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Unformatted text preview: C HAPTER 19 Quick Quizzes 1. (b). The force that a magnetic field exerts on a charged particle moving through it is given by sin F qvB qvB = = , where B is the component of the field perpendicular to the particles velocity. Since the particle moves in a straight line, the magnetic force (and hence B , since qv &#2; ) must be zero. 2. (c). The magnetic force exerted by a magnetic field on a charge is proportional to the charges velocity relative to the field. If the charge is stationary, as in this situation, there is no magnetic force. 3. (a). The magnetic force acting on the particle is always perpendicular to the velocity of the particle, and hence to the displacement the particle is undergoing. Under these conditions, the force does no work on the particle and the particles kinetic energy remains constant. 4. (c). Anytime the velocity of a charged particle is perpendicular to the magnetic field, it will follow a circular path. The two particles will move in opposite directions around their circular paths if their charges have opposite signs, but their charges are unknown so (d) is not an acceptable answer. 5. (b). The two forces are an action-reaction pair. They act on different wires, and have equal magnitudes but opposite directions. 125 C H A P T E R 1 9 Problem Solutions 19.1 The direction in parts (a) through (d) is found by use of the right hand rule. You must remember that the electron is negatively charged, and thus experiences a force in the direction exactly opposite that predicted by the right hand rule for a positively charged particle. (a) h o r i z o n t a l a n d d u e e a s t (b) h o r i z o n t a l a n d 3 0 N o f E &#2; (c) h o r i z o n t a l a n d d u e e a s t (d) z e r o f o r c e , ( 29 sin sin 180 F qvB qvB = = &#0; = 19.2 (a) For a positively charged particle, the direction of the force is that predicted by the right hand rule. These are: (a) i n p l a n e o f p a g e a n d t o l e f t (b) i n t o t h e p a g e (c) o u t o f t h e p a g e (d) i n p l a n e o f p a g e a n d t o w a r d t h e t o p (e) i n t o t h e p a g e (f) o u t o f t h e p a g e (b) For a negatively charged particle, the direction of the force is exactly opposite what the right hand rule predicts for positive charges. Thus, the answers for part (b) are r e v e r s e d f r o m t h o s e g i v e n i n p a r t ( a ) ....
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