Download Developing students` understanding of cause and effect

Developing students’ understanding of cause and effect in
electric circuits
Pamela Mulhall & Brian McKittrick
Faculty of Education, Monash University
What actually happens in an electric circuit when we turn on the switch? We can see the
effect (e.g. the lighting of a globe), but explaining the cause is not simple because we cannot
‘see’ what is happening inside the circuit. This session focussed on the difficulty that students
have in understanding electric circuits and ways that might help improve their understanding.
Over the past six years we have worked with Professor Dick Gunstone at Monash University
on two research projects, one concerned with issues related to teaching physics generally and
the other concerned with teaching and learning electricity in particular. During the session
some findings from the first of these projects were presented to highlight just how difficult
students find it to understand and explain the behaviour of simple DC circuits. In the project,
some 200 Year 11 VCE students from a wide range of schools were given a test involving
qualitative and quantitative questions about simple DC circuits. An example of one of the
qualitative questions is shown in Appendix A. 25% of all students gave correct responses to
both parts (a)(i) and (b)(i), i.e. the multiple choice sections of the question, while 10%
showed correct understandings in both parts (a)(ii) and (b)(ii), i.e. the sections asking for
explanations. (The difference in these figures underlines the doubtful value of using multiple
choice questions alone to test student understanding.) These data took into account
consequential errors, with allowances also being made for students who obviously misread
the question, so that their response for part (a) referred to the reading on voltmeter V2, and
vice versa for part (b). The data suggest poor student understanding of electric circuits, a
conclusion supported by the weak overall performance of the cohort across all questions
tested. Generalising these findings to all Year 11 physics students, we list below what appear
to be some common shortcomings in student understanding.
Features of students’ (mis) understandings about electric circuits
Some central science ideas often seem to be missing in student understanding. These include:
1 The moving charged particles that form a current are in and from the materials that
make up the circuit.
2 The moving charged particles (e.g. free electrons in a metallic wire) that form an
electric current are affected by the push and pull of:
 the battery
 other ‘fixed’ charged particles within the circuit (e.g. positive ions in a metal
wire)
in ways that can be considered simultaneous.
3 The battery generates throughout the circuit an electric field which enables the
battery to affect all the moving charged particles in the circuit simultaneously.
(Although ‘electric field’ is not specified in Unit 2 of the VCE Physics Study
Design, it is useful for explaining a range of phenomena in DC circuits.)
4 The effects of the ‘fixed’ charged particles differs at different points in the circuit
(e.g. if a copper wire is connected to a nichrome wire, the electric resistive forces on
the current electrons will be different in the two wires)
5 The hotter a resistor or the brighter the globe in a circuit, the higher is the time rate
of energy transfer from the battery to that resistor/globe.
Misconceptions were common, too. Some of these related to:
1 Students having no clear idea of what ‘voltage’ is. Our research suggests that
texts/teachers often give it a variety of meanings (e.g. potential difference, emf,
2
potential, potential drop) which is confusing for students, particularly in terms of
explaining cause and effect. (Indeed we suggest not using the term ‘voltage’ at all,
such is the general confusion about its meaning.)
Students not seeing that a circuit is a system and that changes to one part can affect
another.
In pondering how students’ understanding might be improved, our thinking has been
informed by the second of the projects mentioned above, in which we have looked at the
treatment of electricity in a range of texts and interviewed their authors, and physics and
general science teachers about issues related to teaching electric circuits. What stands out is
the exclusive reliance by many texts, teachers and students on V = IR to explain what happens
in a circuit. Yet while some students are able to successfully use this to predict and explain
various circuit behaviours, as the data we described above shows, the majority are unable to
successfully do this. (Indeed we would argue that even the ability to correctly use V = IR in an
explanation may reflect the ability to think logically rather than demonstrate understanding
per se, an issue we did not explore in the session.) Thus we suggested that, in addition to V =
IR, students need exposure to alternative models and analogies for electric circuits so they
have a range of ways of thinking about and explaining circuit phenomena – ‘one size’ does
not fit all! A variety of models and analogies, each having their own particular strengths and
weaknesses, were presented at the conclusion of the session, and ways of using these
suggested, as outlined below.
Developing students’ understanding of electric circuits
1
1
Use a variety of models/analogies:
 Bicycle chain: useful for developing the idea of a system; that the emf gives
energy to the circuit as a whole; and that energy is expended by the circuit as a
whole (see diagram in Appendix B)
 Jelly bean role play:
In one version, students, representing charged particles, collect jelly beans,
representing energy, as they pass through the ‘battery’ and ‘give up’ some of
this ‘energy’ as they reach/ pass through different obstacles or ‘resistors’. If
each student represents a ‘coulomb’ of charge and the jelly beans represent
‘joules’ of energy, the number of ‘joules’ given up by each student ‘coulomb’
when they reach a ‘resistor’ can be linked with the idea of ‘volts.’
 Rope: helpful for developing the idea of a system and energy transfer within a
circuit (see diagram in Appendix B)
 Bowling ball: essentially this is a gravitational ‘circuit’. Note that the bowling
balls are falling at terminal velocity in a viscous medium (see diagram in
Appendix B)
 Water in pipes: note that the water ‘circuit’ used in this model is ‘closed’, unlike
the ‘open’ domestic system that supplies water to houses which students are
familiar with (see diagram in Appendix B)
 Electron drift: useful for emphasising that the source of the moving charged
particles in a current is the conductor itself and for distinguishing between the
drift speed of these particles and the speed at which the electric field travels along
a wire (see diagram in Appendix B)
 Traffic – e.g. cars on 4 lane freeway driving onto 2 lane road
 Pushing two cars which are touching each other bumper to bumper1: the energy a
person uses to push two cars (!) is distributed evenly between the two (i.e. half to
each) – an analogy for how a current ‘knows’ how much energy to give each
resistor in the circuit
etc etc
This analogy was given to us by Daniel Gooding, Wantirna College, Wantirna, Vic
(Note that each model/analogy can be used to help explain more ideas than we have
indicated above.)
2
Class discussion identifying links between a specific model/analogy and central science
ideas/concepts. The process of thinking about, and discussing, possible links in itself can
be helpful in clarifying one’s understanding.
3
Class (or small group) discussion identifying limitations of a specific model/analogy as a
means of explaining electrical behaviour. Again, thinking about, and discussing, how a
model/analogy differs from a real circuit can be promote better understanding.
Appendix A
A fixed and a variable resistor are connected in series with a cell. A voltmeter is
connected across each of the two resistors.
V
V
1
2
The resistance of the variable resistor is INCREASED.
(a) (i) Will the reading on voltmeter V1
increase
decrease
remain
unchanged?
(ii) Explain your answer.
(b) (i) Will the reading on voltmeter V2
increase
decrease
remain
unchanged?
(ii) Explain your answer.