CollegePhysics20

# CollegePhysics20 - Chapter 20 ELECTROMAGNETIC INDUCTION...

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780 Chapter 20 ELECTROMAGNETIC INDUCTION Conceptual Questions 1. As the loop rotates, the magnitude of the flux increases as it approaches the maximum positive value with the loop perpendicular to the field and then decreases to zero as the loop becomes parallel to the field. As it turns through the maximum position and the flux changes from increasing to decreasing, the induced current reverses direction, because the sign of the change it’s opposing has flipped. As the loop continues to rotate, the flux reaches its maximum negative value when the loop is once again perpendicular to the field but facing the opposite way. As it rotates through this position and the flux changes from decreasing to increasing, the induced current will once again reverse direction. Thus, the induced current reverses its direction twice per rotation. 2. The alternating current produces an oscillating magnetic field in the coil accompanied by an oscillating electric field. If the core were made of solid iron the electric field would cause eddy currents to flow, which would dissipate energy. Making the core out of laminated iron or a bundle of insulated wires significantly reduces the flow of eddy currents, thereby increasing the efficiency of the transformer. 3. The energy stored in an inductor is given by 2 / 2. U LI = Therefore, it takes 3 times as much energy to increase the current from 10 mA to 20 mA as it does to increase it from 0 mA to 10 mA. 4. There would be a very brief emf induced as the current in the primary builds up to its final value. Once the current in the primary reaches its final value, there would no longer be a change in flux through the secondary and no emf induced. 5. (a) In position 1, there is an eddy current circulating clockwise to oppose the increase in magnetic flux as the plate enters the magnetic field. (b) In position 2, there is an eddy current circulating counterclockwise to oppose the decrease in magnetic flux as the plate leaves the magnetic field. (c) From the right hand rule, the induced magnetic field due to the induced current produces a magnetic force on the metal plate that opposes the motion of the plate. This force slows the pendulum each time it enters or leaves the magnetic field and rapidly brings the plate to rest. 6. (a) As the poles of the horseshoe magnet move past the disk, eddy currents are set up in the disk in such a way as to oppose the change in flux through the disk. The net effect is a drag force on the disk—i.e., eddy current damping. The disk feels a torque that tends to make it rotate with the magnet. (b) Now the disk is constrained so that it can’t rotate freely with the magnet. It will rotate until the torque produced by the eddy current drag force equals the torque of the hairspring. The torque produced by the eddy current damping is proportional to the angular speed of the magnet. The angular position of the pointer then indicates the speed of the vehicle, as it is also proportional to the angular speed of the magnet.

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## This note was uploaded on 04/07/2009 for the course PHY PHY2140 taught by Professor Talagala during the Spring '09 term at Wayne State University.

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CollegePhysics20 - Chapter 20 ELECTROMAGNETIC INDUCTION...

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