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Two identical bulbs (e.g. 6V/0,3A, 6V/0,1A), two identical bulbs (e.g. 6V/0,05A), power source (6V), leads, computer, interface and software (e.g. CMA Coach6), current sensor, voltage sensor differential (if computer with interface is not available, ammeter and voltmeter can be used) |
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Before starting this activity students are already introduced the series connection of resistors and the concept of energy and power dissipated in the resistor. In this activity students can solve two problems to investigate on the behaviour of bulbs in a dc circuit. Both of them can be solved as guided (or bounded) inquiry activities depending on the students´ level. In both cases students work in groups of 2-3. Supposing we have two different bulbs and a power source with voltage standard to light each separate bulb we can light the bulbs separately as well as connecting them in series. From understanding these simple experiments new problems can emerge. Problem 1, activity 1,2: If we put two identical bulbs in the holders they shine equally brightly. If we put two identical bulbs (different from the first ones) they also shine equally brightly. When we put two different ones in the holders than one lights up but the other does not (or very faintly) If we let students do guided inquiry they carry out experiments according to the instructions in their worksheets. They investigate what physical quantity is responsible for the bulb brightness in order to find out that energy (or power) dissipated is crucial. They do measurement with the help of sensors on the current through the bulb and voltage across it to determine the power dissipated in each of the bulb. If two bulbs are connected in series, the power dissipated in each of them drops. In steady state, the one that draws power closer to its normal condition will shine; the other one will barely glow at all. Fig. Example of measurement results for two bulbs connected to 6V power source In case of bounded inquiry students get the problem and materials available and they design their own experiment to reason the intriguing behaviour of bulbs. Problem 2, activity 3, 4: If we put two different bulbs in the holders one of them will light up later then the other. There is a noticeable delay between the two bulbs. If they did the previous investigation, they could do this activity as a bounded inquiry to design their own experiment and decide about measurement and analysis of the relevant quantities. Analysing the current, voltage, resistance and power diagrams students can draw conclusions. When connected in series, bulbs are heated gradually, so their resistances change after some time and hence the voltage and power is redistributed. As a result, one bulb gradually lights up (power rises) while the other bulb fades a little (power decreases). Fig. Example of measurement results for two bulbs in series connected to 6V power source The investigation can be extended to exploration of identically labelled bulbs from different sources or even from the same producer but different batches. It can be pointed out that even if the label presents identical parameters, the reality can be different in some cases. The industrial content can be illustrated by examples of bulbs with different energy consumption. The incandescent bulbs are gradually replaced by other types of electric lights towards more energy-efficient lighting. Students can look up some other electric devices and compare the same kind of device made by different producers in terms of the power input. |
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