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Unformatted text preview: WebAssign Ch16 HW1 S2010 (Homework) Kiyaniah Tilghman PY 208, section 003M, Spring 2010 Instructor: Keith Weninger Current Score : 21.5 / 22.5 Due : Tuesday, February 2, 2010 12:30 PM EST About this Assignment Extension Requests Question Points 12 3 8.5 4 9/10 Total 21.5/22.5 (95.6%) Description Electric fields due to distributed charges; electric field of a uniformly charged thin rod Instructions Reading: Sec. 16.1-16.3 The due date for this assignment is past. Your work can be viewed below, but no changes can be made. 1. 8.5/8.5 points All Submissions Notes Question: MI3 16.X.019 A plastic rod 1.6 m long is rubbed all over with wool, and acquires a charge of - 3e-08 coulombs. We choose the center of the rod to be the origin of our coordinate system, with the x-axis extending to the right, the y-axis extending up, and the z-axis out of the page. In order to calculate the electric field at location A = < 0.7, 0, 0 > m, we divide the rod into 8 pieces, and approximate each piece as a point charge located at the center of the piece. (a) What is the length of one of these pieces? .2 0.2 m (b) What is the location of the center of piece number 3? <0 0 , .3 0.3 , 0 0 > m. (c) How much charge is on piece number 3? (Remember that the charge is negative.) - 3.75e-9 - 3.75e-09 coulombs (d) Approximating piece 3 as a point charge, what is the electric field at location A due only to piece 3? - 53.5 , 22.92 22.9 , 0 0 > N/C 3 = < - 53.48 (e) To get the net electric field at location A, we would need to calculate due each of the 8 pieces, and add up these contributions. If we did that, which arrow above would best represent the direction of the net electric field at location A? g g 2. 4/4 points All Submissions Notes Question: MI3 16.X.020 A thin plastic rod of length 2.6 m is rubbed all over with wool, and acquires a charge of 76 nC, distributed uniformly over its surface. Calculate the magnitude of the electric field due to the rod at a location 11 cm from the midpoint of the rod. Do the calculation two ways, first using the exact formula for a rod of any length, and second using the approximate formula for a long rod. (a) exact formula E = 4.766e3 4770 N/C (b) approximate formula E = 4.783e3 4780 N/C 3. 9/10 points All Submissions Notes Question: MI3 16.P.069.alt02 A strip of invisible tape 0.16 m long by 0.017 m wide is charged uniformly with a total net charge of 5 nC (nano = 1e-9) and is suspended horizontally, so it lies along the x axis, with its center at the origin, as shown in the diagram. Calculate the approximate electric field at location < 0, 0.03 , 0 > m (location A) due to the strip of tape. Do this by dividing the strip into three equal sections, as shown in the diagram, and approximating each section as a point charge. What is the approximate electric field at A due to piece #1? < - 3.519e3 3490 , 1.992e3 1960 , 0 0 > N/C What is the approximate electric field at A due to piece #2? <0 0 , 1.6666e4 16700 , 0 0 > N/C What is the approximate electric field at A due to piece #3? < - 3.519e3 - 3490 , 1.992e3 1960 , 0 0 > N/C What is the approximate net electric field at A? <0 0 , 2.065e4 20600 , 0 0 > N/C What could you do to improve the accuracy of your calculation? Use three pieces but give many more significant figures in the answer. Divide the tape into more pieces. Divide the tape into fewer pieces. Solution or Explanation Each piece has 1/3 of the charge of the entire tape. To treat each piece as a point charge located at the center of the piece, find the location of the center of each piece. Calculate the electric field at A due to each piece, then add these contributions to get the net field. ...
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