Notebook EP - Electric potential and fields draft

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Iowa State University *

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132L

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Electrical Engineering

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Apr 3, 2024

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docx

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Lab EP - Electric potentials and fields During this lab you will explore the electric potential and electric fields produced by charged conductors with different geometries. Major equipment  Large graphic tablet (this is in addition to the usual small tablet)  Custom-made plastic base with electric connections  Set of silver-ink prints on conductive carbon paper  Digital multimeter.  DC power source The large drawing tablet is expensive and delicate. Treat it with care! Lab EP – Page 1

Working with two tablets Today, you will have two tablets on your table. The small tablet is for sketches, as usual. The large tablet will be used to collect data as described below. 1. Setting up the apparatus The apparatus to be used is depicted below. Both the power source and the voltmeter should be off for now. Check all the wiring in your setup. Make sure that the multimeter is set to function as a DC voltmeter before you turn on the power source. Both devices can stay off until section 1.3. The bottom surface of the plastic board is lined with rubber to protect the tablet. Use care when moving the board, to avoid scratching the tablet. Lab EP – Page 2 Do not drag the pen or the probe over the paper. Avoid scratching the carbon paper and the silver electrodes. Lift and reposition the tip instead.

1.1. The large drawing tablet The drawing tablet for this experiment is much larger than the one we normally use in these labs. It will be used to record the location of the points where the potential is measured.  Download and double-click the Tablet Preferences file. A dialog box will ask you if you want to continue: select “Replace”.  Download and open Blank canvas . Maximize the window so it takes the whole screen. (This is necessary to obtain the correct mapping between the tablet and the screen!)  On the left-hand corner of the screen, select “Layer 2”.  The stylus only writes when the lower button on the side of the pen is pressed. Otherwise, it just moves the cursor around the screen. Test your pen: make some markings and check that the cursor correctly maps the motion of the pen on the tablet (left/right and up/down) 1 .  If not pre-selected, choose Paintbrush as your writing tool. On the top toolbar, set the Brush size to 4. During today’s experiment, it will be useful to use different colors: click on the desired color on the color wheel before writing.  To erase, you may use the Eraser tool, or select the area and click Delete. 1.2. The plastic board A custom-made plastic board provides an easy way to connect the electrodes to the power source. For this shortened version of the experiment, the electrode sheet with two parallel lines has been pre-installed. If you removed the sheet (don’t), you would see what is shown to the right. The parallel line electrodes are connected to sockets 4 and 5, which means each is connected to one of the terminals of the power source. 1 The mapping should work fine when the button panel is on the right. If it doesn’t, just turn the tablet around! Lab EP – Page 3

How to install an electrode sheet:  Find the sheet with two parallel straight lines. Describe the 3D system that these 2D electrodes represent. It represents two charges with an electric field between them.  Each sheet has a pre-punctured hole for each electrode. Thread a washer into each of the screws and then gently insert the screws in the holes in the electrodes, doing your very best to not make the hole larger! The washer should lay between the head of the screw and the paper.  Place the paper on the board so that the numbers are upright, and so that the screws are aligned with the corresponding sockets. Secure the screws without overtightening. The washers should not protrude outside of the silver painted area.  Once the electrode sheet is in place and the board is on the tablet, tap all four corners of the paper with the pen to make sure that the entire paper is being mapped onto the writable area. Adjust the position of the board on the tablet if needed. Do not drag the stylus over the paper (or the tablet!). Avoid scratching the carbon paper and the silver electrodes. Lift and reposition the tip instead. 1.3. The power source and the multimeter  Make sure the multimeter is set to work as a DC voltmeter  Set the Range of the multimeter to Auto .  Turn on the power source and set the output to 10 V. Touch each of the electrodes with the pen. One of them should give a reading of 0 V, and the other a reading of about 10 V.  Adjust the output of the power source to exactly 10.0 V as read on the multimeter (the multimeter is a more reliable instrument than the voltmeter built into the power source).  Then, while the 10.0 V reading is on the screen of the multimeter, change the Range to Hold. This will force the device to round values to only 2 significant figures, the appropriate precision in this experiment. Lab EP – Page 4

Whenever you measure potentials with the probe on one of the carbon paper samples, try to not rest your hands Also, avoid the using the probme right over unused screw holes –they produce bad data. 2. The parallel electrodes 2.1. Potential and electric field in a conductor Prediction: How does the potential depend on position for points on one of the electrodes? Potential depends on the distance between the electrodes and the charge of those electrodes. Test it! Place the probe at different locations on each electrode (not around). Describe what you observe. As we move away from negative charge, the potential increases towards the positive charge. What can you conclude about the electric field within the electrodes? Explain. The field is going into the negative electrodes and out of the positive ones. Lab EP – Page 5

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