Lect08 handout
- 1. Magnetism Physics 102: Lecture 08 This material is NOT on exam 1!
- 2. Lecture Overview Magnetic fields Magnetic forces on moving charges Direction: “Right hand rule” Magnitude
- 3. Magnets & magnetic fields North Pole and South Pole Opposites Attract Likes Repel Magnetic Field B Units = Tesla (T) Like E, vector at a location Points N to S Magnetic Field Lines Arrows give direction Density gives strength Looks like dipole!
- 4. The Earth is a Magnet
- 5. No Magnetic Charges N and S poles always go together Magnetic Fields are created by moving electric charge! Where is the moving charge? We’ll return to this in Lecture 25 & 26 Orbits of electrons about nuclei Intrinsic “spin” of electrons (more important effect)
- 6. Magnetic Fields and Forces Magnetic field B exerts force on moving charge Magnitude Direction Magnetic force is perpendicular to both B and v “ Right-hand rule” (RHR): Thumb of right hand along v Fingers of right hand along B Out-of-palm points in the direction of F for + charge opposite to F for – charge Note: there are different versions of RHR
- 7. ACT: Direction of Magnetic Force on + Moving Charge Velocity B Force out of page right up out of page left down 1) Up 2) Down 3) Right 4) Left 5) Zero out of page up
- 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 out of page down right left
- 9. Preflight 8.3 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 1 2 v = 75 m/s q = +25 mC 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.
- 10. Preflight 8.4 What is the direction of the magnetic field in region 1? 1) up 2) down 3) left 4) right 5) into page 6) out of page 1 2 v = 75 m/s q = +25 mC 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. B 1
- 11. ACT: 2 Chambers What is the direction of the magnetic field in region 2? A) down B) left C) right D) into page E) out of page 1 2 v = 75 m/s q = +25 mC 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. B 2
- 12. Magnitude of Magnetic Force on Moving Charges Only component of v ⊥ to B (or B ⊥ to v) matters If v is parallel to B then F = 0 Does not matter whether you use or 180 – B V Force depends on magnitude of charge, velocity, and magnetic field B ⊥ V ⊥
- 13. 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 B 1 2 3 F = qvBsin( )
- 14. Comparison Electric vs. Magnetic Electric Magnetic Source: Charges Moving Charges Act on: Charges Moving Charges Magnitude: F = q E F = q v B sin( ) Direction: Parallel to E Perpendicular to v, B
- 15. Velocity Selector 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. v E For straight line, need |F E |= |F B | 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 F E would be up so F B must be down. Electric force is down, so need magnetic force up. By RHR, B must be into page Example F E F B
- 16. Motion of q in uniform B field Force is perpendicular to B,v B does no work ! (W=F d cos ) Speed is constant (W= K.E.) Motion is circular Uniform B into page Force is perpendicular to B,v Solve for R: Recall circular motion from Phys 101 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 v F v F v F v F v F v F R
- 17. Preflight 8.6 What is the speed of the particle when it leaves chamber 2? 1) v 2 < v 1 2) v 2 = v 1 3) v 2 > v 1 1 2 v = 75 m/s q = +25 mC Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v 1 = 75 m/s up, and follows the dashed trajectory.
- 18. Preflight 8.8 Compare the magnitude of the magnetic field in chambers 1 and 2 1) B 1 > B 2 2) B 1 = B 2 . 3) B 1 < B 2 1 2 v = 75 m/s q = +25 mC Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v 1 = 75 m/s up, and follows the dashed trajectory.
- 19. ACT: 2 chambers 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 1 2 v = 75 m/s q = ?? mC Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v 1 = 75 m/s up, and follows the dashed trajectory.
- 20. Summary We learned about magnetic fields B We learned about magnetic forces on moving charged particles