PHYSICS 202 EXAM 1 February 1, 2007 NAME: SECTION: 501 502 503 504 509 510 511 512 Note: 501 Recitation Mon 8:00 509 Recitation Tues 8 :00 502 Recitation Mon 9:10 510 Recitation Tues 9:35 503 Recitation Mon 10:20 511 Recitation Tues 11 :10 504 Recitation Mon 11:30 512 Recitation Tues 12:45 There are a total of 9 problems on this test. For each problem, write your answer in the space provided. You may use the backs of the pages for scratch calculations if you wish, but only the work in the spaces provided on the front of the pages will be graded. Problems A1 and A2 are worth 5 points each, A3 is worth 7 points, and A4 is worth 8 points. For these four problems, you do not need to show calculations or explain your reasoning unless requested. Also, no partial credit will be awarded for incorrect answers. Problems B1-B5 are worth 15 points each. For these five problems, you must show your work and/or explain your reasoning to receive any credit for a problem. Merely stating the answer is NOT sufficient. Partial credit will be awarded where appropriate. Also, to receive full credit in this part: Numerical answers should be given with 3 significant figures and be correct to within 2%. An answer can not be completely correct if it has the wrong units! G O O D L U C K!!!!!
Section A: Encircle the correct answer(s). Note that there may be more than one correct answer to a question. No partial credit will be awarded. A1: (5 points) An electron is released from rest between the plates of a charged parallel plate capacitor very close to the right-hand plate. Just as it reaches the left-hand plate, its speed is v. If the distance between the plates were halved without changing the electric potential difference between them, then the speed of the electron when it reached the left-hand plate would be (encircle the correct answer): 2v v 2 v v/ 2 v/2 A2: (5 points) The figure shows a gold leaf electroscope. It consists of a metal tube with a metal ball at the top and a sheet of extremely thin gold leaf fastened at the other end. The gold leaf is fastened in such a way that it is free to pivot about its upper edge. Encircle the correct answers: (a) If a charged rod is brought close to, but does not touch, the ball at the top, the gold leaf: Pivots outward Hangs straight downward (b) If charged rod touches the metal ball for a second or so and is then removed, the gold leaf: Pivots outward Hangs straight downward A3: (7 points) In the figure below, three small identical metal spheres have initial charges q 1 = 1 μc, q 2 = 2 μc, and q 3 = +3 μc. Draw the electric field lines for this charge configuration on the figure, using a total of 6 field lines. A4: (8 points) A parallel plate air-filled capacitor is charged by a battery. The battery is then disconnected from the charged capacitor. A piece of Teflon (K=2.1) is then inserted between the plates of the capacitor. Encircle the correct answers: (a) The capacitance Increases Stays the same Decreases (b) The charge on the plates Increases Stays the same Decreases (c) The potential difference between the plates Increases Stays the same Decreases (d) The electric energy stored Increases Stays the same Decreases
Section B: Must show your work. Partial credit will be awarded where appropriate. B1: (15 points) If the central charge shown in the figure below is displaced 0.30 nm to the right while the two other charges are held in place, find the magnitude and direction of the net force that the other two charges exert on it. Direction: Up Down Left Right Into page Out of page Magnitude B2: (15 points) Two stationary point charges of +4.00 nc and +3.00 nc are separated by a distance of 60.0 cm. An electron is released from rest at a point midway between the charges and moves along the line connecting them. What is the electric potential energy for the electron when it is: (a) At the midpoint (b) 15.0 cm from the +4.00 nc charge
B3: (15 points) At a certain distance from a point charge, the potential and electric-field magnitude due to that charge are 4.93 V and 12.5 V/m, respectively. (Take the potential to be zero at infinity.) (a) What is the distance to the point charge? (b) What is the magnitude of the charge? (c) Is the electric field directed toward or away from the point charge? (Encircle the correct answer.) Distance Magnitude Direction: Toward Away B4: (15 points) For the capacitor network shown below, the potential difference across ac is 22.0 V. Find: (a) The total energy stored in this network (b) The energy stored in the 15 pf capacitor
B5: (15 points) In a rectangular coordinate system, a positive point charge q = 3.00 nc is placed at the point x = +0.150 m, y = 0, and an identical point charge is placed at x = 0.150 m, y = 0. Find the x and y components of the electric field at the point x = +0.150 m, y = 0.380 m. E x E y