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Unformatted text preview: benavides (jjb2356) homework 17 Turner (59130) 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. None of these 2. yes for both 3. yes; no 4. no; yes correct 5. no for both 6. It depends on the intensity of the fields, which is not provided in the problem. 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. They have opposite charges if their initial velocities are in the same direction. correct 2. One particle is an electron and the other is a positive ion. 3. Their velocities have opposite direc- tions. 4. One particle comes from nature; the other is man-made. 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 . 00195 T that points straight down. The charged particles in the beam move in a circular path of radius 2 . 83 cm. If the charged particles in the beam were accelerated through a potential difference of 307 V, determine the charge to mass ratio of the charged particles in the beam. Correct answer: 2 . 01617 10 11 C / kg. Explanation: Let : B = 0 . 00195 T , r = 2 . 83 cm = 0 . 0283 m , and V = 307 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 . benavides (jjb2356) homework 17 Turner (59130) 2 Thus , the charge-to-mass ratio is R = q m = 2 V B 2 r 2 = 2(307 V) (0 . 00195 T) 2 (0 . 0283 m) 2 = 2 . 01617 10 11 C / kg ....
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This note was uploaded on 03/26/2012 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
- Spring '08