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Unformatted text preview: 453 Chapter 26 The Magnetic Field Conceptual Problems *1 • Determine the Concept Because the electrons are initially moving at 90 ° to the magnetic field, they will be deflected in the direction of the magnetic force acting on them. Use the right-hand rule based on the expression for the magnetic force acting on a moving charge B v F r r r × = q , remembering that, for a negative charge, the force is in the direction opposite that indicated by the right-hand rule, to convince yourself that the particle will follow the path whose terminal point on the screen is 2. correct. is ) ( b 2 • Determine the Concept One cannot define the direction of the force arbitrarily. By experiment, F r is perpendicular to B r . 3 • Determine the Concept False. An object experiences acceleration if either its speed changes or the direction it is moving changes. The magnetic force, acting perpendicular to the direction a charged particle is moving, changes the particle’s velocity by changing the direction it is moving and hence accelerates the particle. 4 • Determine the Concept Yes; it will be deflected upward. Because the beam passes through undeflected when traveling from left to right, we know that the upward electric force must be balanced by a downward magnetic force. Application of the right-hand rule tells us that the magnetic field must be out of the page. When the beam is reversed, the magnetic force (as well as the electric force) acting on it is upward. *5 • Determine the Concept The alternating current running through the filament is changing direction every 1/60 s, so in a magnetic field the filament experiences a force which alternates in direction at that frequency. 6 • Determine the Concept The magnitude of the torque on a current loop is given by , sin θ µ τ B = where θ is the angle between the magnetic field and a normal to the surface of the loop. To maximize τ , sin θ = 1 and θ = 90 ° . Hence the normal to the plane of the loop should be perpendicular to B r . Chapter 26 454 7 • ( a ) True. This is an experimental fact and is the basis for the definition of the magnetic force on a moving charged particle being expressed in terms of the cross product of v r and B v ; i.e. B v F r r v × = q . ( b ) True. This is another experimental fact. The torque on a magnet is a restoring torque, i.e., one that acts in such a direction as to align the magnet with magnetic field. ( c ) True. We can use a right-hand rule to relate the direction of the magnetic field around the loop to the direction of the current. Doing so indicates that one side of the loop acts like a north pole and the other like a south pole. ( d ) False. The period of a particle moving in a circular path in a magnetic field is given by qvB mr T π 2 = and, hence, is proportional to the square root of the radius of the circle....
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- Spring '07
- Force, Magnetic Field, numerical values