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Unformatted text preview: CHAPTER 21 MAGNETIC FORCES AND MAGNETIC FIELDS CONCEPTUAL QUESTIONS ____________________________________________________________________________________________ 1. REASONING AND SOLUTION Magnetic field lines, like electric field lines, never intersect. When a moving test charge is placed in a magnetic field so that its velocity vector has a component perpendicular to the field, the particle will experience a force. That force is perpendicular to both the direction of the field and the direction of the velocity. If it were possible for magnetic field lines to intersect, then there would be a different force associated with each of the two intersecting field lines; the particle could be pushed in two directions. Since the force on a particle always has a unique direction, we can conclude that magnetic field lines can never cross. ____________________________________________________________________________________________ 2 . REASONING AND SOLUTION If you accidentally use your left hand, instead of your right hand, to determine the direction of the magnetic force on a positive charge moving in a magnetic field, the direction that you determine will be exactly opposite to the correct direction. ____________________________________________________________________________________________ 3 . SSM REASONING AND SOLUTION A charged particle, passing through a certain region of space, has a velocity whose magnitude and direction remain constant. a. If it is known that the external magnetic field is zero everywhere in the region, we can conclude that the electric field is also zero. Any charged particle placed in an electric field will experience a force given by F = q E , where q is the charge and E is the electric field. If the magnitude and direction of the velocity of the particle are constant, then the particle has zero acceleration. From Newton's second law, we know that the net force on the particle is zero. But there is no magnetic field and, hence, no magnetic force. Therefore, the net force is the electric force. Since the electric force is zero, the electric field must be zero. b. If it is known that the external electric field is zero everywhere, we cannot conclude that the external magnetic field is also zero. In order for a moving charged particle to experience a magnetic force when it is placed in a magnetic field, the velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field. If the moving charged particle enters the region such that its velocity is parallel or antiparallel to the magnetic field, it will experience no magnetic force, even though a magnetic field is present. In the absence of an external electric field, there is no electric force either. Thus, there is no net force, and the velocity vector will not change in any way....
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