Comparing historical models and pupils’ external representations of DC-circuit phenomena

Both historical models and pupils’ external representations of the DC-circuit phenomena are basically qualitative and descriptive explanations of observed phenomena. These models are also based on the same kinds of simple circuit experiments like constructing a circuit, adding more batteries to the circuit, lengthening the circuit somehow, or changing the circuit in some other way. Pupils’ representations are based on everyday life observations and school learning, whereas the historical models were based on systematic empirical experiments (see section 5).

In this chapter the similarities and differences of the pupils’ representations and historical models of DC-circuit phenomena will be discussed. The comparisons will be done on the basis of descriptions given in the context of pupils’ representations (Kärrqvist 1985; Borges and Gilbert 1999) and historical models (chapter 5). However, the information used is not comparable in every way. The studies reporting pupils’

representations generally are concentrated on causal connections, identified properties, and theoretically orientated explanations of DC-circuit properties. They do not include information on comparative properties or empirical laws, which play an important part in the historical models. Thus, the comparisons are made only when applicable.

Electric current like a substance

Borges’ and Gilbert’s first model (BG1) “Electricity as flow” defines electric current as a flowing material substance, which moves in the circuit like water in a hydraulic circuit.

This model is according to Borges and Gilbert particularly popular among children, who have not schooled yet. The same representation also appears in Kärrqvist’s classification in the Unipolar model (K1). Furthermore, a substance-like picture of electric current can be seen in the models of K2 and BG2.

These pupils’ representations include a picture of the electric current, which is very similar to Volta’s historical model of an electric fluid. As discussed in section 5.3 the historical model of electric fluid includes a conception of an electrified substance, which moves as “a package” in the circuit.

Volta’s model of electric fluid also speaks about the number of existing fluids or electric current. The model supports the old controversy of a one-fluid model against a two-fluid model. In this respect pupils’ representation of two opposing currents, which appear in many studies and has been called the ”two-component model” (Clashing currents, Electricity as opposing currents), is very natural. The step from a two-component model to one single electric current can be seen as part of the evolution of an external representation for electric current.

In the historical model of electric fluid an understanding of the instant nature of the phenomenon is very clear. The electric current is understood to be generated instantly all

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over the circuit. This conception may not be obvious in pupils’ representations, except in K6 the highest level of models. Thinking according to representations K1, K2, K4 or K5 (or BG1, BG2) may lead a pupil to the incorrect conclusion that the substance-like electric fluid does not arrive simultaneously at different parts of the circuit, but in chronological sequences as it is represented in K5.

In respect to the nature of the electric current the theoretical explanation of the phenomenon the representations of pupils and historical models are close. Though Volta’s representations are much more developed in many other aspects of the DC-circuit phenomena his theoretical explanations bear close similarities to the pupils’

representations. On the other hand, it is very understandable that at the beginning of the era of DC-circuit phenomena, there were so many views to think about them that all fields could not evolve at the same speed.

Causal connections between the electric current and its effects

At least models K1 and K2 offer information of pupils’ representations concerning the causal connections between the electric current and effects observed. In pupils’

representations the electric current is either transformed into light and heat in a bulb (K1) or produced by two electric currents in a bulb (K2). In addition, model BG2 describes the heat and light produced when positive and negative currents meet at the bulb.

According to Volta’s historical model of electric fluid (see 5.3.3) the electric current causes observed effect phenomena like contractions and convulsions, different sensations and chemical phenomena. In this phase the historical model does not comment exactly on the process of the effect phenomena: the electric current just excites the effects observed.

Modelling an electric circuit

The representations of the electric circuit vary in pupils’ different models: the K1 electric circuit is the most incomplete regarding only one wire for the electric current. K2 requires two wires, one for each current, and the circulation of the current is not included. The K3-K6 models understand that a closed circuit is a necessity for DC-circuit phenomena: in K3 the stress is on the technical level of model and all that really means is the need to find the closed circuit. The K4 model sees the circuit in an atomistic way looking at the parts of the circuit piece by piece, whereas K5 understands the circuit partly in a holistic way emphasising the need for sharing the current. According to K5 the current strength is the same all over the circuit except for the bulbs.

In the historical model of the closed circuit the circulation of the electric current is clear. Also, the different components of the circuit are seen to be necessary. The circuit is understood as a path or route for the phenomenon, and the model emphasises that all the components are needed in the circuit. In the pupils’ representations the roles of different components of the circuit are not so obvious, not battery is understood to be an important component of the circuit. However, the utilities of the wire and the circular path of the electric current are unclear, this can be seen in the different versions of the required number of wires, as discussed above.

The distinguishing point of historical models and described pupils’ external representations is in underlining the components of the circuit in the historical model. The

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historical model includes a clear picture of the meaning of different parts of the circuit, whereas in the pupils’ models the meanings are weaker.

As frequently reported (1.2.1) students often have a tendency to think of a circuit more locally than as a whole. Model K4 is a clear example of an atomistic representation: the parts of the circuit are looked at piece by piece, not as a whole. In contrast, model K5, includes quite a holistic way of representation. The circuit is seen as a system, which shares the electric current. There exists however a discontinuation point in K5, because the flow of electric current is understood to take place in chronological sequences.

There is also discontinuation in Volta’s historical model of the electric circuit.

Although the electric current is understood to be generated simultaneously in different parts of the circuit, the flow of the current is however thought of locally: In Volta’s representation there is a clear picture of the electric current, which transfers as a package and arrives after a while at another part of the circuit. This is an example of a stage of conceptualisation when the model used is not comprehensive yet, but there are properties in the model, which are inconsistent with each other.

The model of source voltage and battery

It can be said that in the most of pupils’ external representations the meaning of the battery is little overemphasised as the most important part of the circuit. Like Volta, pupils understand the battery as a cause of the current phenomena, but in their models current is too closely linked to the battery. Many pupils think that the battery is a store of current, not a generator of it (K1 and BG1). Moreover according to K5, the battery is a constant current source.

Contrary to pupils’ external representations of the functioning of the battery, the historical contact electricity model clearly attributes the electric-current phenomenon to the contact of two dissimilar metals, and the role of battery as a electromotive agent instead of its store.

From the standpoint of the causal connections of the whole system, the electric circuit, pupils’ representations and historical models have similarities and differences. They are similar to the direction of the cause and effect chain, the cause is in the battery and the effect is in a bulb. According to pupils’ representations the battery is a source and the bulb is a receiver (K1, BG1, BG2, K2, K3), whereas the historical model of contact electricity regards the battery as a mover of the electric current and the light of the bulb to be an effect phenomenon of the electric current.

Current strength and magnitude of source voltage in models

Contrary to historical models of electric current and source voltage, in reported pupils’

representations of DC-circuit phenomena there are no reported results of pupils’

representations of magnitudes for the electric current or magnitude for the source voltage.

In summary, it can be concluded that there are similarities between pupils’ external representations and historical models of DC-circuit phenomena. These similarities show that ways of thinking in science and learning are at least partially parallel, so the historical models of DC-circuit phenomena can really be used as sources of innovation for teaching electricity at school.

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