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UW-Madison PHYSICS 208 - Lab 4 - Electric Fields and Electric Potentials

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Name________________________________________ Section___________ Physics 208 Fall 2008 Lab 4: Electric Fields and Electric Potentials Your TA will use this sheet to score your lab. It is to be turned in at the end of lab. You must use complete sentences and clearly explain your reasoning to receive full credit. What are we doing this time? You will complete two related investigations. PART A: Use a numerical simulation to plot on the screen equipotentials and electric field vectors various charge distributions, and see how the presence of additional neutral conductors . PART B: Use the field plotting board to map the equipotentials of a dipole, and to determine how the potential difference ‘across’ a dipole depends on the angle with respect to the dipole axis. PART C: Use the field-plotting board and the torso cutout to understand how an electrocardiogram measures properties of the heart electric dipole. Why are we doing this? To understand the electric potential energy around charges and conducting objects, and how to apply this understanding to interpreting an electrocardiogram What should I be thinking about before I start this lab? You should be thinking about the relation between electric potential, work, and energy. Any safety issues? No2 A. Numerical Simulation Click on “EM Simulator” in the applets column for Lab 4 on the course web site “Laboratories” page. The electric field at each point is shown as a vector, but all the vectors have the same length: the magnitude of the electric field is indicated by color. White = large electric field Light Green = medium electric field Dark Green = small electric field You should be able to click and drag the positive charge around on the screen. A1. The white line contours are equipotentials, connecting points in space that have the same electric potential. Each contour is a different electric potential, and the electric potential difference between adjacent contours is a constant value ΔV. Why do the equipotentials get farther apart as you move away from the charge? (Answer in terms of the relation between electric field and electric potential). A2. Conducting Plates Under the setup menu choose ‘Conducting Plates’. Two plates appear, with equal and opposite electric potential. Move the plates around with your mouse to see the effects on the field lines and equipotentials. When the plates are aligned, the equipotential lines are approximately equally-spaced between them. Explain why this is so.3 Yellow indicates positive charge, and blue indicates negative charge. Explain how the charge arrangement is consistent with the direction of the electric field between the plates. Explain how the relative magnitude of the electric fields between the plates and outside of the plates is consistent with the charge distribution. A3. Move the plates to the top and bottom of the screen. Select “Mouse=Add Conductor (Gnd)” from the Mouse dropdown menu. Draw an empty box as indicated. Select “Mouse=Make Floater” and convert the grounded conductor to a floating conductor by putting your mouse over it and clicking. It is now an isolated conductor with zero net charge. Explain why the charge is distributed as it is on the box that you drew. Select “Mouse=Move Object” from the Mouse dropdown menu, and move your box around the screen. What is the electric field inside the box? Explain4 A4. Dipole and induced charges Under the setup menu choose ‘Dipole’. You should see + and – point charges with the corresponding field lines and equipotentials. Click and drag the charges so that the dipole is horizontal near the bottom of the screen, and takes up most of the screen. Select ‘Mouse=Add Conductor (Gnd)’ and draw a filled rectangle near the top of the screen. Select ‘Mouse=Make Floater’ and convert the grounded conductor to a floating conductor by putting your mouse over it and clicking. It is now an isolated conductor with zero net charge. Select ‘Mouse=Move Object’ and drag the conductor around on the screen. The local charge density on the conductor is color coded, blue for negative and yellow for positive. i) Drag the conductor down near or between the dipole charges. Describe what happens to the charge distribution on the conductor, and how the electric fields change. What value do you think the electric field has inside the conductor? Explain what is going on. ii) How can the presence of the conducting object affect the fields near the dipole? (Hint: how would you describe the induced charge distribution on the conducting object, and how would this effect the fields? )5 iii) Suppose the dipole is an electrogenic fish, i.e. a fish that can cause a charge separation in its own body between its head and tail. Suppose that the conducting object is its (conducting) prey. The electrogenic fish senses its prey by detecting changes in electric fields on its own skin caused by the conducting prey. Move the prey around and watch the electric fields in the region of the dipole. What do you think are some of the factors that affect how close the conductor must be to the fish before it noticeably affects the electric fields at the fish?6 B: Analog simulation Here you use a piece of carbonized paper in which currents flow to simulate electric fields and equipotential surfaces in vacuum. Field plotting board: Get a piece of graphite paper with two silver dots (representing conducting spheres), one on each end. On the field plotting board, first put down a sheet of white printer paper, then a sheet of carbon paper (carbon side down), and finally the graphite paper on top. Power supply: Use a red banana-plug cable to attach the +30V output (red) of the DC power supply to one connector on the field plotting board, and a black cable to attach the ground output (black) to the other. This maintains a constant potential difference between the two painted conductors on the graphite sheet. Digital multimeter: Attach the red and black voltage probes to the Keithley digital multimeter (DMM) by attaching a BNC to banana-plug adaptor to each probe. Then connect a banana plug cable from the red


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UW-Madison PHYSICS 208 - Lab 4 - Electric Fields and Electric Potentials

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