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HW7 - oliver(jmo673 HW07 Tsoi(59090 This print-out should...

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oliver (jmo673) – HW07 – Tsoi – (59090) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A particle with charge q and mass m is un- dergoing circular motion with speed v . At t = 0, the particle is moving along the nega- tive x axis in the plane perpendicular to the magnetic field vector B , which points in the positive z direction in the figure below. x y z vectorv vector B Find the direction of the instantaneous ac- celeration hatwide a at t = 0 if q is negative. 1. hatwide a = ˆ j + ˆ k 2. hatwide a = ˆ k 3. hatwide a = ˆ j correct 4. hatwide a = ˆ i + ˆ j 5. hatwide a = ˆ i 6. hatwide a = ˆ k + ˆ i 7. hatwide a = ˆ i 8. hatwide a = ˆ j 9. hatwide a = ˆ k 10. hatwide a = ˆ k + ˆ i Explanation: The particle is moving along the negative x -axis in this instant vectorv = v ˆ i ; since it is moving in a circle, we need to talk about instantaneous direction. The force F B is equal to q vectorv × vector B at all times. We know that vector B is pointing in the z direction, so vector B = B ˆ k , and therefore vector F B = q v ( ˆ i ) × B ˆ k = q v B ( ˆ i × ˆ k ) = q v B ˆ j . The charge q is negative ( q = −| q | ) , so vector F B = −| q | v B ˆ j = | q | v B ( ˆ j ) . All quantities are positive, so the actual di- rection in which vector F B points is the negative y direction, or hatwide a = ˆ j . 002 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; no 3. no for both 4. None of these 5. It depends on the intensity of the fields, which is not provided in the problem. 6. yes for both 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. 003 10.0 points Two charged particles are projected into a magnetic field that is perpendicular to their initial velocities.

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oliver (jmo673) – HW07 – Tsoi – (59090) 2 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. Their velocities have opposite direc- tions. 2. One particle comes from nature; the other is man-made. 3. One particle is an electron and the other is a positive ion. 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.
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