1202_LabManual.pdf

# After a few moments open the circuit is the capacitor

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After a few moments, open the circuit. Is the capacitor charged or not? To determine if the capacitor is charged, carefully (and safely) remove the battery from the circuit and reconnect the circuit without the battery. With only the capacitor, and bulb (no battery) 75

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QUALITATIVE CAPACITORS – 1202Lab3Prob3 in the circuit, will the bulb light if you close the circuit and the capacitor is charged? Uncharged? Try it. Was the capacitor charged before you closed the circuit? Was the capacitor still charged long after the circuit was closed? Use conservation of energy to explain your results. C ONCLUSION Was your friend right about how the brightness of the bulb changed over time? Sketch a qualitative graph of the brightness of the bulb as a function of time after you complete the circuit. How does this compare to your prediction? 76
PROBLEM #4: QUANTITATIVE CAPACITORS You and a friend are discussing how ion concentrations on either side of a cell membrane change with time. In particular you want to investigate how ions (say Na+) migrate and how voltage across the membrane builds up over time. Now you are wondering how the properties of the membrane affect the migration process. You decide to model the cell membrane, very crudely, as a capacitor in series with a light bulb and a battery. A capacitor can be thought of as a device used to hold separated charges (similar to the cell membrane). You decide to get a quantitative understanding of the rate at which a capacitor charges by using a capacitor in series with a light bulb and battery. How does the time that the light bulb is lit depend on the capacitance of the capacitor connected in series with it? NOTE: This problem, and the problems Qualitative Capacitors and Rates of Energy Transfer in RC Circuits are fundamentally similar. This problem involves a quantitative analysis of an RC circuit, Qualitative Capacitors involves a qualitative analysis, and Rates of Energy Transfer in RC Circuits analyzes RC circuit behavior from the point of view of energy transfer to or from the capacitor. Read Sternheim & Kane sections 16.4, 16.9, 17.7 and 18.4. E QUIPMENT Build the circuit shown using wires, bulbs, capacitors, and batteries. Use the accompanying legend to help you build the circuit. You will also have a stopwatch and a digital multimeter (DMM). If you need assistance, send an email to [email protected] . Include the room number and brief description of the problem. P REDICTION From your experience, make an educated guess about how the time that the light bulb is lit depends on the capacitance of the capacitor. Sketch a graph of the time it takes for the light bulb to turn completely off as a function of the capacitor’s capacitance. Assume the capacitor is initially uncharged. Write down what you mean when you say the light bulb is completely off.

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