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

Checking Understanding Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P? Slide 24-19

Answer Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P? Slide 24-20

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Currents Also Create Magnetic Fields A long, straight wire A current loopA solenoid Slide 24-15

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Magnitude of the Field due to a Long, Straight, Current-Carrying Wire Slide 24-25

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Magnetic Field of a Current Loop Magnetic field at the center of a current loop of radius R Magnetic field at the center of a current loop with N turns Slide 24-29

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding The diagram below shows a current loop perpendicular to the page; the view is a “slice” through the loop. The direction of the current in the wire at the top and the bottom is shown. What is the direction of the magnetic field at a point in the center of the loop? A.To the left B.Up C.To the right D.Down Slide 24-35

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Answer The diagram below shows a current loop perpendicular to the page; the view is a “slice” through the loop. The direction of the current in the wire at the top and the bottom is shown. What is the direction of the magnetic field at a point in the center of the loop? A.To the left B.Up C.To the right D.Down Slide 24-36

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Magnetic Field Inside a Solenoid Magnetic field inside a solenoid of length L with N turns. Slide 24-31

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Example What is the direction and magnitude of the magnetic field at point P, at the center of the loop? Slide 24-30

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Right Hand Rules for Magnetism Right-hand rule 1 (RHR 1) => for finding magnetic force F B = q*v_vector x B_vector (Cross-Product Rule) 1.Point right hand in the direction the charges are moving (current or velocity) 2.Rotate your right hand until you can point your fingers in the direction of the magnetic Field 3.Thumb points in direction of force for + charge Force is in opposite direction for - charges Right-hand rule 2 (RHR 2) => Finding direction of B from I Point thumb of right hand in direction of current I, B-field lines curl in direction of fingers Right-hand rule 3 (RHR 3) => Finding direction of current in a loop from direction of B-field Point thumb of right hand in direction of B-field Fingers of right hand curl in direction of current Slide 24-2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Force on a Charged Particle Moving in a Magnetic Field Slide 24-32

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Right-Hand Rule for Forces Slide 24-33

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 24-34

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Paths of Charged Particles in Magnetic Fields Slide 24-35

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Mass Spectrometer Slide 24-36

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Velocity Selector Slide 24-37

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. Checking Understanding A.10 A to the right. B.5 A to the right. C.2.5 A to the right. D.10 A to the left. E.5 A to the left. F.2.5 A to the left. The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? Slide 24-27

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. E.5 A to the left. The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? Slide Answer

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. 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