It is often useful to use ex cel to perform data

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Unformatted text preview: ediction 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 Ex cel to perform data analysis, rather than doing it by hand. EQUIPMEN T Build the circuit show n u sing o f w ires, bulbs, capacitors and batteries. U se the accom panying legen d to help you build t he circuit . You w ill also h ave a stopw atch and a d igital m ultim eter (DMM). Legend: light bulb A + - battery capacitor switch wire C ircuit I Read the section T he Digital M ultimeter (DM M ) in the Equipment a ppend ix. If equipment is missing or broken, submit a problem report by sending an email to l abhelp@physics.umn.edu . Include the room number and brief description of the p roblem. PRED ICTION Restate the problem . Do you agree w ith your friend ? If not, d escribe w hat you think the behavior of the circuit w ill be. Give your reasoning. Explain w hat is going on in each com ponent of the circuit. Sketch a qualitative graph of the bulb’s brightness vs. tim e. 63 SIMPLE CIRCUITS WITH CAPACITORS – 1302Lab3Prob2 EXPLORATION WARN IN G: A charged capacitor can d ischarge quickly prod ucing a painfu l spark. D o not h and le the capacitors by their electrical term inals or conne cted w ires by their m etal end s. A lw ays discharge a capacitor before you use it and w hen you are finished using it. To d ischarge a capacitor, use an insulated w ire t o briefly connect one of the term inals to the other. Exam ine each elem ent of the circuit b efore y ou build it. H ow d o you know if the battery is "good "? Is the capacitor charged ? Carefully connect the tw o term inals of the capacitor to ensure it is uncharged . After you are convinced that all of the circuit elem ents are w orking and that the capacitor is uncharged , build the circuit but d o not close it yet. N ote: Make sure the + terminal of the battery is connected to the + terminal of the capacitor! These capacitors are only d esigned to charge one w ay. If you connect the capacitors up the w rong w ay, the capacitance w ill change in an unpred ictable m anner. N ow , close the circuit and observe how the brightness of the bulb changes over tim e. From your observation of the bulb's brightness, how d oes the charge flow ing through the bulb change over tim e? You can check this using the DMM set for current (Am ps). See the Equipm ent append ix for instruction on the use of the DMM. Using a m ental m od el of the capacitor as tw o parallel plates separated by a short d istance, how d oes the charge accum ulated on each plate of the capacitor change as the bulb’s brightness changes? Can you m easure this w ith the DMM? Use conservation of charge to explain w hat you observe. From w hat you know about a battery, how d oes the potential d ifference (voltage) acros s the battery change over tim e? Check this using the DMM set for potential d ifference (Volts). From your observations of the brightness of the bulb, how d oes the potential d ifference across the bulb change over tim e? Check this using the DMM. What can you infer about the change of voltage (change of potential d ifference) across the cap acitor over tim e? Can you check w ith a DMM? Use the concept of conservation of energy to explain w hat you observe. After a few m om ents, open the circuit. Is the capacitor charged or uncharged ? To d eterm ine if the capacitor is charged , carefully (and safely) rem ove the battery from the circuit and reconnect the circuit w ithout the battery. With only the capacitor and bulb (no battery) in the circuit, d oes the bulb light? Use the result to answ er the follow ing questions. Was the capacitor charged before you closed the circu it? Was the capacitor 64 SIMPLE CIRCUITS WITH CAPACITORS – 1302Lab3Prob2 still charged long after the circuit w as closed ? conservation of energy to explain your results. Use conservation of charge and CON CLUSION W as your friend correct about how the brightness of the bulb changed ? Sketch a qualitative graph of the brightness of the bulb as a function of tim e after you com plete the circuit consisting of the initially discharged capacitor, battery and light bulb. H ow d oes this com pare to your pred iction? Sketch a qualitative graph of the charge on the capacitor as a function of tim e for this situation. Sketch a qualitative graph of the brightness of the bulb as a function of tim e after you com plete the circuit consisting of the initially charged capacitor, N O battery and light bulb. Sketch a qualitative graph of the charge on the capacitor as a function of tim e for this situation. Describe how this graph relates to the changing potential d ifference across the capacitor, and to the changing am ounts of charge on each plate of the capacitor. 65 SIMPLE CIRCUITS WITH CAPACITORS – 1302Lab3Prob2 66 PROBLEM #3: CAPACITAN CE You have a part tim e job as a special effects technician at a local theater. As...
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