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CHAPTER 22 ELECTROMAGNETIC INDUCTION CONCEPTUAL QUESTIONS ____________________________________________________________________________________________ 1. REASONING AND SOLUTION If the coil and the magnet in Figure 22.1 a were each moving with the same velocity relative to the earth, there would be no relative motion between the magnet and the coil. The magnetic flux through the coil due to the bar magnet would be constant and, therefore, the combined motion of the bar magnet and the coil would not result in an induced current in the coil. We are ignoring here any effect due to the earth's magnetic field. ____________________________________________________________________________________________ 2. REASONING AND SOLUTION In the discussion concerning Figure 22.5, we saw that a force of 0.086 N from an external agent was required to keep the rod moving at constant speed. Suppose the light bulb in the figure is unscrewed from its socket. Once the light bulb is unscrewed, the conducting rails and the rod are no longer part of a complete circuit (the resistance of the empty socket is infinite). Therefore, even though there will be a charge separation and a motional emf in the rod given by vBL , there will be no current in the rod. Since there is no current, there is no magnetic force to resist the motion of the rod. From Newton's first law, the rod will continue to move with constant velocity v without the application of an external force. ____________________________________________________________________________________________ 3. REASONING AND SOLUTION A metal sheet moves to the right at a velocity v in a magnetic field B that is directed into the sheet. At the instant shown in the figure, the magnetic field only extends over half of the sheet. An induced emf leads to the eddy current shown. (into page) B v Metal sheet Eddy current x xx x x x xxx We can apply RHR-1 (modified for currents) to the portion of the eddy current that exists in the portion of the sheet that is in the magnetic field. With the thumb of the right hand pointing toward the top of the page (direction of I ), and the fingers of the right hand pointing into the page (direction of B ), the palm of the right hand faces the left (direction of F ). Thus, there is a retarding magnetic force F that acts on the sheet due to the interaction of the eddy current with the magnetic field. Hence, the eddy current causes the sheet to slow down. ____________________________________________________________________________________________
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Chapter 22 Conceptual Questions 1163 4. SSM REASONING AND SOLUTION A magnetic field B is necessary if there is to be a magnetic flux Φ passing through a coil of wire. Yet, just because there is a magnetic field does not mean that a magnetic flux will pass through a coil. The general expression for magnetic flux is given by Equation 22.2: ( c o s ) B A φ Φ= , where B is the magnitude of the magnetic field, A is the cross-sectional area of the coil, and is the angle between the magnetic field B and the normal to the surface of the coil. Equation 22.2 shows that the flux depends only on the component of the magnetic field that is perpendicular to the surface of the coil. As shown in Example 4 and in Figure 22.10, when the coil is oriented so that it is parallel to the field, = 90°, B has no component normal to the surface of the coil, and the magnetic flux through the coil is zero. Therefore, the magnetic flux through a coil can be zero even though there is a magnetic field present.
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This note was uploaded on 05/09/2008 for the course PHYS 25 taught by Professor Holland during the Spring '08 term at Pacific.

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