Solutions for conceptual questions UMD Physics（解决方案概念物理UMD格式的问题）.pdfVIP

Solutions for conceptual questions 34.1 The induced current will be counterclockwise (ccw). As the bar moves upward through the constant magnetic field region, the area of the loop decreases, so the flux through the loop decreases. By Lenz’s law, any induced current will tend to oppose the decrease. Using right hand rule we know that magnetic field inside the loop due to a ccw current comes out of the plane. Therefore a ccw current will create a stronger magnetic field inside the loop, tending to increase the flux. 34.2 One has to push the loop against a repulsive force. As the loop enters the region between the poles the magnetic flux linked with the loop increases. Therefore the motion of the loop in the magnetic field induces a current in the loop in the direction shown. This induced current tends to oppose the flux increase by creating a magnetic field opposite in direction to that which existed due to the magnetic poles. In other words the induced current is such that the force the loop experiences due to this current opposes the loop to enter the region between the poles(Lenzes law) . The magnetic poles of the induced current loop are also shown in the diagram . The net magnetic force on the loops is to the right. 34.3 Yes. As the loop falls the magnetic flux linked with the loop decreases hence there is an induced current in the loop in the direction shown in the diagram. The net force due to this current is up since there is only an upward force on the top side of the loop and no force on the bottom side since it is not in a magnetic field. Whereas the other two sides experience forces which are equal and opposite in direction and hence they cancel each other.