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Transcript
Chapter 22: Magnetism Answers to Even-Numbered Conceptual Questions 2. Yes. If an electric field exists in this region of space, and no magnetic field is present, the electric field will exert a force on the electron and cause it to accelerate. 4. In a uniform electric field the force on a charged particle is always in one fixed direction, as with gravity near the earth's surface, leading to parabolic trajectories. In a uniform magnetic field the force of a charged particle is always at right angles to the motion, resulting in circular or helical trajectories. Perhaps even more important, a charged particle experiences a force due to an electric field whether it is moving or at rest; in a magnetic field, the particle must be moving to experience a force. 6. A current-carrying wire in a uniform magnetic field can experience zero force only if the wire points in the same or opposite direction as the magnetic field. In such a case, the angle θ in equation 22-4 will be either 0˚ or 180˚, in which case F = ILB sinθ = 0. Chapter 23: Magnetic Flux and Faraday’s Law of Induction Answers to Even-Numbered Conceptual Questions 2. The magnetic field will have little apparent effect, because the break in the ring will prevent a current from flowing around its circumference. What the magnetic field will do, however, is produce a nonzero emf between the two sides of the break. 4. The metal plate moving between the poles of a magnet experiences eddy currents that retard its motion. This helps to damp out oscillations in the balance, resulting in more accurate readings. 6. Nothing. In this case, the break prevents a current from circulating around the ring. This, in turn, prevents the ring from experiencing a magnetic force that would propel it into the air. 8. As the penny begins to tip over, there is a large change in the magnetic flux through its surface, due to the great intensity of the MRI magnetic field. This change in magnetic flux generates an induced current in the penny that opposes its motion. As a result, the penny falls over slowly, as if it were immersed in molasses. 10. When the angular speed of the coil in an electric generator is increased, the rate at which the magnetic flux changes increases as well. As a result, the magnitude of the induced emf produced by the generator increases. Of course, the frequency of the induced emf increases as well. 12. When the switch is opened in a circuit with an inductor, the inductor tries to maintain the original current. (In general, an inductor acts to resist any change in the current—whether the current is increasing or decreasing.) Therefore, the continuing current may cause a spark to jump the gap. Chapter 25: Electromagnetic Waves Answers to Even-Numbered Conceptual Questions 2. We can tell if a weather system is approaching or receding by noting if the frequency of the reflected radar beam has been shifted up or down. The system is approaching if the frequency is higher; it is receding if the frequency is lower. 4. Light reflected from a horizontal surface has a polarization in the horizontal direction. It follows that when you sit upright, with the transmission axis of your glasses in the vertical direction, they will block most of the reflected light. When you lie on your side, however, the transmission axis is horizontal. This allows most of the reflected light to enter your eyes. 6. The light from the sky is polarized at right angles to the direction of the Sun; therefore, the amount of light received by each of the two polarizing eyes will depend on the orientation of the spider relative to the Sun. By monitoring the amount of light received by each eye, the spider can maintain a course on a given heading relative to the Sun. 8. View the light reflected from a horizontal surface, such as a tabletop. This light is polarized primarily in the horizontal direction. Therefore, if you rotate the sheet of polarizing material until you receive a maximum amount of reflected light, you will know that its transmission axis is horizontal. 10. As mentioned in the answer to conceptual question 4, the light reflected from a horizontal surface is polarized primarily in the horizontal direction. If the glasses are merely tinted, reflected light will have the same intensity no matter how the glasses are rotated. If they are Polaroid, however, you will notice a striking difference in reflected intensity as you rotate the glasses.