C alculate the electric potential at a point in space

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Unformatted text preview: POTEN TIALS Q ualitatively d eterm ine the electric potential at a point in space caused by a configuration of charged objects based on the geom etry of those objects. C alculate the electric potential at a point in space cau sed by a configuration of charged objects based on the geom etry of those objects. Relate the electric field cau sed by charged objects to the electric potential caused by charged objects. PREPARATION Read : Tipler & Mosca Chapters 21, 22 & 23. Before com ing to lab you should be able to: A d d vectors in tw o d im ensions. C alculate the electric field d ue to a point charge. C alculate the electric potential d ue to a point charge. U se the com puter sim ulation program , EM Field . 36 PROBLEM #1: THE ELECTRIC FIELD FROM MULTIPLE POIN T CHARGES You w ork w ith a biochem ical engineering group investigating new insulin -fabrication techniques. Part of your task is to calculate electric field s prod uced by com plex m olecules. The team has d ecid ed to use a com puter sim ulation to calculate the field s. Your task is to d etermine if the sim ulation agrees w ith the physics that you know . You d ecid e to d eterm ine the electric field at a point from a set of charged objects that is com plex enough to test the sim ulation but sim ple enough to m ake d irect calculation possible. The first configuration you try is a square w ith tw o equal negatively charged point objects in opposite corners and a positively charged point object of 1/ 3 the m agnitud e of the negative charges in a third corner. You w ill calcu late the electr ic field at the rem aining corner of the square and com pare your result to that of the com puter sim ulation of the sam e configuration. Instructions: Before lab, read the required reading from the textbook and the laboratory in its entirety. 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. A t the end of lab, disseminate any electronic copies of your results to each member of your group. Read : Tipler & Mosca Chapter 21. Read carefully Sections 21.4 and 21.5 and Exam ples 21-7, 21-8 and 21-9. EQUIPMEN T The com puter program EM Field a nd a ruler. 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. WARM UP 1. Make a picture of the situation , carefully labeling the objects and their charges and all d istances and angles. At the point of interest (the point w here you are calcu lating the electric field ), d raw and label electric field vector for each of the charged objects. Finally, place a usefu l coord inate system on your d raw ing. 2. Determ ine the m agnitud e and d irection of each of these vectors. (You m ay need geom etry and trigonom etry to d eterm ine d istances and d irections.) 3. C alculate the com ponents of each vector w ith respect to your coord inate system . 37 THE ELECTRIC FIELD FROM MULTIPLE POINT CHARGES – 1302Lab2Prob1 4. Find the com ponents of the total (net) electric field vector at the point of interest, and then use them to w rite an expression for both the m agnitud e and d irection of the total electric field vector at the point of interest. (Recall w hat you read about ad d ition of vectors in the textbook.) PRED ICTION D eterm ine the physics task from the problem statem ent, and then in one or a few sentences, equations, d raw ings, and / or graphs, m ake a clear and concise pred iction that solves the task. EXPLORATION In the fold er P hysLab o n the d esktop, open EM Field a nd click anyw here in t he w ind ow for the instructions. From the S ources p ulld ow n m enu, select 3D point charges. Drag a ny positive charge to the center of the w ind ow of EM Field . From the Field and Potential p u ll-d ow n m enu (show n to the r ight), select Field vectors. You can reveal electric field by clicking on the locations w here you w ould like the electric field to rem ain d isplayed . Look at the D isplay d rop -d ow n m enu and explore its options. To place objects at precise points on the screen you can u se the ‘show grid’ a nd ‘constrain to grid’ features from the display p ull-d ow n m enu. Expand the d isplay w ind ow to fill the entire com puter screen. Measure the length of the electric field vector at several locations, as w ell as the d istance from the locations to the center of the charged point object. You can rem ove the d isplayed vectors using the ‘clean up screen’ o ption from the D isplay d rop -d ow n m enu. Try using d ifferent m agnitud es of charge. What range of charge values allow s you to accurately m eas...
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