CollegePhysics19

# CollegePhysics19 - Chapter 19 MAGNETIC FORCES AND FIELDS...

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749 Chapter 19 MAGNETIC FORCES AND FIELDS Conceptual Questions 1. The magnetic field cannot be described as the magnetic force per unit charge because unlike the electric force, the magnetic force depends upon the velocity of the charge. 2. (a) It is possible for a charge to move with constant velocity through a region of space that contains only a magnetic field if the charge’s velocity is directed precisely along the direction of the magnetic field. (b) It is not possible for a charge to move with constant velocity through a region of space that contains only an electric field. (c) It is possible for a charge to move with constant velocity through a region of space containing both a magnetic field and an electric field if the magnetic field is aligned 90 degrees clockwise to the electric field as seen by an observer standing behind the moving charge. The charge’s velocity must also be equal to the ratio of the electric to the magnetic field strengths. 3. For a horizontal beam of electrons to be deflected to the right by a uniform magnetic field, the field must point in the downward direction. With this orientation, the magnetic force would be to the left for positively charged particles and to the right for those with a negative charge. 4. The large arrows depict the direction of the magnetic field vectors at the indicated points around the loop. I I A B C D E 5. A constant magnetic field does zero work on a moving charge and therefore cannot change its kinetic energy—the speed of a particle with constant kinetic energy must also be constant. 6. (a) The electron must be moving up or down along the direction of the magnetic field. (b) The electron must be moving in the horizontal plane perpendicular to the magnetic field. 7. In a velocity selector, the velocity vector must be perpendicular to both the electric and the magnetic field vectors. There are therefore two possible orientations for the velocity vector—in the direction of × EB !! or in the direction of . If the velocity were directed along the latter, the direction of the electric force would be parallel to the direction of the magnetic force. The two forces must be antiparallel to negate each other—the velocity vector must therefore be directed parallel to . × ! !

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Chapter 19: Magnetic Forces and Fields College Physics 750 8. The magnitude of the magnetic force is directly proportional to the charge of the moving particle. Thus, a particle that is doubly charged experiences twice the magnetic force as one that is singly charged. The radius of an object in centripetal orbit is equal to the ratio of the square of the orbital velocity to the magnitude of the centripetal acceleration. The magnetic force does not change the velocity of either charge, and thus, it is only the value of the acceleration that determines the radius. The doubly-charged particle experiences a larger acceleration, and therefore, has a radius half the size of the singly-charged particle.
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CollegePhysics19 - Chapter 19 MAGNETIC FORCES AND FIELDS...

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