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A New Analogy Between Mechanics and Electricity: Pupil's

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A New Analogy Between Mechanics and Electricity: Pupil's A NEW ANALOGY BETWEEN MECHANICS AND ELECTRICITY: PUPIL’S MISCONCEPTIONS, PHYSICAL QUANTITIES AND ELECTRICAL COMPONENTS Morge Ludovic and Mercier-Dequidt Clotilde Laboratoire ACTé EA 4281, ESPE Clermont-Auvergne, Clermont Université, Université Blaise Pascal, France. Abstract: The teaching of electrocinetics has always given rise to recurrent learning difficulties, from junior high school to university. Over the last thirty years, research has made it possible to identify families of errors and to infer from these the spontaneous reasoning which causes them to be made (for summaries see e.g. Duit & vonRhöneck, 1998, Mulhall & al., 2001). This paper proposes a new mechanical analogy. The device consists of a string in a closed loop, which has been tied tightly around four pulleys fixed to a square or rectangular base. A weight hanging from a wire that has been wound round a pulley puts the string in the closed loop into motion. Friction can be applied to the string by pinching it. The speed at which the string moves depends on both the weight hanging from the pulley and the elements that are put in the circuit. The analogy aims to enable pupils to change their conceptual approach (from local reasoning to global reasoning). In this paper, the analogies (physical quantities and electrical components) are established for series circuits. Keywords : Analogy, Electricity, Mechanics, Misconceptions. INTRODUCTION : MISCONCEPTIONS IN ELECTROCINETICS Research in science education has led to the identification of several misconceptions in electrocinetics: - The single wire: some pupils think current is carried by only one wire from the generator to the receiver. This misconception is less frequent when pupils work on the notion of a closed circuit, but may recur when they start studying the capacitor charge (Clement & Steinberg, 2008). - Clashing currents: some pupils assume that two currents leave the generator and join up at the receiver, which creates a sort of ‘clashing current’ (Osborne, 1983). - Current is used up as it circulates: some pupils assume that the current gets used up, that its intensity diminishes once it has gone through a dipole receiver in the circuit (Duit & Rhöneck, 1998). - Some pupils think that the battery always gives out the same amount of current, as there is some confusion in their minds between a current generator and an intensity generator. Most of generators are voltage generators. - For pupils, the current reacts as it moves through the circuit in the conventional sense, that's mean it doesn't consider what there is downstream. This misconception explains why pupils think the order of the electrical dipole have an importance andreinforces the idea that the battery always gives off the same amount of current. Pupils here use local reasoning (Closset, 1983), which is also called linear causal reasoning (Viennot, 1996) when pupils describe a succession of elements of local reasoning. - One of the most difficult concepts in electrocinetics is the notion of voltage. Pupils often can’t see the difference between voltage and intensity, and consider them to have the same characteristics (Duit, R & von Rhöneck, 1998). The similarity between the formulae in the Kirchhoff's Current Law (I(A) = I1 + I2) and the Kirchhoff’s voltage law (U(V) = U1 + U2) add to this confusion. AVAILABLE ANALOGIES Analogies have been devised in order to help pupils to reason more like scientists. Johsua and Dupin, (1989) suggested an analogy using rail trucks (which represent electric charges) which workers set in motion by pushing them (picture 1). The trucks are slowed down by a retarder. But this analogy does not allow pupils to work on the difference between voltage and intensity, and leaves no room for experimentation. Figure 1. The analogy of rail trucks Hind, Leach, Lewis and Scott (2005) suggested an analogy (figure 2) with vans delivering bread (which represents energy) to a supermarket (which rerepresents receptor). But this analogy has the disadvantage of substantializing (Bachelard, 1938) energy in the form of bread, and , as in the previous case, does not allow any experimenting. Figure 2. The analogy of vans Hydraulic analogies are frequently used, but may well reinforce pupils’ inaccurate beliefs: for exemple, that the battery (which is represented by the mountain stream) always puts out the same amount of current even if there is a resistor (an obstacle) in the circuit (figure 3). Figure 3. Exemple of hydraulic analogy (Physique, classe de seconde, Nathan, 1997). As for the Hydraulic closed cicuit, the notion of pressure (which is comparable with the notion of voltage (Closset, 1992) is no better understood by pupils than the notion of voltage and pupils have as much difficulty understanding hydraulics as they do electricity. In this analogy, pupils can’t see whether the liquid moves quickly or slowly. So to sum up, we can say that the analogies available to us, have several drawbacks. They do not allow any concrete manipulations and are incomplete or may be just as complex as the target subject. In this context, we have devised a new analogy, the closed loop string analogy. A NEW ANALOGY FOR TEACHING ELECTRICITY: CORDELEC The device consists of a string in a closed loop, which has been tied tightly around four pulleys fixed to a square or rectangular base. Each pulley is fixed to a corner of the base, as can be seen in the diagram below. This device has been invented by Ludovic Morge and Sébastien Masson and patented by Blaise Pascal University (Morge et Masson, 2010). Figure 4. Tthe closed loop string analogy A weight hanging from a wire that has been wound round a pulley puts the string in the closed loop into motion. Friction can be applied to the string by pinching it. The speed at which the string moves depends on both the weight hanging from the pulley and the elements that are put in the circuit. After gradually accelerating, the device settles into a regular speed, with the force of the friction compensating for the weight hanging from the pulley: the total amount of forces exerted on the string is null. If the string is pinched hard, the string stops moving. The string is not equally taut on each side of where it is pinched. It is tauter on the side in which the string is moving. If the string compresses a spring, the string continues moving until the sum of the forces exerted on the string is cancelled out. If the string pulls an inertia disk, the string will move more slowly. In this device, there is an analogy between the movement of the string and current. Similarly, there is an analogy between electric intensity and the movement of the string. Electric current is invisible, but pupils can observe the movement of the string. Electric voltage is analogous to the difference in the tension in the string. ESTABLISHING THE ANALOGIES The tables below show the main analogies which can be established for physical quantities (table 1), components (table 2) and reasoning (table 3). Table 1 Physical quantities Electricity Mechanics Voltage Difference in string tension Intensity Speed at which the string moves/goes round Energy of the generator Potential energy of the mass of the mechanical generator Electrical Work Mechanical work Battery strength Weight of the mass Temperature increasing by Joules effect Temperature increasing by mechanical rubbing Table 2 Electrical components and their analogous Electrical component Mechanical analogy Resistor/Resistance The string is pinched, causing friction (a finger or a weight pressing down on the string) Coil Flywheel Capacitor Spring Transformer 2 coaxial pulleys with 2 strings running through Diode Clam cleat Voltage generator A mass going down sets a pulley in motion Intensity generator A motor continually turning at the same speed Switch Pliers Table 3 Misconceptions that can be worked on with this analogy Pupils think that… Closed loop string analogy shows that… Energy is a matter which disappears through Heat appears at the receiver without matter the receiver disappearing Electricity moves in two different directions The current moves in only one direction in from the battery the circuit There is less current after than before the The intensity is the same before and after the receiver receiver If you put a receiver in a circuit there is no If you put a receiver in a circuit, there is an effect before the receiver effect before and after the receiver Voltage is the same as intensity: no intensity You can have tension without having means no tension. intensity (and the contrary). The battery always gives the same amount of The battery delivers less current if you add a current resistor in the circuit When the capacitor is charging, there is no The intensity is always the same after and current after the capacitor before the capacitor The intensity is not always the same in the The intensity is always the same in the different parts of the series circuit different parts of the circuit Local reasoning Global reasoning CONCLUSIONS AND PERSPECTIVES Many analogies are possible with this device and may help teachers in their teaching of electrocinetics all the way through from junior high school to university. Research is underway to assess the way this device impacts teachers’ practices, and the changes in pupils’ misconceptions. Cordelec analogy works for a series circuit, but dosen’t work for a circuit with branch circuit receptors. In this case, hydraulic closed cicuit could be used. In order to know whether or not the field of mechanics is easier to understand than the field of electricity, two questionnaires has been designed with the same informations, the same questions but not in the same field. One is rooted in the field of electricity while the other is rooted in the field of mechanics.
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