PHYS 192 Lab: visualizing electric fields By Marc Favata Name (print): Date: Section #: Lab partners (print]: Objectives: To gain intuition on the properties of electric fields for different types of charge arrangements. Tips: Read and follow the instructions carefully. Read each section completely before you begin. Understand what you are supposed to measure and what you are supposed to calculate. When in doubt, ASK. Make sure this document and any attachments are STAPLED! Equipment: You will need a laptop or mobile device to complete this lab. 1. Download and run the appropriate programs. This lab can be completed with either of the following two sets of web apps. Set A works on most devices, but has more limited functionality. The programs in Set B have more features, but use Java and may be more difficult to get working on your device. Set A: (These will work in any browser that supports HTML5. They work well in the Chrome browser.) Electric Field Lines: Go to http://dragly.org/projects/htmlfieldlines/ For this program, slide the "initial charge for new particles" bar all the way to the left for a negatively charged particle and all the way to the right for a positively charged particle. Do not use that slider in an intermediate position. Keep the "number of steps" slider to the right. In some cases, the field lines are not initially drawn nicely and you may need to adjust the number of field lines or decrease the step length. Electric Field Hockey: http://electricfieldhockey.com/ Set B: PhET simulations: these use java. Click "Run Now!" to play directly in your browser. This might not work unless you adjust your Java security settings. You can also download the program and open it directly on your computer. (On a Mac, I was able to run these by downloading them, right-clicking and opening with the "Jar Launcher" app.) Charges & Fields: http://phet.colorado.edu/en/simulation/charges-and-fields Click on "show E-field". This program will only show the electric-field vectors, not the field lines. However, you should be able to reconstruct the field lines from the pattern of electric field vectors. If you understand the meaning of equipotential lines, you can use the equipotential plotting feature to better help you infer the directions of the field lines. PhET Electric Field Hockey: http://phet.colorado.edu/en/simulation/electric-hockey (This is more challenging and interesting than the HTML5 version above.) Experiment with getting these different programs up and running. Play with them, figure out how they work, and decide which you'd like to use to complete the lab. Do not expect the instructions to be spelled out here; you can figure this out with trial and error.2. Electric fields around charges: a) In the space below, draw two diagrams. On the left half, draw the electric field vectors around a single positive charge. Be sure you draw several field vectors in different directions and at different distances from the charge. Think about how the electric field should behave at large distance from the charges. On the right side of the page, draw the corresponding electric field line diagram. Be sure to include arrows on your lines that indicate their direction. b) In the space below, repeat this exercise for the electric field around a negatively charged particle. c) Circle the correct response: Electric field lines point [away from] [towards] a positive charge, and [away from] [towards] a negative charge. Using either the Electric Field Lines or Charges & Fields program, complete the following exercises. (You will get much prettier field line diagrams using the Electric Field Lines program, but that program does not indicate the field line directions. The Charges & Fields program only produces vector fields from which you'll have to infer the field lines. I recommend you use the Electric Field Lines program. In all cases, indicate the sign of your charges on your diagram and always remember to indicate the correct direction associated with the field lines. d) Place two positive charges a short distance apart (~5 times the "width" of a charge). Draw the resulting electric field pattern below. e) Repeat this exercise, but using one positive and one negative charge. This is an electric dipole. Sketch the resulting field below. (Note that the net charge is zero.)f) Next, place three negative charges very close to each other (within ~2 charge diameters) and place a single positive charge some distance away (greater than ~ 15 charge diameters). Sketch the resulting field lines. g) What happens when you move the positive charge very close to the 3 negative charges? What should the electric field look like far from the charges? (Draw it.) h) Arrange three charges in an isosceles triangle. Let a positive charge mark each of the ends of the short arm of the triangle, while a single negative charge anchors the two longer arms of the triangle. Draw the resulting field.i) Now move the single negative charge so that it lies directly in line (and halfway between) the two positive charges (with all three charges lying on a single line). Draw the resulting field. i) Make a line of 8 positive charges (spaced by about -2 or 3 charge diameters). Draw the resulting field. k) Add to the above picture a line of 8 negative charges. The line should be parallel to the line of positive charges but about 5 charge diameters away. Draw the resulting field.I) Create a square arrangement of charges with two positive and two negative charges. Like charges should lie along diagonals of the square (i.e., the corners should alternate +, -, +, -). This arrangement is called an electric quadrupole. (Note that the net charge and the net dipole moment is zero.) m) Play around and create an arbitrary arrangement of charges of your choosing. Choose something sufficiently different from the cases you considered above. Draw the result here. 3. Electric field hockey: Now you will explore how a charge arrangement affects the motion of a single. The objective is to arrange charges such that a positive charge will be deflected around obstacles and land in the goal. If using the HTML5 program, you should complete the two levels following the practice round. (The first level involves navigating around a red rectangular block; the second level involves navigating around the cowhide pattern shown on the opening webpage for the program. For both cases, take a screenshot of your final charge arrangement (with the charge in the goal) and attach it to this lab handout. If using the PhET Electric Field Hockey program (using Java): after doing the practice round, complete difficulty levels 1 and 2. Turn on the show electric field option. Attach screenshots showing your successful completion of those two levels (resulting in a collision with the goal). [Note, this program is a bit more challenging than the HTML5 version.] Level 3 is much more difficult than levels 1 and 2. Attempt it at your discretion. 5