International Journal of Science Education ISSN: 0950-0693 (Print) 1464-5289 (Online) Journal homepage: https://www.tandfonline.com/loi/tsed20 Mental models of electricity A. Tarciso Borges & John K. Gilbert To cite this article: A. Tarciso Borges & John K. Gilbert (1999) Mental models of electricity, International Journal of Science Education, 21:1, 95-117, DOI: 10.1080/095006999290859 To link to this article: https://doi.org/10.1080/095006999290859 Published online: 29 Jun 2010. Submit your article to this journal Article views: 1118 View related articles Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tsed20 INT. J. SCI. EDUC., 1999, VOL. 21, NO. 1, 95–117 RESEARCH REPORT Mental models of electricity A. Tarciso Borges, Colegio Tecnico, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil and John K. Gilbert* Department of Science and Technology Education, The University of Reading, UK The mental models people use to think about the nature of electric current were investigated. Interviews, based on sequences of prediction–observation–explanation, were conducted with Brazilian secondary students, technical school students, teachers, engineers and practitioners who deal with electricity as part of their daily activities. Four models are reported, showing a possible pattern of progression which may be related to individual’s acquisition of conceptual knowledge about electricity. Introduction Over the last two decades the understanding of physical concepts such as force, energy and motion, have been extensively studied (see Driver et al. 1994). Comparatively few studies of adults’ ideas about physical notions have taken place. Most studies involving adults concern the differences between experts and novices (Chi et al. 1981). Students’ conceptions of electric current have been extensively studied, ranging from the simple notions treated in primary school science up to the more sophisticated notions only addressed in introductory physics courses at university level. The collection edited by Duit et al. (1985) provides an overview of the research conducted up to 1985. Such research has revealed the conceptions that students hold and the difficulties they have in understanding the concept of electric current, even in simple situations. Similar studies have addressed children’ s understandings of the concepts of voltage, energy and resistance (see Duit et al. 1985). The emphasis on an understanding of electricity is justified in terms of its importance as a school science subject and for its pervasiveness in adult life. Most of this research is exploratory and descriptive. The methodologies employed are diverse and are constrained by the age range of the subjects. For instance, most of the studies involving younger children who have not yet studied the subject seek to identify their conceptions of electricity and electric current in simple practical situations. When individuals who have already had instruction in the subject are involved, different approaches can be used. Among them are paper and pencil tests involving diagrams of simple circuits, the solution of problems, and the interviewing and observation of individuals’ actions while completing a task. *Author for correspondence: 0950–0693/99 $12 ´ 00 Ñ 1999 Taylor & Francis Ltd. 96 A. T. BORGES AND J. K. GILBERT This paper discusses what is known about mental models of electricity and presents the findings of an empirical research involving Brazilian students and professionals. Mental models The processes through which we understand a novel situation have been the subject of endless discussion among philosophers, psychologists and cognitive scientists. There is no simple answer to such an issue, though it is broadly accepted that our ability to talk about a phenomenon or an object is closely tied to our understanding of it. Mental models are, therefore, internal representations of objects, state of affairs, of a sequence of events or processes, of how the world is like and of psychological and social actions. They enable individuals to make predictions and inferences, to understand phenomena and events, to make decisions and to control their execution. They are incomplete, despite being structural analogues of the processes taking place in the world (Johnson-Laird 1983). Johnson-Laird’ s concern is not with the processes by which one constructs mental models but with how one uses mental models in giving explanations and making predictions. In his view, our ability to give explanations is intimately tied to understanding: in order to understand any phenomenon or state of affair we must have a ’ working model’ of it. The notion of mental model has been used in research in different areas with different meanings [see Rouse and Morris (1986) and Borges (1996) for a longer discussion of this]. As a result, different terminologies are employed, sometimes leading to confusion. For some researchers a mental model is just a representation of some aspects of the world, whereas for others it is an analogue of objects in the world. The first sense of ’ mental models’ is essentially pragmatic, but weak since it does not suggest any strong epistemological or ontological commitments. We can speak of the model that someone has about a given issue, without being concerned with the origins of such a model or about how the possession of such a model affects thinking about that issue. The concern is mainly with the content of the mental model. On the other hand, the second sense is stronger and implies that mental models represent aspects of an external reality. Mental models serve as means with which to explain the relation between one’ s cognitive activity and the world. In this view, mental models are unstable, naturally evolving and incomplete (Norman 1983). The views adopted by most researchers may be seen as delimited by these two extreme positions. Models and model-based reasoning have mainly been addressed by philosophers. Only recently have educational psychologists and science education researchers begun to address the use of models in education (Mayer 1992 Gilbert 1993) and the issue of learning via model construction (Clement 1989). In the task of constructing a mental model of a given system, one simplifies it, that is one selects only some parts of the situation and the relations between them for representation (Gilbert and Boulter 1995). This initially produced model describes the functioning or the behaviour of the target system by referring to the structures and mechanisms existing or assumed to exist in it. The initial focus of the knower is likely to depend on the purposes intended for the model and on his/her prior knowledge of the domain. MENTAL MODELS OF ELECTRICITY 97 Previous research into people’s ideas about electricity Explaining the functioning of a simple circuit Most of the studies of this issue use the same basic structure. Children are given a battery, some wires and a torch bulb and then are asked to light the bulb. While they are involved in the task, their actions and behaviour are recorded or observed (Tiberghien 1983, Osborne and Freyberg 1985). They are then interviewed and asked to explain what they have done and what they were thinking while they were doing it. From the protocols generated, researchers are able to infer the underlying ideas about simple circuits. This type of task has been used with individuals from primary school up to university level to elicit their understandings of electricity. For instance, Fredette and Lochhead (1980) have used it to assess North-American university students’ conceptions of simple circuits, and Osborne used it in a number of studies involving secondary school students in New Zealand and in the UK (Osborne 1983, Osborne and Freyberg 1985). It was found that, independent of their ages, subjects attempted to connect the bulb to the battery according to one of the diagrams shown in figure 1 (Osborne and Freyberg 1985). These types of diagrams may vary slightly, depending on the level of description of the researchers, but can be found in a number of the studies about children’ s conceptions of electricity. Only diagrams (g) and (h), where the light will glow, are perfectly equivalent from a physicist’ s point of view. Such research has showed that, in children’ s accounts, there is a cause located in the battery and an effect, that is the lighting of the bulb. A causal agent acts in between them. This causal agent is named ’ electricity’ , ’ current’ , ’ energy’ , and these terms are often used interchangeably. This causal agent is endowed with a Figure 1. Methods of connecting a bulb to a battery (Osborne and Freyberg 1985). 98 A. T. BORGES AND J. K. GILBERT number of properties. it is thought to move, normally with great speed, according to one of the following schemas: Leaving one terminal of the battery and moving to the bulb; Leaving both terminals of the battery simultaneously and moving along both wires into the bulb; Moving from one terminal through the circuit to the bulb and hence to the other terminal, in a cycle. Kärrqvist (1985) found six mental models among secondary students, which encompass those proposed by both Osborne (1983) and Shipstone (1984, 1985). These are: Unipolar model: Named by Osborne (1981), in this model there is a flow of current from the positive terminal of the battery to the base of a bulb, where it is all used up. The second wire is seen as unnecessary or just an extra wire required to light the bulb, but with no active role in the circuit. This is similar to Fredette and Lochhead’ s (1980) ’ sink model’ . It is usually found among younger children (Osborne and Freyberg 1985) and tends to disappear with instruction. This model implies that current is not distinguished from energy and that it is not conserved. Furthermore, the bipolarity of both the battery and of the bulb is not acknowledged. Two-component model: ’ Plus’ and ’ minus’ currents travel from the battery terminals to the bulb where they meet and produce energy, lighting the bulb up. It is similar to Osborne’ s (1983) ’ clashing currents’ model and to Shipstone’ s (1984) ’ model 1’ . Such a model is particularly popular among children from 10 to 13 years old, but it is practically absent by the end of secondary education (Osborne and Freyberg 1985). Closed circuit model: According to this model all the circuit elements have two connections. Current circulates around the circuit in a given direction and the circuit only functions when the switch is closed. Current flow through a resistive circuit element liberates energy. There is no equivalent model in the studies by Osborne and by Shipstone. This model recognizes the bipolarity of circuit elements but it suggests that current is not conserved, perhaps because of a lack of differentiation between current and energy. Current consumption model: Current is described by means of a timedependent sequence of events. Current is consumed as it goes through resistive circuit components, though a fraction of it returns to the other end of the battery. This model is similar to Shipstone’ s (1984) ’ sequence model’ and to Osborne’ s (1983) ’ attenuation model’ . It implies a compromise between the notions that a current circulates the circuit and that it is used up as it goes through each component of the circuit. Constant current source model: It encompasses bipolar circuit elements, the circulation of current in a cycle and the need for a closed circuit. However, the battery is seen as a source of constant current. that is, the current supplied by the battery is always the same regardless of the circuit features. It is recognized that the battery ’ wears out’ with time and that this is the only source of current variation. According to this model, two bulbs share the current, whether they are connected in series or parallel. A similar MENTAL MODELS OF ELECTRICITY 99 model is described by Cohen et al. (1983) and it shows most of the features of Shipstone’ s (1984) ’ sharing model’ . According to Shipstone (1984) this model results from the assimilation of some rules about circuit functioning into children’ s models. For instance, the rule that identical bulbs connected in series (or parallel) shine equally brightly. Ohm’ s model: A current flows around the circuit transmitting energy. Current is conserved and well differentiated from energy. The circuit is seen as a whole interacting system, such that a change introduced at one point of the circuit affects the entire system. This corresponds to the ’ scientific view’ and has also been found by Osborne (1983) and Shipstone (1984). This model becomes increasingly popular as students grow older, perhaps as a result of instruction. Mental models of electricity Most studies of children’ s models of electricity so far have been mainly concerned with the functioning of simple circuits. Only a few have examined the nature of electric current, resulting in only a partial picture of students’ mental models of the domain. Many of these studies make use of quite similar probes, leading to very similar outcomes. While this improves the reliability of the findings, it tends to turn them into a standard form of conceptualizing such models. It is possible to infer from such studies, for instance those of Osborne and Freyberg (1985) and Shipstone (1985), that children’ s understanding of simple electrical circuits improves with age and instruction, from simple intuitive mental models towards some version of the consensus, socially agreed, model. Only a few studies, by White and Frederiksen (1987) and by Eylon and Ganiel (1990), address the issue of possible model progression within the context of cross-age studies. Both the studies by Osborne (1983) and by Shipstone (1985) show the popularity of different mental models as a function of age, but they both do not address explicitly what is changing in these models. The problem with the word ’ model’ as it is used in most previous research is that it refers to the content of children’ s knowledge about electricity or is a synonym for particular conceptions of how a circuit works, that is, to refer to the mental models of current circulation. A model of electricity involves several interrelated concepts and science teachers may have little empirical basis on which to decide which concepts are more important and why. There is no doubt that such studies provide priceless information on how students reason about the functioning of simple electric circuits and about their knowledge of scientific vocabulary. However, very little is provided concerning causal or explanatory models in the sense used by Harré (1970), White and Frederiksen (1990), or Brown and Clement (1989). This must be understood as a consequence of the researchers’ purposes. There does seem now to be a growing consensus that, if science education is to provide more than simple knowledge of science content, that is, of specific facts and laws of science, then it must elicit how students acquire and use mental models to think of the physical world and how these models evolve with age. What has emerged from such studies is that children’ s mental models of electricity involve a number of different aspects. These are the: 100 A. T. BORGES AND J. K. GILBERT (1) differentiation of basic terms used to speak about electricity, like current, electricity and energy; (2) recognition of the bipolarity of batteries and other circuit elements; (3) recognition of the necessity of a closed circuit if a current is to circulate in it; (4) issue of the conservation or non-conservation of current; (5) effects of electrical resistance on current; (6) models for current circulation; (7) nature of electric current. Most of the proposed models of electricity deal with only few of such aspects. For instance, items 1, 3 and 5 predominated in the early studies (Osborne 1983, Tiberghien 1983). With the use of slightly more complex circuits, the other aspects were considered a little later (Shipstone 1984, Kärrqvist 1985, McDermott and van Zee 1985). Some aspects are likely to occupy a more central place in children’ s mental models so that instruction may affect them to different degrees. A child who cannot differentiate between current and energy properly is unlikely to adopt a view in which current is conserved, for instance. Research findings also suggest that children are more prone to change their views about some of the above mentioned points after instruction than about others (Osborne 1983, Shipstone 1984, Arnold and Millar 1987). Thus children can, after instruction, recognize batteries and other circuit elements as bipolar devices and the need for a closed circuit if current is to circulate in it (Osborne 1983, Psillos et al. 1987, Cosgrove 1995). That suggests that their models of current circulation have changed, as has been found in a number of studies (Shipstone 1984, Osborne and Freyberg 1985). Other aspects of children’ s models of electricity, however, are more resistant to change, for instance those involving the conservation of current. This becomes a really critical difficulty as students progress in their studies to consider more complex situations, such as those involving combination of resistors in series and parallel (McDermott and van Zee 1985, Shipstone 1985), and when they start to learn about the microscopic processes going on in a circuit (White and Frederiksen 1987, Eylon and Ganiel 1990). Some researchers point out that the problem is with the lack of differentiation between current and energy (Arnold and Millar 1987), while others suggest that what needs to be addressed is how children think of the nature of current (White and Frederiksen 1987, Eylon and Ganiel 1990). The nature of electric current and electric processes as well as the role of other circuit components are addressed in a few studies to a varying degree. Eylon and Ganiel’ s study (1990), although not aiming to describe students’ mental models of electricity, produced a large amount of evidence regarding reasoning about electricity. The emphasis was on the microscopic processes taking place in a circuit passing through transient states and the relationship of such processes to the macroscopic behaviour of the circuit. Stocklmayer and Treagust (1996) focused on the images and metaphors that novices and experts evoke to speak and think of electricity. They found that teachers held a mechanical model of electricity in which electrons are seen as minute hard balls moving along ’ tunnel-like’ wires. Apparently these teachers rely on a functional vocabulary to describe the circuit functioning and do not refer to electric fields. This picture contrasts with the more ’ global and holistic’ views of experts. This study indirectly lends support to our 101 MENTAL MODELS OF ELECTRICITY thesis that subjects’ mental models progress as they acquire more conceptual knowledge of the field. In conclusion, while aspects related to how a circuit works are well understood, more emphasis needs to be put on the study of how subjects’ mental models of electricity evolve as they acquire experience and conceptual knowledge of the subject. That is important for the purposes of informing curriculum planning and the devising of more efficient teaching strategies and assessment. it involves the study of how subjects go about understanding valid scientific explanations of it, for instance, by relating the microscopic processes taking place in a circuit to the observed behaviour of its components or by explaining electric and magnetic interaction in terms of fields. In a certain sense, this calls for an exercise of stretching the notion of model (Gilbert and Boulter 1995) being used in science education research. The study The study reported here involved Brazilian 15- to 17-year-old secondary students and three groups of professionals whose daily jobs involved electricity. The first group comprised physics teachers, the second a group of electrical engineers and the third a group of practitioners. The practitioners were electricians or schoollaboratory assistants, most of whom were partially schooled and had no formal instruction in the subject. They had attended school for four years on average and only two of them have completed their primary education. Table 1 gives the details of the sample population. The younger students were attending the first year of secondary education. In the eight years of elementary and middle school, which is normally called the first grade or primary school in Brazil, they had only met general notions about electricity, but had received no instruction about electric circuits. Electricity and electromagnetism are taught at the second year of secondary education (age 16). The other two student groups differed in the purposes and orientation of their study of electricity. Data were gathered by means of semi-structured interview which consisted of a number of simple experimental situations involving prediction–observation– explanation (White and Gunstone 1992). This type of probe has been shown to be a useful way of uncovering individuals’ understandings of concepts and beliefs Table 1. Population details. Group PAL TAL TAC TEC ENG PRO Description First-year secondary students (age about 15). Had not studied electricity and magnetism yet Third-year secondary students (age 17–18). Had studied electromagnetism in the year before Third-year technical school students (age 17–18). Had studied electromagnetism in the year before Partially schooled practitioners in areas related to electricity. No formal instruction in the subject Electrical engineers with more than two years of work experience Secondary physics teachers, most with long teaching experience Number of subjects 9 9 10 10 7 11 102 A. T. BORGES AND J. K. GILBERT (Champagne et al. 1985). The same set of questions (see Appendix) was presented to all the interviewees, but the follow-up depended on the responses given. The interviews were audio-recorded and transcribed soon after the event. In designing the research probes we were confronted with the need to think of interesting situations which could be answered by all the interviewees independently of their level of schooling. For this reason, only very simple circuits were used. All the interviews were conducted in the Portuguese language, as was the entire analysis. In translating pieces of conversation with particular interviewees, the standard form of English was adopted. We have attempted to portray what the respondents might have meant by particular responses and actions. Four models of electricity were identified among the population studied: electricity as flow, electricity as opposing currents, electricity as moving charges and electricity as a field phenomenon. The models identified in this study are somewhat different from the models of electricity reported in previous research (Osborne 1983, Shipstone 1984, Kärrqvist 1985, Heller and Finley 1992). A number of reasons seem to account for the differences. In first place, the subjects in previous studies were typically secondary students with ages ranging from 11 to 16 years, although students at the end of primary school have also been studied (Tiberghien 1983). None of these studies inquired into how adults, both unschooled and professionals with an university degree, thought about electricity, except those of Stocklmayer and Treagust (1996), and of Heller and Finley (1992). This latter piece of research was exclusively into how elementary and middleschool teachers at rural schools in the USA conceptualize electric current. The study only concerned those individuals holding alternative views of it: it did not include in the sample teachers who already have ’ an essentially correct model’ (Heller and Finley 1992). The study by Stocklmayer and Treagust (1996) involved adult apprentices and technical-school students as well as physics teachers and university lecturers. Their findings support some of the models identified in the present study. Electricity as flow This model is characterized by a poor differentiation between the scientific notions of current, energy, electricity and voltage. Current is seen as ’ something’ flowing through the circuit, from the battery to the bulb very much like water in a hydraulic circuit. This thing flowing through the circuit is sometimes referred to as energy, sometimes as electricity or simply as the current. Subjects frequently refer to its unseen nature. The battery is the source of the energy/electricity which flows through the circuit. The need for a physical link between the source of agency and the object which is acted upon was obvious it is suggested by a number of everyday situations, for example a bulb only lights when properly connected to a battery. Students usually adopted views similar to Andersson’ s (1986) ’ experiential gestalt of causation’ . Practitioners, on the other hand, tended to see the situations from the point of view of what is required for their function. They emphasized some operative rules which contain the knowledge applicable to specific situations and elements for troubleshooting. For instance, the rule that ’ only a complete circuit functions’ . Terms like ’ energy’ and ’ electricity’ were used to designate the material substance flowing in a circuit. The battery was imagined as a passive container that MENTAL MODELS OF ELECTRICITY 103 only stores electricity and wears out as its content is used up in the circuit elements. Current travels quickly around the circuit and is used up in the bulb. Subjects holding this model did not describe a circuit’ s behaviour in terms of internal mechanisms and processes. Their descriptions were rather based on the association of perceived events and effects. This model was found exclusively among first year secondary students (age 15) and partially unschooled practitioners. All the first-year students holding this model tried to connect the bulb to the battery using only one wire, which indicates some form of ’ unipolar’ or ’ sink’ model. They required some support during the interview in order to light the bulb. For instance, when an interviewee faced difficulties he/she was given a bulb socket. The existence of two separated terminals suggested the way to complete the task and all of them succeeded in lighting the bulb in this way. This indicates that the major difficulty for some of them was to recognize the bulb as a dipolar device. None of the practitioners required any kind of support in presenting their ideas. Luiz’s interview Luiz started to work as an electrician some 15 years before these interviews were conducted (1994), after working for some months as an apprentice. He had gone to primary school for two years and he says that he can ’ read, more or less’ . He used to live in a rural area before migrating to a city, where he worked as a labourer on building sites. Luiz has never had any formal instruction in electricity, and has only hands-on experience. He associated electric current with circuits, and with the notion that there is something passing in the conducting wires of a circuit. S: I understand that current . . . is a circuit . . . It is the positive wire . . . the live wire. I: Is there something in the wire? S: Only electrons and copper, isn’ t it? The wire is copper. I: And what is this electron? S: It’ s the voltage that runs . . . It comes from the power station and passes along the wire. I: How is that? S: Oh, that’ s hard to explain. I haven’ t studied this. Luiz was asked to light a torch bulb using a battery and some pieces of wire, but he did not feel confident to do so. He did manage, however, to light the bulb with relative ease. Luiz explained that what causes the bulb to light is the existence of a complete circuit. Current and electricity do not take part in this explanation. S: I have to use two wires, haven’t I. I never tired this before. I: Is this different from a house wiring? S: The voltage is different because this is DC. But the wiring is the same . . . To install a switch I have to do the same as I’ m doing here . . . that is, I think I have [he succeeds]. I: What is it that makes the bulb light? S: What lights it up is the neutral and the live . . . they go into the bulb, then to the switch and returns to the bulb I: do you have neutral and live in here? S: Here? I must have. I: which one is the neutral? S: This, the bottom of the battery . . . but I’ m not sure, ’ cause I have no experience with this. 104 A. T. BORGES AND J. K. GILBERT In Luiz’ s view, the battery only connects one wire to the other. Although he thought of it as a reservoir of electricity, it has only a passive role. The bulb lights up because it completes the circuit, putting the ’ neutral’ wire in contact with the ’ live’ wire. He appeared to suggest that he bulb is short-circuiting the neutral and the live, and in this way producing light. Luiz appeared to think in terms of what is needed for lighting the bulb, which is more effective in practical problem-solving situations. He mentioned a number of technical terms, like current, energy and voltage, as interchangeable. I: What is going on in the battery while the bulb is lit? S: One end is meeting the other and providing passage, or else it wouldn’ t light up. There are two parts here in the battery – top and bottom. If I connect them I’ m closing a short. I: And in the wires, what is going on? S: It’ s passing current through them. I: How is that? S: Electricity . . . the electric part. I: Is there anything passing in this wire? S: Yes, there is electricity. I: Is it the same in both wires? S: No, one is neutral and the other is live. I: What happens when they arrive at the bulb? S: It lights because it closes . . . closes one with the other. I: Where does this electricity come from? S: From the battery . . . it’ s charged but is going to discharge and to wear out. I: How is that? S: Its power will die out. Then it has no more use, it doesn’ t light the bulb any more. Finally, Luiz was asked to consider a normal lighting situation, like in a room in which an incandescent bulb lamp can be turned on and off by means of a switch located in the wall. Luiz explains the switch functioning in terms of completing the circuit. S: The live goes into the switch and form the other end comes out the returning wire that connects to the bulb . . . when you turn it on, you close the live and returning. The neutral is up there in the bulb, then the circuit is complete. I: Why does the bulb light immediately when one turns the switch on? S: Because it’ s closed there, then if you close here it lights quickly. It must light up quickly or else there’ s some problem . . . in the bulb or in the switch. Luiz’ s thinking was procedural. That is, he was concerned with the rules for wiring electrical circuits properly and with safety procedures. His knowledge of the lighting circuit functioning already includes elements for troubleshooting. That is the kind of knowledge that is vital in his work. Electricity as opposing currents In this model current is not clearly differentiated from energy: both terms are sometimes used as equivalents. Current is seen as energy or electricity flowing through the wires in a circuit from both terminals of the battery towards the bulb. It is assumed that positive and negative currents travel along separate wires and that they meet at the bulb to produce heat and light. Thus, this model explicitly assumes a non-conservation of current. The battery is still seen as a reservoir of electricity/energy, which wears out with time as a result of energy consumption in the bulb. A closed circuit is necessary to light the bulb, and current is assumed to MENTAL MODELS OF ELECTRICITY 105 travel fast through the circuit. The role played by a switch in the circuit is not clear: some individuals suggest that it produces current. Students sometimes mention protons and electrons, suggesting that electric current consists of electrical particles moving through a circuit. There is a tendency for students to adopt explanations in the form of a time sequence of events. This seems to arise from the form of system modelling noted by de Kleer and Brown (1981), and by Gutierrez and Ogborn (1992). Practitioners, on the other hand, often refer to a short circuit occurring in the bulb which causes it to light. All the students holding this model attempted to light the bulb using only one connection to the battery. However, all of them succeeded in making it light after being given a bulb socket. Apart from two third-year students, all the other subjects holding this model had not studied electricity before. This model agrees with the well documented ’ clashing currents’ model (Osborne 1983, Shipstone 1984, Kärrqvist 1985). Lucio’s interview Lucio had been working as a electrician for the previous eight years. This involved doing electrical wiring and maintenance of the electrical network in the university buildings. He had worked for some time as a bricklayer helper and then spent about six months as an apprentice electrician on building sites. He had no formal instruction in electricity, only the help of an experienced electrician. Lucio had attended the four initial years in primary school and then dropped out. For him, electricity is a good area to work with: S: . . . But one has to be careful. It’ s a job in which you cannot make mistakes. I: What is electricity for you? S: It’s something that nobody sees. It’ s passing there and no one sees it. I: What comes to your mind when you think of electric current? S: There’ s high and low current. High current is dangerous, depending on the voltage . . . I: You were taking about something that no one can see in the wire. What is it? S: I can’ t explain it rightly . . . because it’s energy we don’ t see it . . . Lucio had no difficult in connecting the torch bulb to the battery. He explained that the bulb lights when it is short-circuited. He meant that the positive and negative wires have to go into the bulb so as to make it light. He thought of current as a sort of energy coming through the wires – positive in one and negative in the other. When asked about what is going on in the battery he answered that he thought that the carbon rod existing within a dry cell is a small generator. Lucio did not refer to any processes taking place inside the battery and so this idea appears to account for the production of electricity in the battery. I: What is it that makes the bulb light? S: For it to light we have to close a short in it. I: what do you mean by closing a short? S: The positive and the negative come together, and then it gets in short . . . I: What is going on in the battery while the bulb is lit? S: I think it has a tiny generator inside . . . which is that carbon stick it has within. I: Have you opened a battery? S: Yes . . . it has carbon which comes right here [at the positive terminal]. I: And in the wires? S: In the wires . . . the current comes from the battery to the bulb. 106 A. T. BORGES AND J. K. GILBERT I: Is it the same thing in both wires? S: No . . . It’ s not the same. One is positive and the other is negative. Lucio did not refer to processes taking place inside the battery and the wires, except that current is passing trough. He thought of a switch in a circuit as a device controlling the passing of current, which feeds the bulb with electricity. He recognized that when the bulb is off there is ’ energy’ in the switch, and thus it may be dangerous to touch it. He explained what happens in the switch in a lighting circuit that causes the bulb to light. S: The bulb lights because the switch sends energy to it . . . When it’ s off it doesn’t send energy. I: Is there electricity in the bulb when the switch is off? S: No, because it is that which sends energy I: Can I handle the circuit without danger? S: No, there’ s energy in the switch. If the bulb is on, then you must turn the switch off before touching the bulb. Electricity as moving charges A number of third-year students, some technicians, and a few engineers, held the view that current consists of electric charges in motion through a conductor. The battery was seen as an active source of electricity, that is, it produces energy which is delivered to the charges by means of a chemical reaction. Bipolarity of circuit elements and the need for a closed circuit were evident. The circuit behaviour was explained in terms of a time-dependent sequence of events, which is likely to have arisen from reasoning in terms of a chain of causes and effects (Gutierrez and Ogborn 1992). The emphasis is on the behaviour of individual components, so that it appeared that subjects holding this model were not able to perceive of the circuit as an interacting system. Energy transformations were frequently described and current was assumed to be conserved. The battery supplied energy to the particles to keep them circulating, and this energy was consumed on passing through resistive elements. This model incorporated simplified mechanisms to account for processes taking place in a circuit. A number of mechanical and anthropomorphic analogies were mentioned to explain the interactions of the particles with the atoms of the circuit components, such as collisions and movement through a viscous medium. Rui’s interview Rui (TAC-09) was in the last year of a secondary technical school doing electronics. He thought of electricity in more practical terms and associated the idea of electricity with devices and apparatus he dealt with in laboratory classes and at home. His view of electric current were operational, that is, related to the kind of knowledge one should have in order to quantify it. S: What comes to my mind when I think of electricity are its everyday uses, mainly in lighting . . . and also its use in electronics in things like a video recorder, compact disc players and liquid-crystal displays . . . When I’ m trying to organize how a given device works . . . then I think of it in terms of components, that is, of coils, transistors and so on . . . I: What comes to your mind when you think of electric current? MENTAL MODELS OF ELECTRICITY 107 S: Electric current? . . . It’ s measured in amps. There’ s a definition we use: it’ s the ordered motion of electrons. But I really think of the measuring unit, the amp . . . It’ s important to know if a given current is large or small and whether it’s AC or DC. Rui completed the practical tasks with ease. He drew on the idea of an electric current as electrons moving through the circuit as being much like cars travelling in a road, one behind the other. His view of electric current implied that electrons move to occupy the place which other electrons have just left. The bulb filament has a capacity of emitting light when crossed by a current. The battery is an active source of electric charges, however Rui only mentioned the chemical reaction occurring in a later part of the interview. S: There’ s a current passing through the filament. Tungsten has the ability of giving off light when circulated by an electric current. I: What is going on in the battery while the bulb is lit? S: It releases negative charges that go on to the positive terminal . . . This current flows through the bulb and goes back to the battery. I: And in the wires? S: The current that just left the battery. I: How do you imagine this? S: Oh, it’ s like lots of cars in a traffic jam, only that they are electrons instead of cars, moving one after the other all over the circuit and returning to the battery. I: What happens when the reach the bulb? S: They reach the bulb . . . then there’ s a swap, that is, while some are coming out others are coming into their place . . . and the whole thing start again in a cycle. I Is it the same thing in both wires? S: Yes, there are electrons in both wires, but they keep moving around the circuit. Rui explained that a switch changed the circuit resistance. This is infinitely large when the switch is open, and then current cannot flow. In this view a bulb lights instantly after the switch is closed because current travels at the speed of light. By this he meant that electrons actually move at the speed of light, as he did not employ the notion of an electric field being established in the whole circuit. S: The switch has a metallic slab which links the two wires connected to it. When it’ s turned off, the slab moves aside and there’ s no contact. Thus the resistance becomes infinite and there’ s current. I: Why is it that the bulb lights immediately when the switch is turned on? S: It completes the circuit and current start to circulate from the positive to the negative. I: Why is it so fast? S: Current travels at the speed of light and then reaches the bulb quickly. I: Do electrons move at this speed? S: Yes, I think they do . . . I cannot remember it but I know it’ s very high. Electricity as a field phenomenon This model comprises some knowledge characteristics of the models presented above. Current is distinguished from energy and it is understood as the movement of electrically charged particles under the action of a potential difference. Current only circulates in a closed circuit and is conserved: the bipolarity of the circuit elements is recognized. The battery maintains a deference of potential between its terminals which creates an electric field. This, in turn, causes electric charges to move along the conductor. People holding this model also referred to energy 108 A. T. BORGES AND J. K. GILBERT transformations, but they explained the processes going on in a circuit in terms of an electric field acting on the charges. The circuit was perceived as a whole interacting system, and when a change is made in it an electric perturbation propagates through the circuit establishing a new steady state situation. Although subjects still thought of electric current as motion of charges, they were able to recall field-like explanation when prompted. This model was found to be held by half of the physics teachers and a few technical school students. Almost all the engineers and some teachers and a few students held a different version of this model. It is similar to the ’ field model’ in most of its aspects. However, these subjects preferred to adopt a functional description of the circuit behaviour in terms of energy and voltage. Thus, these subjects frequently referred to energy transformations, but were not able to describe them. Energy transformation seemed to be a process that does not require further explanation. Individuals holding this model variation were likely to see the circuit in a holistic way, in that they did not use time sequence descriptions of the circuit, nor focus on each component’ s behaviour. Nonetheless they did not explain circuit functioning in terms of electric field. For this reason they could not properly explain why the bulb lights immediately on turning the switch on. Valdo’s interview Valdo had concluded his university course in electrical engineering two years previously. He had little experience in the profession, but has worked for some time as an electronic technician and during his university course. During his university years he worked as a teacher, in an evening course for unschooled electricians. he was now studying for a MSc in electronics. Valdo associated electric current to something flowing in a circuit, which may be treated in mathematical terms as a flow variable. S: when I think of current, two different sorts of ideas emerge . . . Firstly there is the idea of something flowing . . . something which is passing. Current has no meaning – it’s only something flowing. A sort of water flow. That is, the analogy that everybody mentions is that electric current is like water flowing . . . I: It is like that then? S: It’ s similar, but it’ s not the same thing. On the other hand, you can think of it as a mathematical representation of the thing . . . In general terms, it’ s a flow variable, similar to those used to represent fluid and heat flow. He clearly distinguished current from electricity and energy, and had no difficult in completing all the tasks. He explained how the bulb lights up when connected to a battery in terms of electrical energy being transformed into heat and light. The bulb filament functions as a hindrance which the current has to overcome. I: What is it that makes the bulb light? S: have an energy source, this battery . . . There’ s a potential difference between the two terminals. When you create a conducting path between the two potentials where energy is stored . . . It’s as if you had a water reservoir and pipes that allow this water to flow through . . . It goes from the higher to lower potential energy . . . If a path exists then a current is created. the bulb is an obstruction in this path . . . In it electrical energy is transformed into light. I: How does this transformation come about? MENTAL MODELS OF ELECTRICITY 109 S: By heating. When the current pass across the bulb filament it transforms electrical energy into light and heat. first the filament heats a lot, somewhat close to a thousand degrees, I think. Valdo though of the battery as an active reservoir of charges. When a closed circuit is provided, the battery transfers charges from one terminal to the other, supplying them with energy. His model of the battery included an internal resistance which increases with time. To explain what is happening in the battery during the operation of the circuit, Valdo used a simple model in which electrons are treated as small hard particles injected by the battery into a tunnel-like wire. This is similar to the type of description encapsulated in the model of electricity as moving charges, used by some of the third-year secondary students. But he was able to describe the situation in more abstract terms, without resource to such concrete images. I: What is going on in the battery while the bulb is on? S: It’ s leaking charge. There’ s a chemical reaction which moves charges across the battery and where the charges acquire energy from. I: Is there energy stored there? S: It’s stored as chemical energy. When a viable path exists the battery discharges . . . It moves charges from one terminal to the other, that is, from one potential to the other . . . With time the resistance of this battery grows because the carbon electrode oxidizes . . . That is, the battery also offers resistance to the passage of current and for this reason it heats. I: And what is going on in the wires? S: They carry energy. I: Why is that? S: The potential difference causes it . . . It causes charges to circulate. You can think of this in terms of a simple model: imagine that there are many small billiard balls, one behind the other and pushing each other. When more balls are injected at one end of the wire, those in the opposite end move out immediately . . . But there’ s a hindrance in the circuit. In it they must be pushed harder and because of friction that produces heating. Thus the filament gets incandescent and lights up. I: You said that this is a simple model. Is there another more sophisticated model? S: I can describe it mathematically . . . You have to think of electrons is an electric field, created by that potential difference. This electric field is transmitted through the wire and makes the charges to move instantly through the circuit . . . I don’ t need to inject charges into the circuit. Valdo coped well with the remaining situations. He again used the billiard-ball model to distinguish the processes of matter and energy transport in a circuit. He was asked why the bulb lights immediately when the circuit is switched on. S: Let’ s imagine the wire as a tube full of small billiard balls, one after the other. When you close the switch you’ re allowing the energy source to apply an ’ effort’ , available as electrical energy. This push is transmitted through the balls quickly – you push the first ball and this impulse propagates instantly through the whole circuit. In this way, energy is transferred to the bulb . . . Electrons’ speed is very low and is quite different from the speed which energy travels. Discussion The present study inevitably raises the issue of the extent to which the form and substance of the questioning used to elicit people’ s views about science topics affects the forms of explanation produced. Individuals apparently can hold different views or ways of explaining things, and can hold different mental models with 110 A. T. BORGES AND J. K. GILBERT which to cope with events and states of affairs (Viennot 1994). A particular view or form of explanation is triggered by the specific question posed. In this way, it does not make sense for an individual’ s response to be analysed without reference to the question he/she was asked. That would explain why a ’ sharing model’ (Shipstone 1984, Kärrqvist 1985, Heller and Finley 1992) was not found in this study. This model is only needed to explain the behaviour of more complex circuits. In this study more complex questions were deliberately avoided, given the varied prior experience of the subjects with electrical matters. It was decided to ask simpler questions so that everyone would have something substantive to say about them. It was found that some features of an individual’ s model of electricity are linked. The differentiation of current and energy, alternative views of the way that current circulates through a circuit and the lack of conservation of current are always associated with less developed models of the nature of the electric current. Such ideas seem to have little or nothing to do with the practical ability to wire a circuit correctly. Most practitioners, for instance, do not have knowledge of the principles governing the working of circuits, although they can cope with their daily tasks. On the other hand, all those subjects holding more sophisticated models of the nature of the electric current were able to make the connections in order to light the bulb and explain the functioning of simple circuits. That suggests that science teachers should try to help students to develop better models for the nature of current. The mental models of electricity identified in this study exhibit a pattern of changes along several dimensions as individuals’ models evolve over time. In particular they show changes in the scope and limitation of models, in the differentiation of basic notion, in the adoption of a richer vocabulary, in the use of more abstract notions, and in the introduction of new entities (Borges 1996). The case studies reported exhibit many instances of such changes. The distribution of models across the population (see table 2) suggests a rough progression from simple phenomenological models up to the culturally accepted scientific models. This progression suggests that individuals develop simple initial models for some aspects of a given domain. These are constructed from everyday knowledge of how things work in that domain. Initially subjects are likely to hold different models for specific situations and, for this reason, when responding to different probes, their knowledge may appear incomplete and inconsistent. They do not master the specific vocabulary used in that domain and basic concepts are likely Table 2. Distribution of models of electricity across groups. Electricity as Flow Opposing current Moving charges Field Mixed PAL TAL TAC TEC ENG PRO 3 – – 3 – – 4 1 1 2 – – 1 5 6 3 4 3 – 2 3 – 3 7 1 1 – 2 – 1 Total 6 8 22 15 5 MENTAL MODELS OF ELECTRICITY 111 to be little differentiated. In this way, initial models make few claims about the nature of the systems dealt with and have little predictive and explanatory power. As subjects acquire knowledge about that subject matter, new aspects are incorporated to his/her initial models. The vocabulary related to such models becomes progressively richer and the basic concepts differentiated. New ontological entities are introduced in the model for that class of phenomena. They may reclassify the ontological status of existing entities (Chi et al. 1994). Therefore subjects are able to tell more complete and sophisticated stories about the events and phenomena in that domain. Underlying such a progression are changes in individuals’ reasoning. They initially focus on the objects forming a situation which are unproblematic and salient. With time, the focus changes to the consideration of interactions between those objects and to the internal structures and entities that give rise to such interactions. These are posited to account for the observed behaviour. It appears that only with deliberate instruction can learners come to adopt more sophisticated models. This trend has been found in other areas of science education (Driver et al. 1994). In a number of cases it was not possible to assign a model of electricity to each of the respondents. In such cases, the responses appear fragmented and the set of conceptions of particular subjects appears to support more than one kind of model. These may be thought as cases of ’ mixed models’ , or even cases of subjects holding a set of fragment ideas, perhaps strongly dependent on the context. This is not, however, an issue specific to this study. It seems to be a general problem when one attempts to elicit others’ mental models, and has been discussed by a number of researchers (see Shipstone 1984, Gott 1985 and Vosniadou and Brewer 1992). In any case, it is not always possible to trace a sharp line delimiting a model’ s or a category system’ s boundaries. Table 2 shows the distribution of models of electricity among the study population. It suggests that models ’ electricity as flow’ and ’ electricity as opposing currents’ are acquired without the need of explicit instruction. On the other hand the models ’ electricity as moving charges’ and ’ electricity as field’ are associated with physics instruction at secondary school and at university level, respectively. A surprising finding is the number of practitioners holding a moving charges model compared with first years, for only two of the former had concluded primary education. Secondary students should have been taught some notions of electricity at primary school. However, this is not actually the case, because most middle school (ages 11–14) Brazilian science teachers usually have a background in biology or related areas and avoid dedicating much time to topics on physics and chemistry. This also explains why so many 15 year olds faced difficulties in lighting the bulb and appeared initially to hold na¨õ ve models for current circulation, when compared with previous research findings (Shipstone 1984, Osborne and Freyberg 1985). Some of the practitioners revealed their curiosity about electricity during the interview. This had led them to read about electricity in books and magazines. All the three practitioners holding such a model have been working in close co-operation with teachers and engineers. Therefore, knowledge of electricity resulted from their personal engagement and motivation, and was not related to previous academic experience with the subject matter. Mechanical images of electric current, such as the view of electricity as moving charges, predominated among 17-year-old 112 A. T. BORGES AND J. K. GILBERT students and is the main outcome of physics instruction in the subject. The five remaining subjects were found to hold mixed models of electricity. The models ’ electricity as flow’ and ’ electricity as opposing currents’ represent an aggregate view in which the entities assumed to form an electric current are not treated individually. They both make little claim concerning the nature of electricity and are essentially descriptive models and predominate among those who have not studied the subject yet. They are both too limited to raise further questions about the behaviour of electric current and they do not require experimentation. This is different from the other models, ’ electricity as moving charges’ and ’ electricity as field’ , which are more popular among those who have already studied electricity. These models suggest that a number of phenomena related to electric current have to be explained and are amenable to empirical testing. For instance, that there is a relationship between the intensity of current, the potential difference of the battery, and the conservation of current throughout a circuit. The images and metaphors that subjects use to speak about a circuit suggest issues to be inquired into. For instance, how is it that electricity is transformed into heat and light? What parameters affect the electrical resistance of conducting wires? The distribution of models across the groups suggests that the first two models arise from everyday encounters with electricity and from knowledge already available in the common culture, while the other two models are acquired through deliberate instruction. Implications for science teaching and research The findings of this study suggest that what is referred to in the literature as ’ sequential reasoning’ or ’ causal sequential reasoning’ needs be studied in more detail. It has been characterized in different ways, for example by Closset (1983), and Rozier and Viennot (1991), and appears often in the form of explanations involving a time sequence of events. It has been explained as the consequence of the adoption of chains of cause and effect (Rozier and Viennot 1991), and from a view of time being a discrete variable instead of being continuous (de Kleer and Brown 1983). However, it may result from a broadly standard form of discourse adopted by science teachers and textbooks. In this study it was hardly found among practitioners and 15-year-old students. As discussed earlier in this paper, the acquisition of a scientific understanding of a given aspect of the natural world is best conceptualized in terms of developing a mental model of it. Such a model can be run in the mind’ s eye to generate explanations and predictions related to the behaviour of that system. Any strategy intended to help students to generate better mental models of a consensus model must give consideration to the dimensions along which an individual’ s model progresses. The present study suggests (Borges 1996) that models progress by a: change in the scope and limitation of models. More sophisticated models address a larger database of empirical observations in each domain. Models may expand to account for novel phenomena and may specialize to exclude anomalies. In this way, old models may still provide adequate explanations in a narrow domain. This indicates an ability to transform an ill-structured problem into a better structured one; MENTAL MODELS OF ELECTRICITY 113 differentiation of basic concepts and the adoption of a richer vocabulary. The basic notions and concepts become better defined and differentiated in the process of acquiring a common language to speak about phenomena in that domain; shift from qualitative to quantitative models. This is accompanied by the use of more sophisticated notions, amenable to mathematical representation. More sophisticated models do not refer to phenomena as they are perceived, but rather to constructs and entities more detached from everyday experience. Grosslight et al. (1991) suggested that students’ notions of ’ model’ themselves evolve; change in ontology. New models often introduce new entities to account for novel aspects of a given domain. In many situations this implies a move from macroscopic to microscopic models; change in the forms of explanation adopted. Initial models tend to be descriptive in character – knowledge of what happens – and no causal mechanisms are involved. Teachers, instructional material, and activities designed to teach electricity, should give special attention in exploring such aspects. Thus, the model of a system or of a domain should introduce the appropriate vocabulary, define the entities involved in producing the system’ s behaviour, and also define how the parts which form the system are interrelated. Otherwise, learners will find it difficult to construct productive models of that system and to speak meaningfully about it. Model progression seems to be a general feature of learners in different science domains (Driver et al. 1994). On the other hand, recent research suggests that students would profit from learning to see a given domain from different perspectives (Eylon and Ganiel 1990, White and Frederiksen 1990). In particular, they should be able to interpret physical phenomena from a phenomenological or macroscopic, point of view and from a microscopic perspective, and to relate one to the other. The identified mental models could serve as a basis for constructing simple teaching models to introduce younger students to a domain. For instance, the model ’ electricity as flow’ appears to be the basic idea underlying most people’ s models of electricity. Initially, teachers can build upon such a notion to explore simple electrical systems. This view of electricity as a kind of fluid may allow younger students to avoid the traps of attempting to adopt a microscopic model too early without the necessary background. Likewise, simple models for batteries, bulbs and other circuit components may be devised. This sort of model works well for practitioners and by itself it is not an impediment for students to acquire hands-on experience and more developed models later on. This is equivalent to the use of ’ bridging conceptions’ (Clement et al. 1989). The idea is to build upon intuitive models that even young students can accept and help them to develop such models. In a later phase a microscopic model will have to be introduced. The early introduction of such kind of model creates an overload for learners because a great number of processes are involved even in simple situations. Therefore, attention should be given to teachers’ own models and to the ’ teaching models’ (Gilbert and Boulter 1995) they choose to use in classroom because of the impact they have on children’ s ideas about a subject matter. In a teaching situation, the terms used to refer to that class of phenomena, the entities and structures that form the system and the way they interact should be explicitly 114 A. T. BORGES AND J. K. GILBERT introduced. Students must make sense of why the different parts of the system they are learning about interact in that particular way. that will allow them to develop their mental models about the mechanisms acting to produce the system’ s behaviour and to generate explanations for phenomena and events associated to it. Conclusion This study has found four models of electricity among a quite heterogeneous population, both in terms of schooling and of hands-on experience with the subject matter. These models attempted to capture the progression in individuals’ models along a number of dimensions: changes in the scope and limitation of models; differentiation of basic notions and adoption of a richer vocabulary; use of more abstract notions and introduction of new entities. In such a view of progression, individuals start with a general model of ’ electricity as flow’ . It pictures electric current as a material stuff flowing form the battery to the elements of a circuit where it is used up. This is referred to as ’ electricity’ , ’ energy’ , or ’ voltage’ . The battery delivers a ’ substance’ to the circuit and for this reason wears out with time. Subjects holding this view do not refer to unseen entities or mechanisms to account for electrical phenomena. The model ’ electricity as opposing currents’ comprises the notion of two distinct types of electricity flowing in opposite directions towards a light bulb, where they meet to produce light. This view suggests that electric current is not conserved. Terms like ’ current’ and ’ energy’ are not differentiated. The model ’ electricity as moving charges’ appears to be the more likely outcome of secondary instruction about electricity. It comprises a description of electricity as electrons moving under the action of a potential difference. Subjects holding this view normally use mechanistic and anthropomorphic analogies to refer to electric current and electric resistance. This model includes new entities and mechanisms to account for some of the microscopic processes taking place in a circuit. Finally, the model ’ electricity as on field phenomena’ includes all of the previous model. However, individuals use on the notion of an electric field or an electric signal which travels through the circuit to explain how a change at a point in a circuit achieves a new steady state and why electric current travels so fast. The approximate sequence of models identified corresponds to different views of the nature of electricity. Model progression appears to be a general trend in other areas of science as well. It exhibits the evolution of learner’ s sense of how things work and why they behave in the ways they do. That suggests that teachers and science educators may profit from knowing typical forms of mental models about each science topic in order to develop new teaching models and teaching sequences to help students to acquire more productive models. Acknowledgements This study would not have been possible without the sponsorship of the Brazilian Educational Agency (CAPES) and from the Universidade Federal de Minas Gerais for A. Tarciso Borges. The people and the organizations involved in this study are warmly thanked for allowing their daily routines to be disturbed in order to conduct the interviews. MENTAL MODELS OF ELECTRICITY 115 References Anderson, B. (1986) The experiential gestalt of causation: a common core to pupils’ preconceptions in science. European Journal of Science Education, 8 (3), 155–171. Arnold, M. and Miller, R. (1987) Being constructive: an alternative approach to the teaching of introductory ideas in electricity. 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(2) You have a bulb, a D-size battery and some wires, I want you to light the bulb up using this material. (a) (b) (c) (d) What What What What makes the bulb light? is going on in the battery while the bulb is lit? is going on in the wires while the circuit is on? is it that sometimes makes bulbs break? (3) Here you have another battery (AA size) which you are going to use in the place of the first battery. (a) What do you expect to happen to the bulb’ s brightness on replacing the battery? (b) Why do you think that? Change the batteries. (c) Does the outcome agree with your prediction? Why is that? (d) What is in the battery that is affected by its size? (4) In a common lighting situation an incandescent lamp is fixed in the ceiling and can be turned on/off by means of a switch on the wall. (a) (b) (c) (d) switch? What does the switch do to make the lamp light when you turn it on? Is there electricity in the lamp when the switch is off? Why do you think so? What is it that makes the lamp light immediately after one closes the (5) What comes to your mind when you think of: (a) Electricity? (b) Electric current? (c) Electrical energy?