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Magnets and the magnetic field Electric currents create magnetic fields

Chapter 24 Magnetic Fields and Forces. Topics:. 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. Sample question:.

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Magnets and the magnetic field Electric currents create magnetic fields

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  1. Chapter 24 Magnetic Fields and Forces Topics: • 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 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

  2. Key Points • Three types of magnetic interactions • no interaction with either pole of a magnet => object is non-magnetic • attracted to both poles of a magnet => object is magnetic • Attracted to one pole and repelled by the other pole=> object is a magnet • Magnetic field vector from a bar magnet is a super position of the magnetic field vectors from the N and S poles: • Vector from N pole points away from N pole • Vector from S pole points towards S pole • Field lines form complete loops inside and outside of magnet • Field lines outside magnet go from N to S poles • Field lines inside magnet go from S to N poles • Magnetic Field vectors at a point are tangential to Magnetic Field Lines

  3. 3-D Arrows, Cross Products, and Right Hand Rule 1 • Showing vectors in 3D • Cross Product • For direction use Right-hand rule 1 • Right-hand rule 1 (RHR 1) => for finding direction of cross-product vector (Cross-Product Rule) • Point right hand in the direction of the first vector (vector A) • Rotate your right hand until you can point your fingers in the direction of the second vector (vector B) • Thumb points in direction the cross-product vector (vector C) Slide 24-2

  4. Right Hand Rules for Magnetism • Right-hand rule 1 (RHR 1) => for finding magnetic forceFB= q*v_vector x B_vector (Cross-Product Rule) • Point right hand in the direction the charges are moving (current or velocity) • Rotate your right hand until you can point your fingers in the direction of the magnetic Field • Thumb points in direction of force for + chargeForce 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

  5. Drawing Field Lines of a Bar Magnet Slide 24-10

  6. Magnetic Fields Produced by Bar Magnets A single bar magnet A single bar magnet (closeup) Slide 24-11

  7. Magnetic Fields Produced by Bar Magnets Two bar magnets, unlike poles facing Two bar magnets, like poles facing Slide 24-12

  8. Magnetic Fields from Two Magnets • Bar Magnets A and B are placed at right angles. Two compasses, X and Y are placed so that they are equidistant from the two magnets as shown • A.) The arrow in compass X indicates the direction in which the North pole of the compass is pointing. Indicate the North and South ends of both magnets in the diagram • B.) Draw an arrow in compass Y to show the direction in which the North pole of the compass needle would point. Slide 24-2

  9. Electric Currents Also Create Magnetic Fields A long, straight wire A current loop A solenoid Slide 24-15

  10. The Magnetic Field of a Straight Current-Carrying Wire Slide 24-16

  11. Slide 24-17

  12. Representing Vectors and Currents That Are Perpendicular to the Page Slide 24-18

  13. 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

  14. 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

  15. Drawing Field Vectors and Field Lines of a Current-Carrying Wire Slide 24-21

  16. Drawing a Current Loop Slide 24-22

  17. The Magnetic Field of a Current Loop Slide 24-23

  18. The Magnetic Field of a Solenoid A short solenoid A long solenoid Slide 24-24

  19. The Magnitude of the Field due to a Long, Straight, Current-Carrying Wire Slide 24-25

  20. Slide 24-26

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

  22. Answer The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? • 5 A to the left. Slide 24-28

  23. Magnetic field at the center of a current loop of radius R Magnetic field at the center of a current loop with N turns The Magnetic Field of a Current Loop Slide 24-29

  24. Example What is the direction and magnitude of the magnetic field at point P, at the center of the loop? Slide 24-30

  25. The Magnetic Field Inside a Solenoid Magnetic field inside a solenoid of length L with N turns. Slide 24-31

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