This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: This printout should have 27 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 6) 5.0 points A plastic rod 1 . 5 m long is rubbed all over with wool, and acquires a charge of 4 10 8 C (see the figure below). 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 , which is at r A = . 9 , , m , we divide the rod into 8 pieces, and approxi mate each piece as a point charge located at the center of the piece. A 1 2 3 4 5 6 7 8 What is the length of one of these pieces? Use the value of k = 8 . 98755 10 9 N m 2 / C 2 . Correct answer: 0 . 1875 m. Explanation: We simply divide the length of the rod by 8: y = (1 . 5 m) / 8 = 0 . 1875 m . 002 (part 2 of 6) 5.0 points What is the y coordinate of the center of piece number 6? Correct answer: . 28125 m. Explanation: Since the center of the rod is at (0,0,0) and it lies along the y axis, we know that only the y coordinate changes as we move along the rod. To find the y coordinate of the center of this particular piece, we just add the lower and upper y bounds of the piece and divide by 2: r center , 6 y = r lower , 6 y + r upper , 6 y 2 = . 375 m + ( . 1875 m) 2 = . 28125 m . 003 (part 3 of 6) 5.0 points How much charge is on piece number 6? (Re member that the charge is negative.) Correct answer: 5 10 9 C. Explanation: As in the first part of this problem, we know the total charge, so to find the charge on any particular piece, we simply divide by 8: Q = 4 10 8 C / 8 = 5 10 9 C . 004 (part 4 of 6) 5.0 points Approximating piece 6 as a point charge, what is the electric field at location A due only to piece 6? This is a vector with x and y components, E ( A ) = E ( A ) x , E ( A ) y , N / C , so begin by finding the x component, E ( A ) x . Correct answer: 48 . 2422 N / C. Explanation: The electric field of a point charge is given by the familiar expression E ( A ) = 1 4 q  r  2 r. So we need the total distance between the piece we are considering and the location A , and also the unit vector pointing from the piece to A . The distance is easy to find:  r  = ( . 28125 m) 2 + (0 . 9 m) 2 = 0 . 942922 m . The unit vectors components are r = . 9 , . 28125 , m . 942922 m = . 95448 , . 298275 , . The x component of the field, then, is E ( A ) x = 1 4  5 10 9 C (0 . 942922 m) 2 (0 . 95448) = 48 . 2422 N / C . 005 (part 5 of 6) 5.0 points Find E ( A ) y . Correct answer: 15 . 0757 N / C. Explanation: This is just like part 4, except with respect to the y components: E ( A ) y = 1 4  5 10 9 C (0 . 942922 m) 2 (0 . 298275) = 15 . 0757 N / C ....
View
Full
Document
This note was uploaded on 10/25/2011 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
 Spring '08
 Turner
 Physics

Click to edit the document details