Physics for Scientists and Engineers, 6eChapter 29 Magnetic Fields

The north-pole end of a bar magnet is held near a positively charged piece of plastic. The plastic isattracted repelledunaffected by the magnet

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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.

A charged particle moves with velocity v in a magnetic field B. The magnetic force on the particle is a maximum when v is parallel to Bperpendicular to Bzero

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The maximum value of sin occurs for = 90.

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 toward the top of the pagetoward the bottom of the pagetoward the left edge of the pagetoward the right edge of the pageupward out of the pagedownward into the page

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The right-hand rule gives the direction. Be sure to account for the negative charge on the electron.

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?b, c, d a, c, b d, c, b c, a, b No force is exerted on any of the wires.

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(a), (b) = (c), (d). The magnitude of the force depends on the value of sin . 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 (2) and (3) 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.

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 in the plane of the page and toward the left edgein the plane of the page and toward the bottom edgeupward out of the pagedownward into the page

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Use the right-hand rule to determine the direction of the magnetic field.

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, b, c b, c, a c, b, a a, c, b. All loops experience zero torque.

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

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, b, cb, c, ac, b, ab, a, cAll loops experience zero net force.

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Because the magnetic field is uniform, there is zero net force on all three loops.

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 smaller larger equal in size

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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 29.13.

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 smaller larger equal in size

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The magnetic force on the particle increases in proportion to B. The result is a smaller radius, as we can see from Equation 29.13.

Three types of particles enter a mass spectrometer like the one shown in your book as Figure 29.24. The figure below shows where the particles strike the detector array. Rank the particles that arrive at a, b, and c by speed.a, b, c b, c, a c, b, a All their speeds are equal.

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The velocity selector ensures that all three types of particles have the same speed.

Rank the particles that arrive at a, b, and c by m/q ratio.a, b, c b, c, ac, b, a All their m/q ratios are equal.

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We cannot determine individual masses or charges, but we can rank the particles by m/q ratio. Equation 29.18 indicates that those particles traveling through the circle of greatest radius have the greatest m/q ratio.