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Unformatted text preview: pan (zp695) Ch17-h3 chiu (56565) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 2) 10.0 points The potential di ff erence from one end of a 1 cm-long wire to the other in a circuit is V = V B- V A = 2 . 1 V , as shown in the figure below. A B 1 cm Which end of the wire is at the higher po- tential? 1. The ends are at the same potential. 2. A 3. B correct Explanation: Since V B- V A is a positive number, it is clear that B is at the higher potential. 002 (part 2 of 2) 10.0 points What is the magnitude and direction of the field E inside the wire? (Since this is a 1D problem, the sign of your answer will indicate the direction.) Correct answer:- 210 V / m. Explanation: It should be clear that E points toward the left, since E points toward the lower potential. For a constant electric field, V =- E d =- E . For a path from A to B, = x, , . V =- E x x E x =- V x =- 2 . 1 V . 01 m =- 210 V / m . 003 10.0 points In a television picture tube, electrons are boiled out of a very hot metal filament placed near a negative metal plate. These electrons start out nearly at rest and are accelerated toward a positive metal plate. They pass through a hole in the positive plate on their way toward the picture screen, as shown in the diagram. Hot filament-------- + + + + + + + + + + Plate Plate F = eE E v = ? L The high-voltage supply in the television set maintains a potential di ff erence of 14500 V between the two plates, what speed do the electrons reach? Use m e = 9 . 11 10- 31 kg and q e = 1 . 6 10- 19 C and assume that this is not relativistic. Correct answer: 7 . 13674 10 7 m / s. Explanation: The net energy remains constant through- out the whole process. We can use the follow- ing train of logic: pan (zp695) Ch17-h3 chiu (56565) 2 E = 0 U + K = 0 q V + K = 0 (- e ) V + K = 0 K = e V = (1 . 6 10- 19 C)(14500 V) = 2 . 32 10- 15 J . Then K = K f- K i = 1 2 mv 2 f- v f = 2 K m = 2(2 . 32 10- 15 J) 9 . 11 10- 31 kg = 7 . 13674 10 7 m / s . 004 (part 1 of 2) 10.0 points What is the maximum possible potential (rel- ative to infinity) of a metal sphere of 19 cm in air? Remember that the breakdown electric field strength for air is roughly 3 10 6 N / C and also take 1 4 = k = 9 10 9 N m 2 / C 2 . Correct answer: 5 . 7 10 5 V. Explanation: The problem indicates that E max at the sur- face of the sphere is 3 10 6 N / C. Now, using the equations for electric field and potential for a spherically distributed charge, we have E = 1 4 Q R 2 Q = (3 10 6 N / C)(19 cm) 2 9 10 9 N m 2 / C 2 = 1 . 20333 10- 5 C ....
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This note was uploaded on 02/21/2012 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
- Spring '08