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PHYS 2020 Class 08 (Laboratory) Electric Fields Adapted from the University of Sharjah We’ve introduced the concepts of electric fields. Today we will explore them in more detail using a simulation program. Materials (see Appendix): • None Activity 1 Since we have limited lab equipment in an online situation, we will be exploring the concepts virtually using a PhET simulation. Click on the following link to use today’s simulation. https://phet.colorado.edu/sims/html/charges-and-fields/latest/charges-and-fields_en.html In the box at the bottom, you can find point charges and a sensor. You can pull those onto the main workspace to see their impact. The charge values cannot be changed, but if you want more charge, you can place several individual charges together to total your desired value. Once a charge is place, the simulation shows the electric field in a grid pattern. Each arrow points in the direction of the field at that point, and the brightness relates to the strength. If you want to know what’s going on with the field at a different point, you can drag/drop a yellow Sensor dot. Take a few minutes and play with the various options to see what they do. Activity 2a: Electric Fields 1. Adjust the settings so only the Grid is active, and drag a red +1 nC charge into the middle of the screen. 2. Using the math definition for the electric field due to a single charge, estimate the value (and direction) of the electric field 1m away from a +1nC charge. 3. Use a yellow sensor to check your prediction. Afterward, turn the Electric Field display back on and see how the resulting arrows compare to the yellow sensor information. Think About It: How did your prediction compare to the simulated value? Calculate the relative uncertainties between the two, assuming your calculated value is correct. 4. Clear the existing charge, drag a blue –1 nC charge onto the screen and repeat the Activity. Think About It: How did your prediction compare to the simulated value? Calculate the relative uncertainties between the two, assuming your calculated value is correct. Did anything else change? Activity 2b 5. Clear the existing charge(s). Place a positive charge and a negative charge 2m apart, horizontally Think About It: Using the sensor tool, is there a point anywhere on the screen where the electric field is zero? Should there be? Where is the electric field the largest value? Think About It: What force (and what direction) would you expect the negative charge to feel from the positive charge? Activity 2c 6. Clear the existing charge(s). Place a positive charge and a positive charge 2m apart, horizontally Think About It: Using the sensor tool, is there a point anywhere on the screen where the electric field is zero? Should there be? 7. Clear the existing charge(s). Place a negative charge and a negative charge 2m apart, horizontally Think About It: Using the sensor tool, is there a point anywhere on the screen where the electric field is zero? Should there be? 8. When you have two of the same charges along a horizontal line, where is the electric field the greatest? Is there ever a point where the field will be zero? 9. Determine what charge/charges (magnitude and positive/negative) would give each the lines of equipotential shown below? Activity 3a: Equipotentials We have seen how electric fields relate to the forces charges will feel, and how they change. Relating to these fields, we can indicate how those fields are changing via voltage potentials. 𝑉 = 𝐸𝑑 or 𝑉=𝑘 𝑄 𝑟 A useful comparison is contour maps. Consider a mountain. If you release a ball on the side of the mountain, it will spontaneously roll downhill along a direct path toward a lower elevation (similar to a proton experiencing an electric field). A contour map of the hill indicates points along the hill at the same height, resulting in an equipotential line. 1. Adjust the settings so only the Grid is active, and drag a red +1 nC charge into the middle of the screen. Bring the voltage meter into play (the blue meter in the box to the right of the screen) Think About It: Move the voltage meter around the screen, reading the value of the voltage at the crosshairs. Where is the voltage the highest? Is this consistent with the math definition for electric voltage? 2. Move the voltage meter to a location where the voltage reads 10.0V, and click the pencil icon on the meter. Think About It: Move the voltage meter along the resulting line. How does the voltage change? What does that imply about the line? Think About It: Create additional equipotentials. Do any of them cross? 3. Clear the existing charge and equipotentials, drag a blue –1 nC charge onto the screen and repeat the Activity. Think About It: What changed, and what stayed the same? Activity 3b: Equipotentials 4. Place two equal and opposite charges along a horizontal line, separated by 1.5m. Create a variety of equipotentials about each charge, and sketch the results. Do any of the equipotentials cross? 5. Clear the existing arrangement, and place two equal charges along a horizontal line, separated by 1.5m. Create a variety of equipotentials about each charge, and sketch the results. Do any of the equipotentials cross? Think About It: Is there any case you can find where equipotentials can cross? Activity 4: Equipotentials and Electric Field Lines 1. Clear the existing arrangement, and as before, bring a single positive charge onto the screen. Adjust the settings so only the Grid is active, and drag a red +1 nC charge into the middle of the screen. Turn on the electric field arrows, and create several equipotentials. Sketch the result Think About It: What can you say about the qualitative relation between the directions of the electric field arrows and the equipotential lines. 2. Clear the existing arrangement, and bring additional charges onto the screen (positive and negative, varying the total charge as you like. Turn on the electric field arrows, and create several equipotentials. Sketch the result. Think About It: Does the resulting arrangement uphold your theory for how equipotentials and electric field arrow directions? Why or why not? Think About It: Can electric field lines cross? Can equipotentials? Deliverables We need an informal report describing what you did and what you learned. Imagine you are talking to your parents or your boss, and describing the activities you just completed. Make sure to include any pictures, images of relevant graphs, and resulting understanding you have gained. Submit a copy of this report for grading. Rubric: Documentation Missing 25 (0.00%) Novice 15 (30.00%) Partial 20 (40.00%) Proficient 25 (50.00%) Did not submit Narrative unclear, incomplete thoughts and/or sentences. Did not include sufficient information for a person to replicate the work Narrative was fairly clear, but left out something significant (i.e, meaning of the results, numbers without units or uncertainties) Ideas were expressed in a clear and organized fashion. It was easy to figure out what was going on, and how to repeat the experiment if desired. Included discussion of results compared to accepted values (with appropriate uncertainties and units) A post-lab quiz will also be required to assess your understanding of the goals for this lab, and will count for half the grade. 1.Given the existence of a charged object, can there be a point with no electric field? A. Yes B. No C. Only in certain circumstances 2. Assuming you have two positive charges along a line, is there a point where the electric field is zero? A. No B. To the side of the pair, specifically on the side with the larger charge C. To the side of the pair, specifically on the side with the smaller charge 3. Assuming you have a large positive and small negative charge along a line, is there a point in between them where the electric field is zero? A. Closer to the larger charge B. Closer to the smaller charge C. No 4. If the electric potential is large in a given region, is there an electric field present? A. Yes, electric fields always appear with potential B. No, electric fields are completely independent of potential C. Depends if the potential is changing in the region, or remains a constant value

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