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Magnetism

Physics 102: Lecture 08. Magnetism. Today’s lecture will cover Textbook Sections 19.1- 4. -. +. Magnets are Cool!. North Pole and South Pole Opposites Attract Likes Repel. Magnetic Field Lines Arrows give direction Density gives strength Looks like dipole!. Lets Break it!.

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Magnetism

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  1. Physics 102:Lecture 08 Magnetism • Today’s lecture will coverTextbook Sections 19.1- 4 Physics 102: Lecture 8, Slide 1

  2. - + Magnets are Cool! • North Pole and South Pole • Opposites Attract • Likes Repel • Magnetic Field Lines • Arrows give direction • Density gives strength • Looks like dipole! Lets Break it! Physics 102: Lecture 8, Slide 2

  3. Field Lines of Bar Magnet S N Magnetic field lines don’t start or stop. There are no magnetic charges (monopoles) Physics 102: Lecture 8, Slide 3

  4. 1 Preflight 8.1 S N 2 S N 3 S N Which drawing shows the correct field lines for a bar magnet? (1) (2) (3) Physics 102: Lecture 8, Slide 4

  5. 1 Preflight 8.1 S N Which drawing shows the correct field lines for a bar magnet? (1) (2) (3) 2 S N • Magnetic field lines are continuous • Arrows go from N to Soutside the magnet (S to N inside). 3 S N Physics 102: Lecture 8, Slide 5

  6. Comparison:Electric Field Lines vs. Magnetic Field Lines • Similarities • Density gives strength • Arrow gives direction • Leave +, North • Enter -, South • Differences • Start/Stop on electric charge • No Magnetic Charge, lines are continuous! • FYI • x x x x x x x INTO Page • • • • • • • • • • OUT of Page Physics 102: Lecture 8, Slide 6

  7. Orbits of electrons about nuclei Intrinsic “spin” of electrons (more important effect) No Magnetic Charges • Magnetic Fields are created by moving electric charge! • Where is the moving charge? Physics 102: Lecture 8, Slide 7

  8. B v F Direction of Magnetic Force on Moving Charges Velocity B Force out of page right out of page left out of page up out of page down Right Hand Rule • Thumb ___ , Fingers ___ , Palm ___ Physics 102: Lecture 8, Slide 8

  9. B v F Direction of Magnetic Force on Moving Charges Velocity B Force out of page right up out of page left down out of page up 1) Up 2) Down 3) Right 4) Left 5) Zero Right Hand Rule • Thumb v, Fingers B, palm F Physics 102: Lecture 8, Slide 9

  10. Direction of Magnetic Force on Moving Charges Velocity B Force out of page right up out of page left down out of page up out of page down left right Right Hand Rule B v • Thumb v, Fingers B, palm F F • Negative charge has opposite F! Physics 102: Lecture 8, Slide 10

  11. Preflight 8.3 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 v = 75 m/s q = +25 mC What is the direction of the force on the particle just as it enters region 1? 1) up 2) down 3) left 4) right 5) into page 6) out of page Physics 102: Lecture 8, Slide 11

  12. Preflight 8.3 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 v = 75 m/s q = +25 mC What is the direction of the force on the particle just as it enters region 1? 1) up 2) down 3) left 4) right 5) into page 6) out of page Particle is moving straight upwards then veers to the right. Physics 102: Lecture 8, Slide 12

  13. Preflight 8.4 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 What is the direction of themagnetic fieldin region 1? 1) up 2) down 3) left 4) right 5) into page 6) out of page v = 75 m/s q = +25 mC Physics 102: Lecture 8, Slide 13

  14. _ v (thumb) points up, F(palm) points right: so B(fingers) must point out. _ Preflight 8.4 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 What is the direction of themagnetic fieldin region 1? 1) up 2) down 3) left 4) right 5) into page 6) out of page v = 75 m/s q = +25 mC Physics 102: Lecture 8, Slide 14

  15. ACT: 2 Chambers Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 What is the direction of the magnetic field in region 2? 1) up 2) down 3) left 4) right 5) into page 6) out of page v = 75 m/s q = +25 mC Physics 102: Lecture 8, Slide 15

  16. ACT: 2 Chambers Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. 2 1 What is the direction of the magnetic field in region 2? 1) up 2) down 3) left 4) right 5) into page 6) out of page v = 75 m/s q = +25 mC v (thumb) points right, F(palm) points up, B(fingers) point in. Physics 102: Lecture 8, Slide 16

  17. Magnitude of Magnetic Force on Moving Charges • The magnetic force on a charge depends on the magnitude of the charge, its velocity, and the magnetic field. F = q v B sin(q) • Direction from RHR • Thumb (__), fingers (__), palm (__) • Note if v is parallel to B then F = 0 V q B Physics 102: Lecture 8, Slide 17

  18. ACT: Moving Charges The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. 1) Which particle experiences the greatest magnetic force? 1) 1 2) 2 ) 3 4) All Same 2) The force on particle 3 is in the same direction as the force on particle 1. 1) True 2) False B 3 2 1 Physics 102: Lecture 8, Slide 18

  19. ACT: Moving Charges The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. 1) Which particle experiences the greatest magnetic force? 1) 1 2) 2 3) 3 4) All Same 2) The force on particle 3 is in the same direction as the force on particle 1. 1) True 2) False F = q v B sin(q) B Thumb (v), fingers (B), palm (F) into page! 3 2 1 Physics 102: Lecture 8, Slide 19

  20. ComparisonElectric vs. Magnetic Electric Magnetic Source: Charges Moving Charges Act on: Charges Moving Charges Magnitude: F = qE F = q v B sin(q) Direction: Parallel to E Perpendicular to v,B Physics 102: Lecture 8, Slide 20

  21. Example Velocity Selector E v Determine magnitude and direction of magnetic field such that a positively charged particle with initial velocity v travels straight through and exits the other side. What direction should B point if you want to select negative charges? 1) Into Page 2) Out of page 3) Left 4) Right Physics 102: Lecture 8, Slide 21

  22. FB FE Example Velocity Selector E v Determine magnitude and direction of magnetic field such that a positively charged particle with initial velocity v travels straight through and exits the other side. Electric force is down, so need magnetic force up. By RHR, B must be into page For straight line, need |FE |= |FB | q E= q v B sin(90) B = E/v What direction should B point if you want to select negative charges? 1) Into Page 2) Out of page 3) Left 4) Right FE would be up so FB must be down. Physics 102: Lecture 8, Slide 22

  23. x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Motion of Q in uniform B field • Force is perpendicular to B,v • Force is perpendicular to B,v • B does no work! (W=F d cos q ) • Speed is constant (W=D K.E.) • Circular motion v • Solve for R: Uniform B into page Physics 102: Lecture 8, Slide 23

  24. R F x x x x x x x x x x x x x x x x x x x x x v F x x x x x x x x x x x x x x x x x x x x x v F F F F v v v v Motion of Q in uniform B field • Force is perpendicular to B,v • Force is perpendicular to B,v • B does no work! (W=F d cos q ) • Speed is constant (W=D K.E.) • Circular motion • Solve for R: Uniform B into page Physics 102: Lecture 8, Slide 24

  25. Preflight 8.6 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 What is the speed of the particle when it leaves chamber 2? 1) v2 < v1 2) v2 = v1 3) v2 > v1 v = 75 m/s q = +25 mC Physics 102: Lecture 8, Slide 25

  26. Preflight 8.6 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 What is the speed of the particle when it leaves chamber 2? 1) v2 < v1 2) v2 = v1 3) v2 > v1 v = 75 m/s q = +25 mC Magnetic force is always perpendicular to velocity, so it changes direction, not speed of particle. 43 Physics 102: Lecture 8, Slide 26

  27. Preflight 8.8 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 Compare the magnitude of the magnetic field in chambers 1 and 2 1) B1 > B2 2) B1 = B2. 3) B1 < B2 v = 75 m/s q = +25 mC Physics 102: Lecture 8, Slide 27

  28. Preflight 8.8 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 Compare the magnitude of the magnetic field in chambers 1 and 2 1) B1 > B2 2) B1 = B2. 3) B1 < B2 v = 75 m/s q = +25 mC Larger B, greater force, smaller R Physics 102: Lecture 8, Slide 28

  29. Preflight 8.9 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 A second particle with mass 2m enters the chamber and follows the same path as the particle with mass m and charge q=25 mC. What is its charge? 1) Q = 12.5 mC 2) Q = 25 mC 3) Q = 50 mC v = 75 m/s q = ?? mC Physics 102: Lecture 8, Slide 29

  30. Preflight 8.9 Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1= 75 m/s up, and follows the dashed trajectory. 2 1 A second particle with mass 2m enters the chamber and follows the same path as the particle with mass m and charge q=25 mC. What is its charge? 1) Q = 12.5 mC 2) Q = 25 mC 3) Q = 50 mC v = 75 m/s q = ?? mC Physics 102: Lecture 8, Slide 30

  31. See you later! • Read Sections 19.6-8 Physics 102: Lecture 8, Slide 31

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