Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Magnets and the magnetic field Electric currents create magnetic fields Magnetic fields of wires, loops, and solenoids Magnetic forces on charges and currents Magnets and magnetic materials Chapter 24 Magnetic Fields and Forces Topics: Sample question: This image of a patient’s knee was made with magnetic fields, not x rays. How can we use magnetic fields to visualize the inside of the body? Slide 24-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Forces between Current Loops Slide 24-40

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A Current Loop Acts like a Bar Magnet Slide 24-41

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Magnetic Fields Exert Torques on Current Loops Slide 24-42

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley Applications: Galvanometers, Motors, Loudspeakers An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Torque on a Dipole in a Magnetic Field Slide 24-43

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Magnetic Resonance Imaging Slide 24-44

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Magnetic Resonance Imaging Slide 24-45

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Reading Quiz 1.Which of the following will cause an induced current in a coil of wire? A. A magnet resting near the coil. B. The constant field of the earth passing through the coil. C. A magnet being moved into or out of the coil. D. A wire carrying a constant current near the coil. Slide 25-2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.Which of the following will cause an induced current in a coil of wire? C. A magnet being moved into or out of the coil. Slide 25-3 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Reading Quiz 2.The speed of electromagnetic waves in a vacuum A. depends upon the wavelength. B. depends on the photon energy. C. is the same as the speed of sound. D. is the same for all waves regardless of wavelength. Slide 25-4

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 2.The speed of electromagnetic waves in a vacuum D. is the same for all waves regardless of wavelength. Slide 25-5 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Currents - moving charges - create a B-field Can magnetic fields create an E-field? What should we try to test this? Slide Minute-Brainstorm

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Hold the coil so it is vertical Bring the north pole of the magnet toward the coil Record the galvanometer reading Pull the magnet away from the coil Record the galvanometer reading Repeat with the south pole of the magnet closest to the coil Slide 25-5 Faraday’s Magic

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Questions 1. When was the galvanometer reading positive? 2. When was the galvanometer reading negative? 3. What happens if the magnet is just held near the loop? What does this mean? Slide 25-5 Faraday’s Magic

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1. List at least 3 factors that affect the direction of the induced current. Report results on the board 2.List at least 3 factors that increase the magnitude of the induced current 3.Find at least 2 ways to make alternating current (AC). Describe what you did in detail Slide 25-5 Faraday’s Magic

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. Motional emf Slide 25-9

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. 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. 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. Using Lenz’s Law Slide 25-14

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. 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