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Unformatted text preview: nguyen (jmn727) – homework 17 – Turner – (59070) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Can an electron at rest in a magnetic field be set into motion by the magnetic field? What if it were at rest in an electric field? 1. no; yes correct 2. yes for both 3. None of these 4. It depends on the intensity of the fields, which is not provided in the problem. 5. no for both 6. yes; no Explanation: An electron has to move across lines of magnetic field in order to feel a magnetic force, so an electron at rest in a stationary magnetic field will feel no force to set it in motion. However, an electron in an electric field will accelerate regardless of its current state of motion. 002 10.0 points Two charged particles are projected into a magnetic field that is perpendicular to their initial velocities. If the charges are deflected in opposite di- rections, what does this tell you about them? (Ignore the interaction between these two par- ticles.) 1. One particle is an electron and the other is a positive ion. 2. Their velocities have opposite direc- tions. 3. One particle comes from nature; the other is man-made. 4. They have opposite charges if their initial velocities are in the same direction. correct Explanation: The magnetic force exerted on a particle depends on the charge of the particle, the velocity of the particle and the magnitude and direction of the magnetic field. If two particles have the same velocities but opposite charges they feel opposite magnetic forces so that they are deflected in opposite directions. But, this does not guarantee that one of them is an electron. 003 10.0 points A charged particle beam (shot horizontally) moves into a region where there is a constant magnetic field of magnitude 0 . 00127 T that points straight down. The charged particles in the beam move in a circular path of radius 2 . 33 cm. If the charged particles in the beam were accelerated through a potential difference of 280 V, determine the charge to mass ratio of the charged particles in the beam. Correct answer: 6 . 39542 × 10 11 C / kg. Explanation: Let : B = 0 . 00127 T , r = 2 . 33 cm = 0 . 0233 m , and V = 280 V . The kinetic energy of the electron after accel- erated through the potential difference is 1 2 mv 2 = q V . And the centrifugal force experienced by the charge particle in the magnetic field is q v B sin(90 ◦ ) = mv 2 r then we have v = q b r m then 1 2 m q 2 B 2 r 2 m 2 = q V . nguyen (jmn727) – homework 17 – Turner – (59070) 2 Thus , the charge-to-mass ratio is R = q m = 2 V B 2 r 2 = 2(280 V) (0 . 00127 T) 2 (0 . 0233 m) 2 = 6 . 39542 × 10 11 C / kg ....
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This note was uploaded on 01/21/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
- Spring '08