Fall 2010[COULOMB’S LAW]1 | P a g eSTUDIO Unit 02PHY 2054 STUDIO College Physics IIThis unit begins our study of Coulomb’s Law and theElectric Field. Be sure to check for any WebAssignmentthat has been posted for this chapter. You are certainlyfree to read ahead in the chapter.Coulombs LawRecord all of your answers and observations either in the space below a question or on aninserted blank sheet. These papers (always) should become a part of your notebook. Always feelfree to add pages as necessary. The ManagementWe begin this session with a game that includes in it, a simulation of Coulombs Law. A copy ofthe game is shown here: http://physics.ucf.edu/~bindell/PHY%202054%20STUDIO%20Spring%202010/electric-hockey_en.jarAt the left is a puck that can be a positive or a negative charge. At the right is a goal that youwant the puck to strike. The puck will move based on a distribution of charges that you canplace on the screen by dragging them from the boxes of charge at the top of the screen. The goaland any other barriers that are placed into the game (by level of difficulty) cannot be penetrated2 | P a g eby the puck but the force created by the charges can penetrate. Your job is to place chargesaround such that the “orbit” of the puck will strike the goal. The easy solution to this practicescreen is to place a (-) charge behind the goal and let it attract the puck. Try it. You candownload the program from the class website (www.physics.ucf.edu/~bindell). Note whathappens when you turn on “trace” or any of the other choices from the bottom of the screen.Notice that the FORCE on the puck (as shown by the arrow) will vary as the distance(s) from thecharge(s) are varied. More on this shortly.Learning from “simulations” is the latest thrust in modern education. A simulationmathematically models a situation in a very real way by including all of the appropriate laws ofphysics in the computer programs code. One has to take this on faith but one can discard theprogram if it ever shows a result that does not sync with reality.START WITH THIS ONEAn example is the “shoot the monkey” simulation at the link:http://jersey.uoregon.edu/vlab/newCannon/NewCannon_plugin.htmlIn this you have to adjust the cannon to aim directly at the monkey. After that, vary the angleand see what happens. Try it for a few minutes. It is assumed that you could do the math if youwere forced to. Right??A. The Attractive/Repulsive Race [Return to the first simulation.]1. We start with a race. When you get the signal (after you have a few minutes to get usedto the game), set the game for level “1” with a (+) puck and start. When you are able tohit the goal, feel free to yell out! You will be asked to share your solution with the class.2. Repeat for level 2, again waiting for the signal from your instructor to start.3. List two things that you have learned from this game (assuming that you have!). Do youknow why these things occur? If you do, write this in your notebook as well.3 | P a g e4. In the last session we looked at bringing charged rods close to charges tapes, etc.Qualitatively, how did the interaction depend upon the spacing between the charges?Was this consistent with what you saw in the hockey game?B. “Measuring: It, sort of.Next, we will look at the “arrow” that represents the force on the puck. You will make some“measurements” of the force on the puck and the distance of the puck from the deposited charge(within limits). You will need a ruler but, if one is not available, fold the one shown here and useit even though it probably isn’t very accurate. Don’t worry about the units of the measurement.We will start by placing a positive charge a small distance from the puck so that the “force”vector remains on the page and its length can be measured with our “ruler”. The distancebetween the “charges” can also be measured. A result such as the following will appear:Move the charge a short distance away from the original one and the diagram will change to:4 | P a g eFor each position of the charge, you can measure a pseudo-force and a table can be prepared thatlooks like the following (use Excel if you wish). Ignore units for this chart.Point # Distance Force1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Here are the next few steps.1. Fill in as much of the table shown above as you can. Make the measurements ascarefully as you can.2. Plot a graph of your data. What kind of graph did you plot? Some graph paper isprovided below for your use. Your instructor may have additional paper if you need it.3. You probably knew that the force is an inverse square (1/r2), but perhaps it is just aninverse distance effect (1/r). Use your knowledge of graphs, or statistics or magic toDEMONSTRATE that your data strongly suggests an inverse square law. You can usethe following graphs for your data. Use either one or both,email your graph to yourself or have one of your group members save it on aFlash Drive and have it emailed to you so you can print it for reference.4. Did you “prove” that the force is an inverse square law? Or have you proven somethingelse?? We will discuss your observations. Outline your “proof” for reference.5 | P a g e.6 | P a g e7 | P a g e8 | P a g eC. Charge MovementCharge can move around in some materials and is relatively fixed in others. We now know thatcharge comes in two flavors: (+) positive and (-) negative. Positive charges are difficult to move(bonding) but the outer electrons in metals are fairly mobile and can be easily moved from oneplace to another. These materials are called conductors. Materials for which these electrons aredifficult to move, the charge is relatively immobile and the materials are called insulators.Consider a metal (conductor) sphere that is neutral (no charge added). Is there ANY charge in or on the sphere?Now, sprinkle the sphere with a bunch of electrons. Draw a diagram of where the charges are inor on the sphere. Use + and – signs to suggest where the charge resides. Just show where theexcess charge winds up. Here is a simple example of a charge diagram:--------------------The diagram shows that excess negative charge is spread over the rod but the excess charge onthe paper is not