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Exam I, Monday, Feb. 22

This lecture covers magnetic fields, magnetic force on charged particles, magnets, and the direction of magnetic force on moving charges. It also discusses the magnitude and direction of magnetic force, comparison of electric and magnetic forces, and motion of charges in a uniform magnetic field.

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Exam I, Monday, Feb. 22

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  1. Physics 102:Lecture 08 Exam I, Monday, Feb. 22 • What will exam cover? • Lectures through #7 (RC circuits) • Homework through #3 • Discussions through #3 • Labs through #3 • Review, Sunday, Feb. 21, 3 PM, 141 Loomis • I will go over old exam problems

  2. Today’s Lecture: MagnetismThis material is NOT on exam 1 • Magnetic fields • Magnetic force on charged particles

  3. - + Magnets • North Pole and South Pole • Opposites Attract • Likes Repel • Magnetic Field Lines • Arrows give direction • Density gives strength • Looks like dipole! 15

  4. The Earth is a Magnet Magnetoreception Magnetotactic bacteria

  5. 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? 21

  6. Magnetic Fields and Forces • Magnetic fields exert forces on moving charge • We will discuss the magnitude of the force shortly • Direction of force is given by a “right-hand rule”

  7. Direction of Magnetic Force on Moving Charges • Magnetic force is perpendicular to both B and v • Right-hand rule: • Point thumb of right hand in v direction • Point fingers of right hand in B direction • Then out-of-palm points • in the direction of F for + charge • opposite to F for – charge • Note: different versions by different books etc. 25 25

  8. Direction of Magnetic Force on Moving Charges Velocity B Force out of page right up out of page left down out of page up ACT: 1) Up 2) Down 3) Right 4) Left 5) Zero 25

  9. 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 25

  10. 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 16% 5% 8% 37% 23% 11% Particle is moving straight upwards then veers to the right. 27

  11. 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 15% 2% 9% 24% 24% 27% 27

  12. 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? A) down B) left C) right D) into page E) out of page v = 75 m/s q = +25 mC 29

  13. 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 • Note if v is parallel to B then F=0 V q B 30

  14. 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? A) 1 B) 2 C) 3 D) All Same 2) The force on particle 3 is in the same direction as the force on particle 1. A) True B) False F = q v B sin(q) B 3 2 1 33

  15. ComparisonElectric vs. Magnetic Electric Magnetic Source: Charges Moving Charges Act on: Charges Moving Charges Magnitude: F = Eq F = q v B sin(q) Direction: Parallel to E Perpendicular to v,B 34

  16. 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? A) Into Page B) Out of page C) Left D) Right FE would be up so FB must be down. 38

  17. 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 41

  18. 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: Recall circular motion from Phys 101 Uniform B into page 41

  19. 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 15% 52% 32% Magnetic force is always perpendicular to velocity, so it changes direction, not speed of particle. 43

  20. 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 36% 41% 24% Larger B, greater force, smaller R 45

  21. ACT 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 48

  22. 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 extra questions to try B • Which angle do you use to determine the magnitude of the force on the charged particle? (q1, q2 , either one). • Below I have drawn the trajectory for two charged particles traveling through a magnetic field. Is particle 1 positive or negatively charged? • If particles 1 and 2 have the same mass and velocity, which has the largest charge? V q1 q2 Either one sin(q) = sin(180-q) 1 Positive Particle 1 2

  23. Summary • We learned about magnetic fields B • We learned about magnetic forces on moving charged particles 50

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