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Chapter 20 Electromagnetic Induction Copyright © 2010 Pearson Education, Inc. Induced Electromotive Force Note the motion of the magnet in each image: Copyright © 2010 Pearson Education, Inc. (PhET Animations) Copyright © 2010 Pearson Education, Inc. Magnetic Flux Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Induced Electromotive Force Faraday’s experiment: closing the switch in the primary circuit induces a current in the secondary circuit, but only while the current in the primary circuit is changing. Copyright © 2010 Pearson Education, Inc. Faraday’s Law of Induction An emf is induced only when the magnetic flux through a loop changes with time. Copyright © 2010 Pearson Education, Inc. Faraday’s Law of Induction There are many devices that operate on the basis of Faraday’s law. An electric guitar pickup: Copyright © 2010 Pearson Education, Inc. Faraday’s Law of Induction Microphone: Copyright © 2010 Pearson Education, Inc. AC vs. DC Copyright © 2010 Pearson Education, Inc. Transformers A transformer is used to change voltage in an alternating current from one value to another. Copyright © 2010 Pearson Education, Inc. Transformers By applying Faraday’s law of induction to both coils, we find: Here, p stands for the primary coil and s for the secondary. Copyright © 2010 Pearson Education, Inc. Transformers The power in both circuits must be the same; therefore, if the voltage is lower, the current must be higher. Copyright © 2010 Pearson Education, Inc. Lenz’s Law An induced current always flows in a direction that opposes the change that caused it. Therefore, if the magnetic field is increasing, the magnetic field created by the induced current will be in the opposite direction; if decreasing, it will be in the same direction. Copyright © 2010 Pearson Education, Inc. Lenz’s Law This conducting rod completes the circuit. As it falls, the magnetic flux decreases, and a current is induced. Copyright © 2010 Pearson Education, Inc. Lenz’s Law The force due to the induced current is upward, slowing the fall. Copyright © 2010 Pearson Education, Inc. Lenz’s Law Currents can also flow in bulk conductors. These induced currents, called eddy currents, can be powerful brakes. Copyright © 2010 Pearson Education, Inc. Mechanical Work and Electrical Energy generator demo Copyright © 2010 Pearson Education, Inc. Generators and Motors An electric motor converts electrical energy into mechanical energy: Copyright © 2010 Pearson Education, Inc. Generators and Motors An electric generator converts mechanical energy into electric energy: Copyright © 2010 Pearson Education, Inc. Homework Continue work on fourth quarter project. First weekly report is due tomorrow. Copyright © 2010 Pearson Education, Inc.