recent work of Bruce Sherwood and Ruth Chabay at Carnegie Mellon University,
•
The work of Samuel Joshua and Jean-Jacques Dupin in France,
•
The Physics Learning Research Group at CRMSE at San Diego State University under the
direction of Fred Goldberg,
•
The Physics Education Research Group at the University of Washington under the direction
of Lillian McDermott, and
•
The work of Arnold Arons, Professor Emeritus, University of Washington.
C. General Safety Considerations
The equipment and its configurations for this unit are similar to everyday uses of
the equipment. It is possible when the batteries are fresh to get a piece of wire
shorted across them that's hot enough to surprise the students or possibly cause
minor burns. While we fear that up-front emphasis of the potential dangers
might add to the fear of electricity already held by some students, it would be
best to use plastic insulated wire and insist that circuits with more than one
battery include a tap switch at all times. The tap switch also serves to help
batteries last the duration of the unit. A second possible problem is loss of
integrity of the battery cases. This happens with faulty batteries and with old
ones. Both should be avoided and any mess cleaned up quickly when it does
occur. We have found that it is best to purchase an adequate amount of quality,
name-brand batteries both to avoid leakage problems and so that they will last
throughout the whole unit. The third potential problem is the danger of cuts
from broken bulbs. If a bulb is broken, the students should have the
opportunity to observe the inside of the bulb, but the bulb and its pieces should
be handled carefully.
©2001 American Association of Physics Teachers
Electricity
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II. STUDENTS' NOTIONS ABOUT ELECTRICITY
A. Students' Prior Beliefs as Described in the Research on
Student Conceptions
Student conceptions about electricity and electric circuits is one of the more
extensively studied topics in physics although student notions about electricity
have not been studied to the same extent as elementary mechanics. From junior
high school to college age, students have been found to display a reasonably
predictable set of ideas about electricity, despite the typical instruction which
they might receive during elementary, junior high school, and high school. In
this section we describe frequently found aspects of student thinking about
electricity.
There are three important aspects of student ideas about electricity which may
explain many of the details described in the research: the undifferentiated nature
of their ideas about these phenomena, the tendency to think as if some material
substance and its motion is responsible for the phenomena, and the tendency to
think locally or sequentially about events in circuits. First, with respect to
electricity in particular students are not thinking about current or electricity as
we think of them, but of some undifferentiated entity, which they call electricity,
that is much more like energy than current—often in interviews they use a
whole list of terms instead of just one, as if they all mean the same thing. Hence,
it is very misleading to suggest that students have notions about electrical current
when they come to class.
Second, students think of this undifferentiated entity, electricity, and the
subentities, current and electrical energy, as if there were a material substance
which flows from a source in the circuit to other points in the circuit and which
gets used by other circuit elements. This tendency to think in terms of material
substances has been noted in studies of student thinking in other areas of
physics, heat and light among them. The "material substance" notion can serve
to explain why direction of flow is important to them and their tendency to
predict bulb brightness based on location in the circuit. The apparent tendency
to think of batteries as sources of constant current or of the same current to all
circuits or as a storage container for electricity and to think of bulbs as
consumers, rather than as resistances is consistent with these general notions.
Because these notions work well enough for everyday activities, a need for a
more sophisticated model of "electricity" flow does not normally occur.
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©2001 American Association of Physics Teachers
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A third, but not independent, tendency in student thinking about electricity
which has been identified is called "sequential" or sometimes "local" reasoning
about circuits. Students tend to think of this substance, electricity, which comes
from a source such as a battery and which encounters various circuit elements
one at a time as it flows from the battery through the wire around the circuit.
What happens at each element seems to depend somewhat on what has already
been encountered, mostly on the specific element in question, but definitely not
on what the substance may encounter next or further down the line. Sometimes
the students' ideas seem as if only the circuit element being directly considered
matters (local consideration) and its relation to the other elements in the circuit
(global consideration) is not important. A manifestation of this is the tendency to
assume that the addition of a resistance always makes the circuit resistance go up
and vice versa for the removal of a resistance.
Conceptions that Characterize Student Thinking about Electricity
1. Electricity as an undifferentiated entity which includes energy, current,
voltage, etc., sorts of ideas
2. Electricity as a "substance" which flows from a source
3. Sequential or local reasoning about events which occur in a circuit
These notions seem to be well entrenched and typical instruction does little to
make changes in these ideas. They even remain in our language about electrical
circuits. For example, it is common among technicians, engineers, and physicists
to refer to a battery or power supply as "delivering" a certain current. Even the
very term "power supply" conveys this notion. Unfortunately, using the term
"generating" in this case hardly helps distinguish the two pictures of electricity.
One has to ask to what extent do many of these more highly trained individuals
still harbor the general kinds of thinking about electricity described above?
On a more pedestrian level, students do not always easily relate aspects of circuit
diagrams with real circuit elements such as wires, bulbs, batteries, etc. This is not
too surprising given that they previously have neither needed to use such a
representation nor had any real experiences which would give them practice at
such connections between the representation and the actual circuit.
From a slightly different, but complementary, point-of-view researchers have
noted that students appear to apply several different models for the functioning
of electricity in circuits. These models are not always explicit to the students
themselves, but student behavior is consistent with some of these models. They
are not necessarily models in the scientists' sense, but they are mechanisms that
can be mentally "run" to make predictions about electric circuits. These models
are listed in the following table:
©2001 American Association of Physics Teachers
Electricity
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Student Models of the Behavior of Electricity in Circuits
1. Unipolar:
electricity comes from only one pole of battery, no return
wire needed (also called total consumption model)
2. Clashing:
electricity comes in opposite directions out of each end of
battery and clashes at the bulb causing light
3. Consumption:
electricity comes out of one end of the battery and gets
"used up" as it passes through circuit elements, some gets
back, so a return wire is needed (also called the partial
consumption model)
4. Sharing:
electricity is shared by all circuit elements
Often the model that scientists use is not in evidence in any student's thinking.
We will call it a constant flow model so as not to prejudice thinking about it. Its
main characteristics are that there is something that seems to circulate around
the circuit essentially unabated (the current) and something else that seems to be
dissipated (energy).
Note that some aspects of the characteristic conceptions about electricity and
some aspects of the models of electrical circuit behavior are compatible with the
ideas about electricity we would like them to have developed by the end of this
unit. Hence, our focus should be to encourage students to consider at all times
what is worth keeping and what should be abandoned about the various ideas
that occur to them.
B. Conceptions that Students Might Develop
In the Electricity unit students are invited to consider a number of specific
situations in which their predictions based on the conceptions they already have
are not in agreement with the outcome in those situations. As a result, the
students, if given encouragement, may modify their ideas to be something more
like the scientists' view. The view that we find students are able to develop in
this unit and we hope to have all students develop at this level goes something
like the description below.
Electricity seems to be a set of phenomena explainable in terms of interactions of
charges and the movement of those charges which seem already to exist in the
wires through which they move. In addition to something that appears to be
used up, there appears to be another aspect of electricity which circulates
completely around a circuit with no losses. This latter we call current and
associate with the rate of passage of charges. Also, bulbs do not consume the
charges, but act to resist the passage of charges.
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©2001 American Association of Physics Teachers
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These charges move in response to forces which, on an individual, microscopic
scale, are like the forces between charged objects, but on a global circuit level this
influence on the motion of the charges is attributed to voltage or potential
difference provided by a battery or other charge-accumulating device such as a
capacitor. In this view batteries do not create the charges, they are already there,
but batteries move the charges. In this role, batteries are not unique. Charges
squeezed together in a storage device such as a capacitor, push each other out
given the opportunity and can thus act as a source of moving charges for a time.
Other mechanical devices apparently accomplish the same thing such as the
Genecons which can be used in the unit.
Unless there is an unbroken path through each of the elements of a circuit, in one
end and out the other, connected by wires in the form of a closed loop around
which the charges can circulate, there will be no long-term circulation as there
will inevitably be a build up of charge somewhere, which will eventually build up
a repulsion to match any other driving forces in the circuit causing the
movement of charge to stop.
This new notion of electricity that differentiates various aspects of electricity,
which we might refer to as energy, current, and potential difference, raises new
issues. The first of these is the relationship between current, resistance, and
voltage. Fortunately, a pervasive notion that most people already have and
routinely use is that the bigger the effort the bigger the result and the larger the
resistance the smaller the result, resistance and effort held constant, respectively.
This generally "ohmic" notion about the relationship between effort, result, and
resistance seems to transfer fairly easily to the electrical situation for the
students.
To go on with the sort of view we find that students can develop: The behavior
of circuits are viewed in terms of each element functioning in relationship to the
sum of the rest of the components. What happens in the circuit due to a change
in one element, bulb for example, depends on that element's relation to the
entirety of the circuit. The everyday view would be that if a bulb is added then
resistance is added and therefore current would
go down in general. The new
view results in considering whether charges passing through other elements of
the circuit must also pass through this new element. Herein lies the essence of
the distinction between parallel and serial configurations.
With this new view students are able to accurately determine relative
brightnesses and changes in brightness of bulbs in circuits containing five bulbs
or so in varying configurations of parallel, serial, and combinations when a bulb
is added, removed, shorted or opened.
©2001 American Association of Physics Teachers
Electricity
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III. COGNITIVE RATIONALE
Investigation E1: Batteries and Bulbs
Our general approach in the first investigation is to ask students to consider their
ideas about electric circuits and then look at the actual behavior of the circuits.
This serves two purposes. First, it is a basis for eliciting student ideas about
electric circuits, and it gives students hands-on experiences with circuits and their
construction. Since we believe that it is useful and a valuable habit of mind to try
to form a more or less mechanical model in order to make sense of physical
phenomena, the approach is to get the students to suggest and then critique
possible models for electricity based on the circuits in this first investigation.
Investigation E2: "Obstacleness" and "Oomph"
In the second investigation, the students are invited to consider aspects of
electrical circuits for which they already have some ideas and will match their
observations on the circuits. We start by referring to these notions using
nontechnical terms, "obstacleness" and "oomph," and after looking at the
behavior of circuits in this context we introduce the terms resistance and voltage.
The students at this point do not have a physicists' view of the nature of these
terms. This is not possible to achieve in the time available, but it is the case that
they do have suitable views at the level to which we need to go with other
aspects of the material on electricity in this unit.
Investigation E3: Capacitors and Their Effect on Electricity
The purpose of the third investigation is to get the students to consider more
about the nature of this "material" which seems to move around the circuit by
confronting the dilemma of the behavior of the circuits with capacitors. What
possible models could account for the behavior of the circuit? What might one
hypothesize about the nature of the "electrical stuff" and or the capacitor itself?
Does one have to conclude that the flow stops eventually, and what significance
might one draw from this? It is intended that the students will decide that
somehow the capacitor stops the electricity, but not right away and that either
the capacitor as a "container" is elastic or that the "stuff" of the electrical current is
compressible.
Electricity
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©2001 American Association of Physics Teachers
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Investigation E4: Electric Charge
The fourth investigation is intended merely to familiarize the students with the
behavior of charged objects. In particular that charged objects can influence each
other at a distance and that they can be squeezed together even when they repel
each other. Activities E4.3, E4.4, E4.7, and E4.8 plus the letters from Benjamin
Franklin would be the basic material here, anything else in the investigation is
extra and, as such, is optional. This investigation is intended to occupy only a
small amount of time and the relevant parts be done in full class, seat work
mode. The basic idea is to be able to end up with a plausible model of what the
"stuff" of the current might be and from that a model of the capacitor. Be
warned, that the scientists' notion of metals as atoms with some electrons free to
move around is not anywhere near the notion of solids that the students come to
class with. That they have had courses in chemistry and can say things about
atoms and electrons notwithstanding, it is painfully clear that more often than
not this plausible (to us the instructors) model of electricity involving moving
electrons in the metals does not find much to relate to in many students'
thinking. One might want to consider whether or not to even deal with the
notions in this investigation. It might suffice to go with what can be concluded in
Investigation 3 as to the nature of the "stuff" of the current.
Investigation E5: Batteries, Bulbs, and Capacitors
In the fifth investigation, we go back to the capacitors, armed with at least the
ideas that can be generated as of the end of the third investigation and look at
other aspects of the behavior of circuits containing capacitors. The intention here
is twofold. One is to get at the idea that whatever it is that is moving in the
circuit, it must already be there when the switch is closed, hence, the battery is
not the source of the current. The wires are already "full" of the stuff that moves.
How else can one explain that the bulb lights on the other side of the capacitor
and between capacitors, if the capacitors themselves are barriers to the
movement of the "stuff" of the current? The second major idea is to look back at
current in the context of different resistances and try to separate the notion of
the amount of "stuff" of the current and the rate at which it moves, i.e., in
scientists' terms charge from current.
Investigation E6: Circuits with Multiple Paths
The sixth investigation is a look at more complicated circuits. The counterintuitive idea that adding bulbs in this setting apparently reduces the resistance is
confronted. The issue of the batteries running down for various of these
configurations against the previous circuits considered is revisited. At this point
it is also possible to get the students to bring out some other common notions
which do not work in the context of the circuits such as: Batteries always supply
the same current.
©2001 American Association of Physics Teachers
Electricity
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Investigation E7: Electric Circuits
In the final investigation, the students consider combination circuits; ones that
are neither simply series or multiple-path (parallel). Here they have to bring
together all the ideas built so far. Their notions at the beginning of the
investigation have come a long way from what they were when they started,
but there are still vestiges of the old ideas, and the new ones are not always
coordinated with each other. In the previous investigation and this one, the
notion of equivalent resistance is an important issue that can be used to aid in the
prediction of what will happen in various combination circuits.
Electricity
Instructor Materials
©2001 American Association of Physics Teachers
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IV. INSTRUCTOR NOTES
A. Equipment List
Equipment
Per lab group
of 4
Aluminum pie plates
2
Battery D-cell
4
Blue foam insulation square
60 cm x 60 cm
2
Bulb (oblong #47)
4
Bulb socket for oblong #47
4
Compass
1
Copper wire (22 gauge, 6 m)
1
Flashlight
1
Four-battery holder
1
NEC Super Cap™ Capacitor
0.49 F, 10 V
1
Rabbit fur
1
Scotch removable magic tape
1
Sponge
1
Table edge or stand and 1 m
length of wood
1
Tap switch
1
Wire with alligator clips
12
Wire with metal ends exposed
1
Per class for
Demonstration
For
unit
ZiplocTM bags for storage of equipment
Note: In activity E3.3, E5.3, E7.1, and E7.2, two groups must share the following equipment in
order to perform the activity:
NEC Super Cap Capacitors
Four-battery holders
D-cell batteries
©2001 American Association of Physics Teachers
Electricity
Instructor Materials