INQUIRY-BASED EXPERIMENTS IN THE INTRODUCTORY PHYSICS LABORATORY

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1 INQUIRY-BASED EXPERIMENTS IN THE INTRODUCTORY PHYSICS LABORATORY Robert Ross 1 Abstract This paper will describe a significant effort to improve the second semester general physics laboratory at the University of Detroit Mercy by incorporating inquirybased laboratory experiments. The inquiry-based laboratory experiments are modeled on published research results. We require the students to confront their misconceptions by directing them through a process of inquiry. The students use simple and inexpensive materials to perform experiments on direct-current circuits, magnets, electromagnetism, and optics. They use these experiments to develop operational definitions for technical terms, construct models of the relevant physical phenomena and apply the models in new situations to test their predictive capability. This paper will describe some of the laboratory experiments, and describe their effect on student outcomes. Student attitudes toward these laboratory exercises will also be presented. Index Terms Circuits, inquiry-based, laboratory, physics. PRESENT SITUATION AND STRATEGY There is no shortage of data that shows that students emerge from introductory physics courses with a poor conceptual understanding. In recent years much effort has gone into trying to understand the causes of these difficulties and to find remedies [1]. The Physics Education Group at the University of Washington has developed a curriculum that is more effective than traditional lecture materials. These materials are inquiry-based and use tutorials to guide students through activities that are shown to develop a solid conceptual framework [2,3]. Laboratory-based curricular materials have been developed by other groups and have been shown to be quite effective [4,5]. To fully implement these types of curricular improvements requires the adoption of laboratory based instruction or tutorial sessions. At the present time our college is unable to implement such massive change. Switching to a laboratory-based curriculum requires adding additional laboratory sessions, which is not practical at the present time. The tutorial sessions require frequent meetings in small groups. These changes would require additional laboratory space and faculty. Instead, our strategy has been to institute a change in our laboratory instruction. We have developed a laboratory curriculum based on the references cited above to improve the instruction of the second semester general physics laboratory. Our introductory physics sequence consists of large group meetings (lecture) 3 to 4 times a week and one 3-hour laboratory session each week. The calculus based engineering and science physics sequence meets 4 times each week while the algebra-based pre-medical/pre-dental physics sequence meets 3 times each week. Electric circuits are introduced very early in the elementary education curriculum. Several times during their primary and secondary schooling students have seen diagrams of complete electric circuits and cross-sectional views of flashlights. They have seen sketches of circuits with batteries and light bulbs. It would seem that the concepts of complete circuits and a rudimentary understanding of current flow would be well understood by college students. Nothing could be further from the truth. Most students, up to 60%, have absolutely no experience with real circuits [6]. Our strategy is to provide students with a hands-on experience. We provide simple, inexpensive materials such as lantern batteries, flashlight bulbs, light bulb sockets, connecting wire and switches and guide the students though a series of activities. The activities are structured to force the students to think about the circuits, develop mental models and apply these models in new situations. We give them an opportunity to set up circuits, observe the behavior of flashlight bulbs in different circumstances and make predictions about what will happen as changes are made to the circuit. They are encouraged to predict honestly and they are not penalized for making incorrect predictions. As they proceed through the workbook they answer questions and describe the reasoning that they use to explain the behavior of the circuits and make their predictions. We believe that for the students to have a functional understanding of new concepts it is generally necessary for them to develop operational definitions of technical terms. They must also explain their reasoning verbally either to their instructor or to their lab partners. We require them to do this during the course of their laboratory assignments. They spend much more time writing down their explanations than they do making calculations. We believe that it is necessary to lead the students from patterns of concrete thinking toward those of formal or abstract reasoning. They need practice distinguishing observation from inference and in applying inductive forms of reasoning instead of only deductive ones. 1 Robert Ross, University of Detroit Mercy, Department of Electrical and Computer Engineering, 4001 W. McNichols, Detroit, MI 48219, rossra@udmercy.edu. F1D-1

2 INQUIRY-BASED EXPERIMENTS Electricity: Students develop operational definitions for complete circuits, conductors, and insulators. They develop the important hypothesis that the brightness of a light bulb is related to the amount of current flowing through it. They explore the concept of current as it relates to flow. The students distinguish between the concepts of speed and the concepts of quantity of flow. They are introduced to schematic representations of electric circuits. Electric Current and Circuits: Students develop an operational definition of electric current based on its effect on a light bulb. They use an indicator bulb to measure the current through a battery and observe the effects of different circuit configurations. They also develop the concepts of resistance and equivalent resistance. Series and Parallel Networks: Students identify series and parallel configurations of light bulbs. They practice relating the physical configurations to the schematic ones, and recognize that they bear little resemblance to each other. Electrical Measurements I Current: Students use a magnetic compass to investigate the relationship between the current flow in a wire and the deflection of a nearby compass needle. This relationship is exploited to develop a model for the behavior of a d Arsonval galvanometer. The ability to measure the amount of current flowing leads to the development of an operational definition for electrical resistance. Electrical Measurements II Voltage and Resistance: The students investigate the properties of a voltmeter and measure the voltage differences between parts of a circuit. They develop the correct definition of resistance and apply it to situations where the resistance is constant, Ohm s Law, and where the resistance depends on the current, a light bulb. Electric Power and Kirchoff s Second Law: The students apply their models for current, voltage and resistance to a new situation, one where light bulbs have different resistance. They recognize that the original current model is inadequate to describe the effect of having bulbs with different resistance in series. They develop the correct relationship that relates the brightness of the bulb to the power it dissipates. They apply Kirchoff s voltage law to a variety of situations and observe that voltage differences often exist across the terminals of an open switch. Circuits Containing Resistors and Capacitors: Students use large valued electrolytic capacitors to visualize the effects of capacitors in circuits. They apply Kirchoff s rules and the definition of capacitance to predict the behavior of circuits with light bulbs and capacitors. They investigate the relationship between the circuit conditions and the time it takes for the capacitor to charge and discharge. Magnetic Fields: The students map the magnetic field due to various configurations of permanent magnets with magnetic compasses. They relate the strength of the field to the configuration of field lines. They develop an operational definition for the field and recognize the applicability of the principle of superposition. Electromagnetic Induction: The students develop a method to determine the magnetic field due to a current carrying coil of wire. They compare the current carrying coil of wire to a bar magnet and develop a rule to relate the direction of the current flow to the direction and strength of the magnetic field. They investigate the effect of a changing magnetic field on a coil of wire. They develop a method to vary the magnetic field through a solenoid and study how the direction of the changing field effects the direction and strength of the induced current. Tracing Light Rays: The students develop the concepts of light rays and practice drawing ray diagrams to locate the image of various objects. They investigate the properties of converging and diverging optical elements such as lenses and mirrors. Image Formation and Lenses: The students develop operational definitions for images and see how images are formed by different optical elements like lenses and pinholes. They investigate the role of the lens in image formation and measure the focal properties of converging lenses. The Optical Principles of the Eye: Using a model of the human eye, the students explore image formation on the retina. They describe typical vision defects. They use lenses to correct the common vision defects. The next sections describe three of the experimental procedures in detail. Some of the results of the student experiments are also reported. EXPERIMENT #1-ELECTRICITY The first experiment that the students perform is designed to develop the concept of a closed or complete electric circuit. The students are given a lantern battery, 20 cm of copper wire, and a flashlight bulb. They are instructed that they need to find three different ways to light the bulb using all three of the components. The students are told that time is not a factor and that the amount of time that it takes to light the bulb does not affect their grade or cast aspersions on their intellect or ability. Each time this activity is performed the instructor is amazed that it takes about 45 minutes before everyone in the class has accomplished the task. Contrary to the observations of some other groups, it appears that the best predictor for the time it takes to complete this activity is prior experience [7]. The students are asked to sketch three different arrangements of the components that cause the bulb to light and also three that do not. They are then asked to write a set of instructions that someone unfamiliar with the equipment could follow to light the bulb. By performing this activity the students have developed an operational definition of a complete circuit. They recognize that the bulb and battery each have two separate ends or terminals. They discover F1D-2

3 that it is necessary to use each of these terminals to complete the circuit. After the students have developed an operational definition of a complete circuit, the students test various materials to observe their effect when placed in the path of the current. This experiment develops the concepts of conductor and nonconductor or insulator. The students recognize that the meaning of these technical terms is contained in the concrete experience of seeing the light bulb glow. There is no meaningless jargon such as conductors allow electrons to pass through. The purpose here is to have the students discover that in order for the bulb to light it is necessary to have a continuous path of metal from one terminal of the battery to one terminal of the bulb and from the other terminal of the battery to the other terminal of the bulb. This concept is crucial to the development of the concept of current. The final part of the introductory experiment is to draw cross-sections of an ordinary household light bulb and light bulb socket. To facilitate these drawings the students are encouraged to break the glass envelope of the light bulb and examine the interior. They are required to pay strict attention to the wires connecting to the filament and to indicate where these wires terminate. They are also provided with a disassembled bulb socket. The result of this activity is that over half of the students are surprised that the filament is connected to the side of the metallic base and also to the metallic tip on the bottom of the bulb. They observe that the socket has two terminals. They see, many for the first time that the socket does not simply allow the tip of the bulb to come into contact with the electricity in the wire. The students discover that the bulb must be in the socket in order to have a continuous path of metal from one side of the socket to the other. Often they verbalize that they now understand how Christmas lights work. The concept of an electric current is introduced with the analogy to the flow of water. The students are familiar with this concept but must be made to carefully distinguish between some closely related concepts. They need to compare how the speed of the water is related to the quantity of flow. They need to state what happens when water flows through a pipe or down a river. They need to recognize that as the water flows its speed can change but the quantity of flow remains constant. As the fluid reaches a branch it has the opportunity to split and the total amount into the junction is the sum of the amounts through each branch. We emphasize that these concepts are all equivalent when we consider an electric current instead of a current of water. We infer that the light bulb lights only when an electric current is present. While we cannot see, hear, or feel the current we hypothesize its existence to explain the observed behavior of the battery and bulb. Furthermore, they hypothesize that the brightness of the bulb is an indication of the amount of current flowing through it. We are careful not to say that we expect that the brightness is proportional to the current, just that brightness is an indication of the amount of flow. The students are asked to connect a bulb to the battery as shown in the figure and to compare the current at different locations in the circuit. We are trying to disabuse them of the notion that the current is used up in certain elements of the circuit. FIGURE 1. A CIRCUIT WITH A BATTERY AND BULB. Next they are asked to predict the relative brightness of two bulbs in a series circuit shown below. They are also asked to predict the brightness of each bulb in relation to the bulb from the single bulb circuit. After they complete the circuit and observe what happens, they are to compare the current flow in the two-bulb circuit with the current flow in the single bulb circuit. FIGURE 2. A CIRCUIT WITH A BATTERY AND TWO BULBS IN SERIES. They observe that the two bulbs are of equal brightness and dimmer than the single bulb case. Many students interpret this as being caused by the bulbs using up some of the current. This misconception is partially due to the belief that the battery always supplies a constant current. They are led to compare the brightness of each bulb and interpret this in accordance to our hypothesis of the relation between the brightness of the bulb and the amount of the current. Then they are asked to infer the amount of current flowing in the battery in this case and compare it to the current flowing through the battery in the case of the single bulb circuit. Finally, they are asked to predict the relative brightness of two bulbs in a parallel circuit. This circuit is shown below in Figure 3. They are also asked to predict the brightness of each bulb in relation to the bulb from the single bulb circuit. After they complete the circuit and observe what happens, they are to compare the current flow in this two-bulb circuit with the current flow in the single bulb circuit. They observe that the two bulbs are of equal brightness and equal to the single bulb case. They are forced to conclude that the current flow through each bulb is the same, and is also the same as that through the single bulb in the one-bulb circuit. They are asked to infer the quantity of current flowing through the battery in this case and compare it to the current through the battery in the single-bulb and series cases. F1D-3

4 that the addition of more resistors in this parallel configuration causes the total resistance to decrease. FIGURE 3. A CIRCUIT WITH A BATTERY AND TWO BULBS IN PARALLEL. To reinforce how their model of electric current can be used to explain the behavior of these circuits, the students are asked to summarize their conclusions about the current flow in each of these cases. They are to clearly discuss how they believe the current flows through the battery, how it splits up at a junction and how the amount of current correlates with the observed brightness. They are asked to estimate the relative time each battery will last in the different cases under consideration. Many students use their model of electric current to correctly predict that the parallel combination will last the least amount of time because it corresponds to the case with the largest amount of current flowing through the battery. It is however, not uncommon for students to predict the lifetime of the battery to be equal in the single-bulb and 2-bulb series cases. They verbalize that since the current is the same in a series circuit, it must be the same as the single bulb case and so the battery should last the same amount of time. It appears that these students misrepresent the meaning of the word same. They interpret same to mean the same at all times, when in fact the correct interpretation is that in a series circuit the current is believed to be the same at all places. EXPERIMENT #2- ELECTRIC CURRENT AND CIRCUITS The second experiment that the students perform continues to reinforce the concept of current. They connect a light bulb to the battery and use this bulb as an indicator bulb. This indicator bulb measures the current flowing through the battery; the brighter the indicator bulb the more current that flows through the battery. Next they build a series circuit with two identical bulbs as they did before, but this time they include a single-pole single-throw switch. They predict what will happen and interpret the results in accordance to their model of current flow. They usually indicate correctly that the indicator bulb will dim. This observation is used to introduce the concept of resistance. Each bulb has resistance; it resists or impedes the flow of current through it. The dimming of the bulb indicates a reduction of current through it and hence through the battery. They explain that adding bulbs in series increases the resistance and causes the current to decrease. They add an additional bulb in parallel with one of the bulbs and predict how the brightness of the bulbs will change. Most students are surprised that the indicator bulb brightens. They are now able to correctly interpret what is happening in this situation. They recognize Indicator bulb FIGURE 4. A CIRCUIT WITH A BATTERY AND AN INDICATOR BULB IN SERIES WITH TWO BULBS IN PARALLEL. The model of electric current flow the students developed is adequate to allow them to predict the relative brightness of the bulbs shown in the circuit below in Figure 5. They can correctly predict that A=B>C=D=E by noting that bulbs A and B each receive half of the current coming out of the battery while C, D and E each receive only onethird. They are then asked to predict what will happen if any of the bulb C, D, or E are unscrewed from their sockets. When the model is applied correctly, the students are able to predict that since we are unscrewing one of the bulbs C, D, or E, we are removing a parallel branch from the circuit and the current through the battery should decrease. Since the current decreases A and B should dim. Furthermore, since the current only splits in half for the remaining branch instead of into thirds, the remaining bulbs (2 of C, D or E) must brighten. A B FIGURE 5. A CIRCUIT WITH A BATTERY AND TWO PARALLEL COMBINATIONS OF BULBS. Each semester a few of the more observant students recognize that something is amiss with this analysis. They realize that removing a parallel branch causes the current to decrease through the battery. If the current decreases and yet a path is removed, how do we know that C, D or E must brighten? Why do they not remain the same? This type of questioning will eventually show why we need to modify our model of current flow to include the concept of potential and potential difference. C D E F1D-4

5 EXPERIMENT #9- ELECTROMAGNETIC INDUCTION Electromagnetic induction is one of the most difficult topics covered in the introductory course. In order to use Lenz Law, students are required to keep track of the strength and direction of a magnetic field, keep track of the magnetic flux, and recognize how the magnetic flux changes. They need to understand how the direction of a magnetic field is related to the direction of the current flow in a wire. Many students cannot develop this pattern of formal reasoning. The situation is complicated by their inability to visualize the multiple changes taking place. Consider the situation shown in Figure 6 where we show an ordinary bar magnet and a coil of wire. The students have previously determined that the magnetic field points away from the north pole of a bar magnet. They have also determined that the field strength is greatest in the region of space near the poles. v v S S N N FIGURE 6. A BAR MAGNET NEXT TO A LOOP OF WIRE. Most of the students can correctly indicate the direction of the field due to the magnet at the location of the loop of wire. They can also correctly determine that the field strength decreases if the magnet is moved to the left. A significant number incorrectly assume that the direction of the field changes to point toward the left if the magnet is moved to the left. It seems as if they are confused the arrows used to represent the direction of the field and the arrows used to represent the direction of motion of the magnet. The students use a solenoid to produce a magnetic field. They use a compass to determine the direction of the field and develop a rule that relates the direction of the current flow to the direction of the field. They are then led to describe what happens to the current though the coil as a current controlling switch is opened and closed. They need to verbalize that the process of closing the switch causes the current to increase and that the increasing current causes the magnetic field to increase. They need to recognize that opening the switch causes a corresponding decrease in the magnetic field strength. Next the students use a galvanometer to measure the current induced in a 1000 turn secondary transformer coil when the primary solenoid is inside of it. They use the galvanometer to determine the direction of current that is induced in the secondary coil. This proves to be especially difficult, the students are able to see the direction in which the coil is wound but they have difficulty in recognizing B B from which terminal the current emerges. Once they have correlated the direction of current flow with the reading on the meter they can proceed to relate how the changing magnetic flux induces current. STUDENT ATTITUDES Students generally seem to like the laboratory experiments. They are required to hand in their work at the end of the session so they have no homework. The students were asked to comment on the question, What factors about the COURSE ITSELF make this class a good learning experience? on the end of the semester course evaluations. Some relevant samples of student comments are included below: Lab was effective in learning concepts for PHY 162 lecture. I think we should take the material before we do the lab. It provided actual hands on learning, was related to classroom teachings. Hands on, interesting, let us think independently. It put the physics learned in class to practical application. Put abstract ideas into perspective. Challenged students to think independently. Labs were sometimes difficult to comprehend. It was set up to make you think. I learned more in this lab than in lecture, and I ll actually remember it because I had to figure things out myself. That is an excellent way to learn. I was forced to apply my intellect in order to succeed in this class. Logic and applied concepts were useful. CONCLUSIONS Inquiry-based laboratory experiments are more useful in engaging the students to use higher level thinking skills then typical cookbook type laboratory experiments. When students use the inquiry methods they seem to be more willing to take responsibility for their own learning. The exercises also provide important experience to students who have never had an opportunity to experiment first hand with common direct current circuits. REFERENCES [1] For a recent review of the literature on physics education research see: McDermott, L.C., Redish, E.F., Resource Letter: PER-1: Physics Education Research, Am. J. Phys., 67, 9, Sept 1999, 755. [2] McDermott, L.C. and the Physics Education Group at the University of Washington, Physics by Inquiry Vols. I and II. Wiley, New York, [3] McDermott, L.C. and the Physics Education Group at the University of Washington, Tutorials in Introductory Physics, preliminary volume. Prentice-Hall, Upper Saddle River, NJ, F1D-5

6 [4] Laws, P., Workshop Physics Activity Guide. Wiley, New York, [5] Laws, P., Calculus-based physics without lectures, Physics Today, 44, 12, 1991, 24. [6] McDermott, L.C., Shaffer, P.S., Research as a guide to curriculum development: An example from introductory electricity. Part I: Investigation of student understanding, Am. J. Phys., 60, 11, Nov 1992, 994. [7] Brown, T.R., Slater, T.F., and Adams. J.P., Gender differences with batteries and bulbs, The Physics Teacher, 36, 9, Dec 1998, 527. F1D-6