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Static Electricity: Coulomb’s Law It is always a good idea to read up on a topic before you conduct a lab exercise. For this lab it is suggested that you review Giancoli Chapter 16 on Static Electricity. A site called “The Physics Classroom” is also a good place to brush up on any topic in Intro Physics that you want to. From early civilizations on, humanity has been aware of the phenomenon of static electricity. Who hasn’t walked on a carpet, touched a door knob and experienced a small electric shock? The first record of the study of static electricity was that of the pre-Socratic philosopher Thales of Miletus (around 600BCE). He found that rubbing amber (a fossilized tree resin) with something (he didn’t say what) caused it to attract bits of leaves and twigs. Twenty-eight hundred years later scientists have a pretty good idea of how rubbing things together generates forces of attraction and repulsion. Charles Coulomb back in 1785, building on the work done by others earlier, was able to formulate and verify a mathematical description of these forces. ( Both as scalar and vector arrangements) Here q is the charge in coulombs, r is the distance between charges in meters and k is the electrostatic constant 9 x 109. C (N m2)/C2. The inverse square arrangement is strikingly similar to the form of Newton’s Law of Universal Gravitation. In this lab exercise, you will create demonstrations of static electricity. In addition, through PhET simulations and using Coulomb’s Law, you will be able to calculate the forces that arise from the interaction of such charges. A. Demonstration of Static Electricity ( S.E.) First go to Youtube to watch a series of S.E. phenomena. In your browser go to Static Electricity Demonstrations Part one Induction // Homemade Science with Bruce Yeany. We will watch this together. You are welcome to view it later on your own time. You can also watch Part 2. However, you need not be restricted to just these demos. There are plenty more. Once you have done viewing S.E. demos, pick two that you will try to replicate. Obviously, you are not in a lab where lab materials can be presented to you. So this exercise entails directing you to actually create objects that can demonstrate how objects become charged and how forces due to S.E. can be created. Make sure that the demos you select can be done in your home with ordinary household material. DO NOT go out and buy expensive parts. Take pictures of what you have made and paste them into your report. Be sure to identify the demos you have created. After your project is complete answer the questions below. 1 If for whatever reason, you are unable to actually build something, say so explicitly. Then describe what you would have done and respond to the questions below as if you had actually built it. Sketch how charges are distributed among the objects in your demo as they are rubbed. Scan and paste your sketches into the report. 2 Describe why an object becomes charged through friction. 3. Explain why some objects become negatively charged while others become positively charged when rubbed. 4. Rubber with Wool and Glass with Silk. They are rubbed together. Which is negative and which is positive. Explain why. 5. Why can a charged object attract a neutral object? 6. Can you support this statement: “An object becomes positively charged when protons migrate to it after being rubbed”? Why or why not? 7. Who first labelled charges Positive and Negative. This person was also ambassador to France (and apparently was engaged in a number of “foreign affairs”.) This person incidentally also was the first to use the term electrical battery. 8. Why does the term “ground” come into play when dealing with electricity of any kind? 9. How does a Wimshurst machine work and why is it useful? 9. What did the people of a Dutch town called Lieden or Leyden do to store S.E? B. Determining the Force Between Charges Copy and paste this PhET url into your address bar at the top. https://phet.colorado.edu/en/simulation/coulombs-law Click the Macro Scale to get to this screen 2 Fool around with the controls to see how the simulation works. 2 As shown below, set the blue charge on the left at -2uC and the red one on the right at 4uC. Using the ruler, arrange the blue one at 0 and the red one at 10. Also be sure to check the Force Values on the right. Record the Distance and Force values on the table below when the centers of the two spheres are 10 centimeters apart. Then move the center of the red sphere to 9 centimeters and record the force again. Continue this way for the other distance values. Distance (cm) Force (Newtons) Distance (cm) 10 6 9 5 8 4 7 3 Force (Newtons) RESPOND TO THIS QUESTION: Why is it that we record distances from the Center of the spheres and not from their edges? GRAPH (Nice and Neat) Now using a full sheet of graph paper, plot the data with F on the y axis and Distance on the x axis. Sketch a line of best fit for the data points. What kind of line of best fit should it be if it is not a straight line? Now work out these problems and show your work! After finding the solution, verify your answer by manipulating the spheres in the simulation adjusting for position and charge. Be sure that you check both the Force and Scientific Notation boxes. How close were you? 3 Post Lab Questions 1. The blue charge on the left ( -3 microcoulombs) is at the 3-centimeter mark. If the red charge on the right (6 microcoulombs) is located at the 9-centimeter mark, what is the magnitude of the attractive force between them? 2. By doing one quick mathematical step, predict what the attractive force between them would be if the red charge were moved to the 6-centimeter mark? Verify your answer in the simulation. 3. How far apart would the two charges have to be if the attractive force between them was -20 newtons? Verify your answer in the simulation. These next problems require that you retrieve your vector analysis skills in order to solve them. They cannot be applied to the simulations 4. What would the net force be (remember signs) on a 2 microcoulomb charge if it were 2 centimeters to the left of the blue charge ( -3 microcoulombs)? The red charge (6 microcoulombs) continues to be 6 centimeters to the right of the blue one. Show your calculations. 5. Two charges are 24 centimeters apart. On the left, the charge is +4 x 10-6 C and, on the right, the charge is +9 x 10-6C. A third charge between them is +3 x 10-6C. How far from the left charge must the middle charge be positioned, so that the net force acting on the middle charge is zero. Show how you would set up the equation, then use Google or a smartphone calculator to determine the actual value. 6. Three charges are placed at the corners of a right triangle as shown in the figure below. The bottom leg is 4 meters wide and the vertical leg is 3 meters long. 2C 3 meters tall -1C 4 meters 8C 4 Find the net force on the 2-coulomb charge at the top. Show how you set it up. Hint: Remember your vectors. You need to find the sum of the x and y forces. Use Google or your smartphone to avoid the drudge arithmetic. Since the net force is a vector, find both the magnitude and angle relative to the VERTICAL axis. 5
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