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Chapter 29 Magnetic Fields

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Quick Quiz 29.1 The north-pole end of a bar magnet is held near a positively charged piece of plastic. The plastic is (a) attracted (b) repelled (c) unaffected by the magnet

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Answer: (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. Quick Quiz 29.1

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Quick Quiz 29.2 A charged particle moves with velocity v in a magnetic field B. The magnetic force on the particle is a maximum when v is (a) parallel to B (b) perpendicular to B (c) zero

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Answer: (b). The maximum value of sin θ occurs for θ = 90°. Quick Quiz 29.2

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Quick Quiz 29.3 An electron moves in the plane of this paper toward the top of the page. A magnetic field is also in the plane of the page and directed toward the right. The direction of the magnetic force on the electron is (a) toward the top of the page (b) toward the bottom of the page (c) toward the left edge of the page (d) toward the right edge of the page (e) upward out of the page (f) downward into the page

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Answer: (e). The right-hand rule gives the direction. Be sure to account for the negative charge on the electron. Quick Quiz 29.3

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Quick Quiz 29.4 The four wires shown below all carry the same current from point A to point B through the same magnetic field. In all four parts of the figure, the points A and B are 10 cm apart. Which of the following ranks wires according to the magnitude of the magnetic force exerted on them, from greatest to least? (a) b, c, d (b) a, c, b (c) d, c, b (d) c, a, b (e) No force is exerted on any of the wires.

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Answer: (a). (a), (b) = (c), (d). The magnitude of the force depends on the value of sin θ. The maximum force occurs when the wire is perpendicular to the field (a), and there is zero force when the wire is parallel (d). Choices (b) and (c) represent the same force because Case 1 tells us that a straight wire between A and B will have the same force on it as the curved wire. Quick Quiz 29.4

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Quick Quiz 29.5 A wire carries current in the plane of this paper toward the top of the page. The wire experiences a magnetic force toward the right edge of the page. The direction of the magnetic field causing this force is (a) in the plane of the page and toward the left edge (b) in the plane of the page and toward the bottom edge (c) upward out of the page (d) downward into the page

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Answer: (c). Use the right-hand rule to determine the direction of the magnetic field. Quick Quiz 29.5

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Quick Quiz 29.6 Rank the magnitudes of the torques acting on the rectangular loops shown in the figure below, from highest to lowest. (All the loops are identical and carry the same current.) (a) a, b, c (b) b, c, a (c) c, b, a (d) a, c, b. (e) All loops experience zero torque.

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Answer: (c). Because all loops enclose the same area and carry the same current, the magnitude of μ is the same for all. For part (c) in the image, μ points upward and is perpendicular to the magnetic field and τ = μB, the maximum torque possible. For the loop in (a), μ points along the direction of B and the torque is zero. For (b), the torque is intermediate between zero and the maximum value. Quick Quiz 29.6

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Quick Quiz 29.7 Rank the magnitudes of the net forces acting on the rectangular loops shown in this figure, from highest to lowest. (All the loops are identical and carry the same current.) (a) a, b, c (b) b, c, a (c) c, b, a (d) b, a, c (e) All loops experience zero net force.

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Answer: (e). Because the magnetic field is uniform, there is zero net force on all three loops. Quick Quiz 29.7

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Quick Quiz 29.8 A charged particle is moving perpendicular to a magnetic field in a circle with a radius r. An identical particle enters the field, with v perpendicular to B, but with a higher speed v than the first particle. Compared to the radius of the circle for the first particle, the radius of the circle for the second particle is (a) smaller (b) larger (c) equal in size

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Answer: (b). The magnetic force on the particle increases in proportion to v, but the centripetal acceleration increases according to the square of v. The result is a larger radius, as we can see from Equation Quick Quiz 29.8

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Quick Quiz 29.9 A charged particle is moving perpendicular to a magnetic field in a circle with a radius r. The magnitude of the magnetic field is increased. Compared to the initial radius of the circular path, the radius of the new path is (a) smaller (b) larger (c) equal in size

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Answer: (a). The magnetic force on the particle increases in proportion to B. The result is a smaller radius, as we can see from Equation Quick Quiz 29.9

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Quick Quiz 29.10a Three types of particles enter a mass spectrometer like the one shown in your book as Figure The figure below shows where the particles strike the detector array. Rank the particles that arrive at a, b, and c by speed. (a) a, b, c (b) b, c, a (c) c, b, a (d) All their speeds are equal.

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Answer: (d). The velocity selector ensures that all three types of particles have the same speed. Quick Quiz 29.10a

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Quick Quiz 29.10b Rank the particles that arrive at a, b, and c by m/q ratio. (a) a, b, c (b) b, c, a (c) c, b, a (d) All their m/q ratios are equal.

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Answer: (c). We cannot determine individual masses or charges, but we can rank the particles by m/q ratio. Equation indicates that those particles traveling through the circle of greatest radius have the greatest m/q ratio. Quick Quiz 29.10b

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