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. Magnetic Fields Exert Forces on Currents Magnitude of the force on a current segment of length L perpendicular to a magnetic field Slide 24-37

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Forces between Currents Slide Magnetic force between two parallel current-carrying wires

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 24-39

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. Electron Magnetic Moments: Ferromagnetism A nonmagnetic solid (copper)A ferromagnetic solid (iron) Slide 24-47

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Inducing a Magnetic Moment in a Piece of Iron Slide 24-48

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Summary of Chapter 20 Magnets have north and south poles Like poles repel, unlike attract Unit of magnetic field: tesla Electric currents produce magnetic fields A magnetic field exerts a force on an electric current:

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Summary of Chapter 20 A magnetic field exerts a force on a moving charge: Magnitude of the field of a long, straight current-carrying wire: Parallel currents attract; antiparallel currents repel

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Summary of Chapter 20 Magnetic field inside a solenoid: Ampère’s law: Torque on a current loop:

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding The diagram below shows slices through two adjacent current loops. Think about the force exerted on the loop on the right due to the loop on the left. The force on the right loop is directed A.to the left. B.up. C.to the right. D.down. Slide 24-37

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Answer The diagram below shows slices through two adjacent current loops. Think about the force exerted on the loop on the right due to the loop on the left. The force on the right loop is directed A.to the left. B.up. C.to the right. D.down. Slide 24-38

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Additional Questions 1.A loop carrying a current as shown rests in a uniform magnetic field directed to the right. If the loop is free to rotate, A.it will rotate clockwise. B.it will not rotate. C.it will rotate counterclockwise. Slide 24-66

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Answer 1.A loop carrying a current as shown rests in a uniform magnetic field directed to the right. If the loop is free to rotate, A.it will rotate clockwise. B.it will not rotate. C.it will rotate counterclockwise. Slide 24-67

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. 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. Clicker Question 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. 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. Magnetic Flux Slide 25-10

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 Slide 25-15

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Motional emf Slide 25-12

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. 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. 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

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is pulled out of the coil. What can we say about the current in the meter? A.The current goes from right to left. B.The current goes from left to right. C.There is no current in the meter. Additional Clicker Questions Slide 25-44

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is pulled out of the coil. What can we say about the current in the meter? A.The current goes from right to left. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The figure shows a 10-cm-diameter loop in three different magnetic fields. The loop’s resistance is 0.1 Ω. For each situation, determine the strength and direction of the induced current. Slide 25-32

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide Into the Field A five-turn rectangular loop is moved through a uniform field at 2 m/s as shown below. 1.What is the maximum magnetic flux through the loop during its motion through the field? The loop is 5 cm long and 3 cm wide. 2.The loop takes 100 ms to completely enter the field. Sketch a graph of the magnetic flux through the loop in the interval from t=0 to t=150 ms. Label values of flux. (Assume the loop begins to enter the magnetic field at t = 0 s)