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Unformatted text preview: this experiment? How do contributions to the systematic uncertainty from the equipment imperfections and from human error compare? Provide detailed explanations. If needed, review the appendix material on uncertainty. 108 PROBLEM #8: RESISTORS AND LIGHT BULBS Your research team has built a device for monitoring the ozone content in the atmosphere to determine the extent of the ozone holes over the poles. You have been assigned the job of keeping the equipment at the South Pole running during the winter months when no supplies can get in. When a piece of equipment fails, you need to replace two resistors. Unfortunately you have only one. You do have a light bulb but wonder how well a light bulb can substitute for a resistor in the circuit. You decide to make a direct comparison. Instructions: Before lab, read the laboratory in its entirety as well as the required reading in the textbook. In your lab notebook, respond to the warm up questions and derive a specific prediction for the outcome of the lab. During lab, compare your warm up responses and prediction in your group. Then, work through the exploration, measurement, analysis, and conclusion sections in sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to perform data analysis, rather than doing it by hand. Read: Tipler & Mosca section 25.2. EQUIPMENT You have banana wires, an 18V/5A power supply, a digital multimeter (DMM), a light bulb, and a resistor. The power supply has the same function as a battery, to supply energy to the circuit by maintaining a constant voltage or potential difference. Because this voltage is not the result of chemical reactions, it is easy to change the voltage across the power supply within some range. Read the sections The Digital Multimeter (DMM) and Resistor Codes in the Equipment appendix. Read the appendices Significant Figures, Review of Graphs and Accuracy, Precision and Uncertainty to help you take data effectively. If equipment is missing or broken, submit a problem report by sending an email to labhelp@physics.umn.edu. Include the room number and brief description of the problem. WARM UP 1. What is the relationship between the current through a resistor and the potential difference (voltage) across the resistor if the resistor is made of ohmic material? Draw a graph of voltage versus current for this resistor. How is the slope of the graph related to its resistance? 109 RESISTORS AND LIGHT BULBS – 1302Lab4Prob8 2. As more current goes through a light bulb, it gets brighter. As it gets brighter, it gets hotter. Do you expect the increasing temperature to affect the resistance of the bulb? If so, how? 3. Sketch a qualitative graph of voltage across a light bulb versus current through the light bulb. PREDICTION Restate what elements of electric circuits (and by what criteria/parameters) you are to compare in this problem. Use your experience to draw a graph of voltage versus current for (a) a standard resistor, and (b) a light bulb. Explain your reasoning, including physical assumptions you have made. EXPLORATION WARNING: You will be working with a power supply that can generate large electric voltages. Improper use can cause painful burns. To avoid danger, the power must be turned OFF and you must wait at least one minute before any wires are disconnected from or connected to the power supply. Never grasp a wire by its metal end. Sketch the circuit you will build to check your prediction. Can you test both the light bulb and the resistor at the same time? Is this a good idea? If necessary, read about how to use the DMM in the Equipment appendix. Pay special attention to the connections and settings that are used to measure voltages and currents, and why the DMM should be connected in the circuit differently for voltage and current measurements. Do you know why we should connect them in these ways? MEASUREMENT There are three methods for determining the electrical resistance of a resistor. 1. Use the chart provided in the appendix to determine the resistance of your resistor based on its color code. What is the uncertainty in this value? 2. Use the DMM set to ohms to measure the resistance of the resistor. What is the uncertainty in this value? Why is this procedure not helpful with a light bulb? 3. Use your power supply, DMM, and resistor to determine the voltage across the resistor and measure the current through the resistor for several different voltages. What is the uncertainty in the value of the resistance obtained by this method? 110 RESISTORS AND LIGHT BULBS – 1302Lab4Prob8 ANALYSIS Make a graph of voltage versus current for your resistor and light bulb. How do the values of the resistance compare for the different methods used? CONCLUSION Are the color-coded resistor and light bulb both ohmic resistors? If so, what are their resistances? Did your prediction match your results? If not, can you use the bulb over some limited range of voltages? What range? Explain your reasoning. What are possible sources of systematic uncertainty? (If you need help determ...
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