Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electromagnetic induction Lenz’s law Faraday’s law The nature of electromagnetic waves The spectrum of electromagnetic waves Electromagnetic Induction and Electromagnetic Waves Topics: Sample question: The ultraviolet view of the flowers on the right shows markings that cannot be seen in the visible region of the spectrum. Whose eyes are these markings intended for? Slide 25-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Aurora Borealis Slide 25-8

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Magnetic Flux Slide 25-10

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A loop of wire of area A is tipped at an angle to a uniform magnetic field B. The maximum flux occurs for an angle. What angle will give a flux that is ½ of this maximum value? A. B. C. D. Slide 25-11

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A loop of wire of area A is tipped at an angle to a uniform magnetic field B. The maximum flux occurs for an angle. What angle will give a flux that is ½ of this maximum value? C. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Faraday’s Law Faraday’s law is a basic law of electromagnetic induction. It says that the magnitude of the induced emf is the rate of change of the magnetic flux through the loop:

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Faraday’s Law A coil wire consisting of N turns acts like N batteries in series, so the induced emf in the coil is

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Faraday’s Laws There are two fundamentally different ways to change the magnetic flux through a conducting loop: 1. The loop can move or expand or rotate, creating a motional emf. 2. The magnetic field can change. The induced emf is the rate of change of the magnetic flux through the loop, regardless of what causes the flux to change.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Faraday’s Law Slide 25-15

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Faraday’s Law An induced emf ℇ is the emf associated with a changing magnetic flux. The direction of the current is determined by Lenz’s law. The size of the induced emf is determined by Faraday’s law.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electromagnetic Induction Slide 25-8

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law Slide Lenz’s law There is an induced current in a closed, conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field opposes the change in the flux.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law Text: p. 813

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law Text: p. 813

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law Text: p. 813

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Using Lenz’s Law Slide 25-14

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. QuickCheck The bar magnet is pushed toward the center of a wire loop. Which is true? There is a clockwise induced current in the loop. There is a counterclockwise induced current in the loop. There is no induced current in the loop.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. QuickCheck The bar magnet is pushed toward the center of a wire loop. Which is true? There is a clockwise induced current in the loop. There is a counterclockwise induced current in the loop. There is no induced current in the loop. 1.Upward flux from magnet is increasing. 2.To oppose the increase, the field of the induced current points down. 3.From the right-hand rule, a downward field needs a cw current.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. QuickCheck The bar magnet is pushed toward the center of a wire loop. Which is true? There is a clockwise induced current in the loop. There is a counterclockwise induced current in the loop. There is no induced current in the loop.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. QuickCheck The bar magnet is pushed toward the center of a wire loop. Which is true? There is a clockwise induced current in the loop. There is a counterclockwise induced current in the loop. There is no induced current in the loop. Magnetic flux is zero, so there’s no change of flux.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.Predict the direction of the induced current if the north end of the magnet is moved into the coil 2.Test your prediction about the direction of the induced current using Lenz's Law. (Be sure to determine which direction the coil is wound as well as which direction of current a positive reading on the galvanometer indicates) 3.Place a second coil of wire next to the coil that is connected to the galvanometer. Connect the second coil to a battery and record the galvanometer reading 4.Now open the circuit. Record the galvanometer reading 5.Switch the connection to the battery so the current flows in the opposite direction 6.Record the maximum reading and the direction of the current in the second coil 7.Now widen the gap between the two coils and record the maximum reading and the direction of the current in the second coil Slide 25-5 Lenz’s Law

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.When was the galvanometer reading positive? 2.When was the galvanometer reading negative? 3.Is your data consistent with Lenz' law? Justify your answer. 4.Make a drawing of the set up that shows the battery connection, the direction of the current in the first wire and the direction of the current in the second coil. 5.What conclusions can you make from these observations? Slide 25-5 Lenz’s Law

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Motional emf Slide Delta V induced = vlB AKA induced EMF ε

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Induced Current in a Circuit Slide 25-13

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Eddy Currents Slide 25-35

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Eddy Currents In a technique called transcranial magnetic stimulation (TMS), a large oscillating magnetic field is applied to the head via a current carrying-coil. The field produces small eddy currents on the brain, inhibiting the neurons in the stimulated region. This technique can be used to determine the importance of the stimulated region in certain perceptions or tasks.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Eddy Currents

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is increasing, what can we say about the current in the loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is increasing, what can we say about the current in the loop? B.The loop has a counterclockwise current. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is decreasing, what can we say about the current in the loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is decreasing, what can we say about the current in the loop? A.The loop has a clockwise current. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Long after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Long after the switch is closed, what can we say about the current in the lower loop? C.The loop has no current. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is reopened, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is reopened, what can we say about the current in the lower loop? B.The loop has a counterclockwise current. Slide Answer