Physics 102 Lecture 08 Todays Lecture Magnetism Magnetic

Physics 102: Lecture 08 Today’s Lecture: Magnetism • • Magnetic fields Magnetic force on charged particles Physics 102: Lecture 8, Slide 1

Magnets • North Pole and South Pole – Opposites Attract – Likes Repel • Magnetic Field Lines – Arrows give direction – Density gives strength – Looks like dipole! Physics 102: Lecture 8, Slide 2 15 - +

The Earth is a Magnet Physics 102: Lecture 8, Slide 3

Physics 102: Lecture 8, Slide 4

No Magnetic Charges • Magnetic Fields are created by moving electric charge! • Where is the moving charge? Orbits of electrons about nuclei Intrinsic “spin” of electrons (more important effect) Physics 102: Lecture 8, Slide 5

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” Physics 102: Lecture 8, Slide 6

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. Physics 102: Lecture 8, Slide 7

Physics 102: Lecture 8, Slide 8

Direction of Magnetic Force on Moving Charges Velocity out of page B right left out of page up Force up down ACT: 1) Up 2) Down Physics 102: Lecture 8, Slide 9 3) Right 4) Left 5) Zero

Direction of Magnetic Force on Moving Charges Velocity out of page Physics 102: Lecture 8, Slide 10 B right left up down Force up down left right

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 m. C 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

Physics 102: Lecture 8, Slide 12

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 v = 75 m/s q = +25 m. C 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 Physics 102: Lecture 8, Slide 13

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 v = 75 m/s q = +25 m. C What is the direction of the magnetic field in region 2? A) down B) left C) right D) into page E) out of page Physics 102: Lecture 8, Slide 14

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 Physics 102: Lecture 8, Slide 15 V q B

Physics 102: Lecture 8, Slide 16

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 F= q v B sin(q) 2) The force on particle 3 is in the same direction as the force on particle 1. A) True B) False B 3 2 1 Physics 102: Lecture 8, Slide 17

Comparison Electric vs. Magnetic Electric Source: Charges Act on: Charges Magnitude: F = Eq Direction: Parallel to E Physics 102: Lecture 8, Slide 18 Magnetic Moving Charges F = q v B sin(q) Perpendicular to v, B

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. FB v FE 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? FE would be up so FB must be down. A) Into Page Physics 102: Lecture 8, Slide 19 B) Out of page C) Left D) Right E

Physics 102: Lecture 8, Slide 20

Motion of Q in uniform B field • Force is perpendicular to B, v x x x x Uniform B into page x v x F F x v • Solve for R: x F – B does no work! (W=F d cos q ) x x x x x v x FRx x – Speed is constant (W=D K. E. ) F x x x x F v – Circular motion 41 Physics 102: Lecture 8, Slide 21 x

Motion of Q in uniform B field • Force is perpendicular to B, v Physics 102: Lecture 8, Slide 22 x v x x x x Uniform B into page x v x F F x v • Solve for R: x F – B does no work! (W=F d cos q ) x x x x x v x FRx x – Speed is constant (W=D K. E. ) F x x x x F v – Circular motion x

Preflight 8. 6 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. 2 1 v = 75 m/s q = +25 m. C 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 Physics 102: Lecture 8, Slide 23

Physics 102: Lecture 8, Slide 24

Preflight 8. 8 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. 2 1 v = 75 m/s q = +25 m. C 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 Physics 102: Lecture 8, Slide 25

ACT 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. 2 1 v = 75 m/s q = ? ? m. C A second particle with mass 2 m enters the chamber and follows the same path as the particle with mass m and charge q=25 m. C. What is its charge? 1) Q = 12. 5 m. C 2) Q = 25 m. C 3) Q = 50 m. C 48 Physics 102: Lecture 8, Slide 26

extra questions to try B 1. 2. 3. Which angle do you use to determine the magnitude of the force on the charged particle? (q 1, q 2 , 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? Physics 102: Lecture 8, Slide 27 V q 1 q 2 x x 1 x x x x x x 2 x

Summary • We learned about magnetic fields B • We learned about magnetic forces on moving charged particles Physics 102: Lecture 8, Slide 28