SOTM LAB: P16 OHM S LAW I. TEACHER NOTES & GUIDELINES TITLE OF LAB: Ohm s Law DEVELOPERS OF LAB: John Lane, JD853@maristb.marist.edu Taylor Pancoast, JD573@maristb.marist.edu OVERVIEW OF LAB DESCRIPTION The lab investigates the relationship between voltage and current to demonstrate Ohm s Law for resistive elements. The lab builds on naïve conceptions of current flow (movement of charges), voltage (force pushing the charges) and resistance ( frictional opposition to the current.) In the pre-lab students will use a simple light bulb circuit to investigate the conducting properties of some common substances. They will then be asked to describe what they have observed and suggest explanations. The students will then be shown how to use a lap top computer interfaced with voltage and current sensors to monitor voltage, current and (by calculation) resistance. Finally the students will be asked to plan and carry out an investigation of the relationship between current, voltage and resistance using the demonstrated technology. CURRICULUM CONSIDERATIONS This lab is intended to be an introduction to voltage, resistance, current and Ohm s Law. It assumes that students have studied static electricity, know what a simple circuit is and have some concept of current. If the students are not familiar with circuits, an exercise such as connecting a bulb, battery and wire to light the bulb could be completed to introduce circuits. Setting up the circuit for testing conductivity can also be used as an exercise to review how to connect a circuit. SAFETY CONSIDERATIONS Although the voltages and currents used are small, care must be taken to instill in students proper respect for the hazards of electrical shock along with safe handling habits. Students should also be cautioned about the fragility of computer and electrical equipment.
BACKGROUND INFORMATION A. SCIENTIFIC VIEWPOINT The underlying concept is Ohm s Law, V = IR, where R is the slope of the V vs I curve. Devices which obey Ohm s Law are called ohmic; those which don t obey Ohm s Law are called non-ohmic. B. COMMON MISCONCEPTION 1. Objects are either perfect conductors or perfect insulators. (There is a continuum of conductivity.) 2. The battery provides all the electrons flowing in the circuit. (All matter provides electrons; the battery provides the voltage to move them.) 3. Protons flow in a circuit, not electrons. (Electrons flow.) 4. All objects/devices are ohmic. (Some things are ohmic, some are not.) OBJECTIVES After completing the lab the student should be able to: 1. State the relationship between the variables V, I and R (Ohm s Law.) 2. Use Ohm s Law to predict a missing variable 3. Give an example of an ohmic and a non-ohmic device/object. 4. Find R from data on V vs. I (usually a graph.) EQUIPMENT/MATERIALS PROVIDED BY SOTM Laptop computer with Science Workshop installed Science Workshop 750 Interface including Patch Cord for analog A channel (voltage) Patch Cord for analog B channel (current) constructed using a 1 ohm resistor, voltage patch cord and two banana plugs (2) Patch cords with banana plugs for voltage output AC/DC Electronics Laboratory PASCO EM-8656 including (2) D batteries light bulb (3 volt) (3) wire leads resistors,(eg.1000 ohm, 100 ohm, etc.) dual LED, LED PROVIDED LOCALLY Various objects to test conductivity including metal such as coins, aluminum foil nonmetal such as paper, plastic graphite pencil with both ends exposed
ADVANCE PREPARATION PRE-LAB To set up the conductivity test circuit, use the Electronics Laboratory patch board with two batteries and a bulb in series with the two 10-inch wire leads as in Figure 1. Use the leads to test various objects. The circuit for testing conductivity should be set up beforehand (or setting up can be used as an exercise to review how to connect a circuit.) If desired an exercise such as connecting a bulb, battery and wire to light the bulb could be completed to introduce circuits. (Note: metal objects should cause full brightness, nonmetal should cause no light, and the pencil should cause a dim light) Figure 1 Pre-lab Patch Board Setup
EXPLORATION Have the materials available to set up the circuit as shown in Figure 2 to measure voltage and current for a 1000-ohm resistor. Make sure the experiment file, OHMSLAW.SWS, is on all computers. FIGURE 2. Exploration and Elaboration Patch Board Setup ELABORATION Have non-ohmic objects available for student elaboration using the measurement procedure from the EXPLORATION. PROCEDURE HOW TO USE OHMSLAW.SWS PART I: Computer Setup 1. Connect the Science Workshop interface to the computer, turn on the 750 interface, and turn on the computer. 2. Plug a voltage Patch Cord into Analog Channel A and a Patch Cord modified for current into Analog Channel B. Plug banana plugs into the Output sockets. Connect the other ends of each of these to the PASCO patchboard in accordance with Figure 2. 3. Open the document titled OHMSLAW.SWS The document opens with windows labeled OHM, GRAPH DISPLAY, TABLE, SIGNAL GENERATOR, VOLTAGE AND CURRENT.
4. The Sampling Option is set on keyboard and will collect data at a rate of once per second. 5. The Signal Generator is set to DC (direct current) and zero volts. It will turn on automatically when you collect data. Part II: Data Recording 1. In the OHM window click the MON button to begin monitoring data (display data without recording it permanently). This calls up the KEYBOARD SAMPLING window which will enable you to see data points one at a time. To see a data point (v & I), first set the voltage as desired (see #2 below) and then hit ENTER. Ignore the numbers inside the sampling box. Your data will appear in the TABLE screen and in the VOLTAGE and CURRENT display screens. To see the next data point reset the voltage and again hit ENTER. To stop seeing data click on the STOP COLLECTING button. 2. The voltage can be regulated by clicking on the signal generator and then either clicking on the up and down arrows to change the voltage by 1.0 volt increments or by clicking on the displayed voltage and typing in the new voltage desired. 3. In the OHM window click the REC button to begin recording data. This calls up the KEYBOARD SAMPLING window which will enable you to record data points one at a time. To record a data point (v & I), first set the voltage as desired (see#2 above) and then hit ENTER. Ignore the numbers inside the sampling box. Your data will appear in the TABLE screen and in the VOLTAGE and CURRENT display screens. To record the next data point reset the voltage and again hit ENTER. To stop collecting data click on the STOP COLLECTING button. Part III: Data Analysis 1. On the GRAPH DISPLAY click on the statistics symbol (Σ), a window will open, click on the new statistics symbol, then choose CURVE FIT and then LINEAR FIT.
II. PRE-LAB PRE-LAB EXERCISE TO ELICIT STUDENTS PRIOR KNOWLEDGE AND MISCONCEPTIONS Have the students fill out the pre-lab questionnaire. Small groups of students will be presented with an open light bulb circuit (see Figure 1) consisting of two 1.5 volt batteries in series (3 volt source), a 3 volt light bulb, and two probes that can be used to close the circuit about various objects to be tested. (At the instructor s discretion the students may be asked to construct the circuit.) The students will be given a variety of materials (tin foil, pencil lead, metal objects, wooden objects, etc.) to test for conductivity by including them in the circuit. Students should record their observations (how bright the light bulb is) for the conductivity of the various objects and then propose and record explanations of their observations. The student handout will contain the following suggested questions: 1. How do results for various objects differ (if they do?) 2. Are there different types of objects? 3. Can you devise a scheme for predicting results for new objects? 4. Can you suggest any explanations of generalizations for how electricity flows? 5. What is the function of the battery? 6. How could you test your ideas? 7. If you wanted to measure the flow of electricity what aspects of the flow might you measure? DISCUSSION OF PRECONCEPTIONS The class as a whole can compare observations for the various objects. Note that some objects are intermediate in conductivity. The instructor can summarize results on the board. Each group can then share with the class their explanations for what was observed. The teacher should point out what the explanations have in common and how they are different. Students can be asked to suggest ways to test their explanations. As students consider ways to test explanations, they may become more aware of the implications of those explanations, which may lead to revisions prior to testing. The need for ways to talk about and measure quantities such as current, voltage and resistance may become apparent. (The instructor should refrain from using the terms current, voltage and resistance during the pre lab exercise.)
PRE-LAB QUESTIONAIRE Pre-lab Questionnaire (with possible public scientific knowledge responses) 1. What is electricity? (flow of charges through a conductor) 2. What happens when an object conducts electricity? (electrons (usually) travel through the object) 3. Give an example of an object that conducts electricity. (penny) 4. What kinds of objects conduct electricity? (metallic) 5. Give an example of an object that does not conduct electricity.(paper) 6. What kinds of objects do not conduct electricity? (non-metallic) 7. What is an electrical circuit? (closed loop in which a charges flows) 8. What is required for a circuit to conduct electricity? (a force on the charges and a path for them to flow through) 9. How do you measure the amount of electricity conducted? (current in terms of # of charges passing a point in a circuit per unit time) 10. What controls how much electricity an object conducts? (the force on the charges measured by the potential difference between points in the circuit in volts, and the resistance of the object measured in ohms) 11. Please list and define any terms you would use to discuss electricity.
III. EXPLORATION OF SCIENTIFIC PRINCIPLE & INTRODUCTION OF EXPERIMENTAL PROTOCOL PROBLEM The instructor should now introduce the terms current (movement of charges), voltage (force pushing the current) and resistance ( frictional opposition to the current), if they have not already been used by the students. These terms may need clarification even if the students have been using them. The initial problem is: How can we quantitatively measure current, voltage and resistance? EXPERIMENT AND TECHNICAL OPERATION OF EQUIPMENT Set up the Electronics Laboratory patch board as shown in Figure 2. Load OHMSLAW.SWS from the hard disk. PROVIDED BY SOTM Laptop computer with Science Workshop installed Science Workshop 750 Interface including Patch Cord for analog A channel (voltage) Patch Cord for analog B channel (current) constructed using 1 ohm resistor, voltage patch cord and two banana plugs (2) Patch cords with banana plugs for voltage output AC/DC Electronics Laboratory PASCO EM-8656 including wire lead, 3 inch various resistors, 1000 ohm, 500 ohm, etc The teacher will demonstrate the measurement of one voltage-current point for the 1000 ohm resistor. Ask the students the following: 1. What will happen to the current if you increase the voltage? 2. What will happen to the current if you decrease the voltage? 3. What happens if you change the sign of the voltage? 4. What is the shape of the curve? 5. How does the slope of the curve compare at different currents? 6. What happens in the circuit when the slope increases? Decreases? (Note: Usually we define the independent variable as the one we are varying and look for the response of the dependent variable. The data is then graphed with the independent variable on the x-axis. Due to the definition of resistance, we plot V on the y-axis and I on the x-axis, so that the slope is R. The interface, however, cannot easily vary current, so we change the voltage and measure the current as the response. The graphs are set up to look at voltage versus current.) The expected product for this part of the lab is to recognize that V vs. I is linear and the slope, R, is a property of the object being measured. Different groups should be given different values of resistor and the results compared to determine what R means. If a group completes the curve for a single resistor they could work on a second resistor in order to answer question.
IV. ELABORATION OF SCIENTIFIC PRINCIPLE: INQUIRY-BASED STUDENT INVESTIGATION PROBLEM The instructor should now ask each student group to write down at least two questions about electrical current flow. They should write down ways to investigate answers to the questions using the equipment provided. The students should be able to investigate the following sorts of questions. 1. How is voltage related to light bulb brightness? 2. What is the shape of the V-I curve for various objects? 3. Does it make a difference in which direction the current is flowing? HYPOTHESIS OR PREDICTION It is intended that students will discover the non-ohmic nature of some of the available objects. EXPERIMENTAL DESIGN Questions to encourage good design could include: 1. What variables need to be considered? 2. What variable do you wish to change? 3. What variable(s) will you measure? Set up a procedure for your lab and have the teacher check it before you begin. Checkpoint (Teacher checks students experimental design for feasibility. Such things as limits of range of voltage, changing one variable at a time, having sufficient detail so that someone can repeat and remember the experiment should be checked) PLAN FOR DATA COLLECTION & ANALYSIS Checkpoint (Teacher checks students plan for feasibility.) CONDUCTING THE EXPERIMENT Checkpoint (Teacher monitors students investigations in progress. ANALYSIS OF DATA Checkpoint (Teacher checks students analysis.)
DISCUSSION OF RESULTS Groups may compare notes either as an entire class moderated by the instructor or starting with pairs of groups and working up to the entire class. This process might consist of: 1. sharing results 2. comparing results (recognizing similarities and differences) 3. resolving differences 4. relating findings to everyday life Possible questions might include: 1. How could you make use of the non-ohmic object? (find direction of current flow, keep current from flowing in a direction) 2. Have you seen any applications of the object you evaluated? (indicators, clock and electronics displays.) V. EVALUATION POST-LAB SURVEY OF STUDENTS CONCEPTIONS Have students retake the Pre-Lab Exercise. Compare pre-lab and post-lab responses. TRADITIONAL (Regents questions on E&M) ALTERNATIVE Hookes s Law describes the relationship between extension and applied force for a linear spring. An object which obeys Hooke s Law is called elastic. Have the students propose an analogy between elastic objects and ohmic objects including an experiment to determine which objects are elastic and which are inelastic. F = k x vs V = R I. This material is based upon work supported by the National Science Foundation under Grant No. ESI 9618936. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
SOTM LAB: P16 STUDENT LAB: OHM S LAW DEVELOPERS OF LAB: John Lane, JD853@maristb.marist.edu Taylor Pancoast, JD573@maristb.marist.edu OVERVIEW OF LAB This lab is about how electricity moves. SAFETY CONSIDERATIONS Whenever electrical devices are used there is a potential for electrical shock. Work areas must be kept clear of liquids (no drinks), and care must be taken to disconnect any devices or circuits from all power sources before working on them. Be aware that computer and electrical equipment can be fragile. PART I: PRE-LAB A conductivity test circuit has been set up using an Electronics Laboratory patch board with two batteries and a bulb in series with the two 10-inch wire leads as in Figure 1. Use the leads to test various objects available from your instructor. For each object write down the brightness of the light bulb. As a group, write down one or more explanations of your recorded results to be shared with the entire class. QUESTIONS TO CONSIDER: Figure 1 Pre-lab Patch Board Setup 1. How do the results for various objects differ (if they do)?
2. Are there different types of objects? 3. Can you devise a scheme for predicting the results for new objects? (If so do so.) 4. Can you suggest any explanations or generalizations for how electricity flows? (If so do so.) 5. What is the function of the battery? 6. How could you test your ideas? 7. If you wanted to measure the flow of electricity what aspects of the flow might you measure? PART II: EXPLORATION: After the class has discussed the results of part I, your instructor will demonstrate how to measure quantitatively three aspects of electrical flow using the circuit shown below in Figure 2. FIGURE 2. Exploration and Elaboration Patch Board Setup QUESTIONS TO CONSIDER : 7. What will happen to the current if you increase the voltage? 8. What will happen to the current if you decrease the voltage? 9. What happens if you change the sign of the voltage? 10. What is the shape of the curve 11. How does the slope of the curve compare at different currents? 12. What happens in the circuit when the slope increases? Decreases?
PART III: ELABORATION PROBLEM Write down at least two questions about electrical current flow. Write down ways to investigate the answers to those questions using the equipment provided. THINGS TO KEEP IN MIND: 1. It may be possible to use additional equipment if it is readily available. (Check with your instructor.) 2. What variables need to be considered? 3. What variable do you wish to change? 4. What variable(s) will you measure? 5. Before writing out your procedure, check with your instructor. HYPOTHESIS OR PREDICTION Predict the results of your proposed experiment. EXPERIMENTAL DESIGN Write out a procedure for your lab and have the teacher check it before you set up. CONDUCTING THE EXPERIMENT Set up your experiment and check with your instructor before you start to collect data. ANALYSIS OF DATA How do you wish to present your data? Does your data agree with you prediction? DISCUSSION OF RESULTS 5. Prepare your question, prediction and data for presentation. 6. Compare your results with those of other groups (what are the similarities and differences?) 7. Resolve any differences. 8. Give examples of how these results help you to understand things in everyday life. 9. Suggest some possible uses for the object(s) investigated. This material is based upon work supported by the National Science Foundation under Grant No. ESI 9618936. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
PRE-LAB QUESTIONNAIRE 12. What is electricity? 13. What happens when an object conducts electricity? 14. Give an example of an object that conducts electricity. 15. What kinds of objects conduct electricity? 16. Give an example of an object that does not conduct electricity. 17. What kinds of objects do not conduct electricity? 18. What is an electrical circuit? 19. What is required for a circuit to conduct electricity? 20. How do you measure the amount of electricity conducted? 21. What controls how much electricity an object conducts? 22. Please list and define any terms you would use to discuss electricity.