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Physics 102: Lecture 08 Magnetism • Today’s lecture will cover Textbook Sections 19.1- 4 Physics 102: Lecture 8, Slide 1 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 S N S N S N 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 Preflight 8.1 1 Which drawing shows the correct field lines for a bar magnet? (1) (2) S N S N S N 2 (3) 3 Physics 102: Lecture 8, Slide 4 Preflight 8.1 1 Which drawing shows the correct field lines for a bar magnet? (1) (2) S N S N S N 2 (3) • Magnetic field lines are continuous • Arrows go from N to S outside the magnet (S to N inside). Physics 102: Lecture 8, Slide 5 3 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 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 7 Direction of Magnetic Force on Moving Charges Velocity out of page out of page B right left out of page out of page up down Force F qv B Right Hand Rule • Thumb ___ , Fingers ___ , Palm ___ Physics 102: Lecture 8, Slide 8 B F v Direction of Magnetic Force on Moving Charges 1) Up Velocity out of page out of page B right left out of page up 2) Down 3) Right Force up down 4) Left 5) Zero B Right Hand Rule • Thumb v, Fingers B, palm F Physics 102: Lecture 8, Slide 9 F v Direction of Magnetic Force on Moving Charges Velocity out of page out of page out of page out of page B right left up down Force up down left right B Right Hand Rule • Thumb v, Fingers B, palm F • Negative charge has opposite F! Physics 102: Lecture 8, Slide 10 F v 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 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 Particle is moving straight upwards 2) down then veers to the right. 3) left 4) right 5) into page 6) out of page 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 mC 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 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 mC What is the direction of the magnetic field in region 1? _ 1) up 2) down v (thumb) points up, F(palm) points right: so B(fingers) must point out. 3) left _ 4) right 5) into page 6) out of page Physics 102: Lecture 8, Slide 14 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 mC 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 Physics 102: Lecture 8, Slide 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 v = 75 m/s q = +25 mC What is the direction of the magnetic field in region 2? 1) up 2) down v (thumb) points right, F(palm) points up, B(fingers) point in. 3) left 4) right 5) into page 6) out of page Physics 102: Lecture 8, Slide 16 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 Physics 102: Lecture 8, Slide 17 V q B 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 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 F = q v B sin(q) 2) The force on particle 3 is in the same direction as the force on particle 1. 1) True 2) False B Thumb (v), fingers (B), palm (F) 3 2 1 Physics 102: Lecture 8, Slide 19 into page! Comparison Electric vs. Magnetic Source: Act on: Magnitude: Direction: Physics 102: Lecture 8, Slide 20 Electric Magnetic Charges Charges F = qE Parallel to E Moving Charges 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. E v What direction should B point if you want to select negative charges? 1) Into Page Physics 102: Lecture 8, Slide 21 2) Out of page 3) Left 4) Right 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 E 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. 1) Into Page Physics 102: Lecture 8, Slide 22 2) Out of page 3) Left 4) Right Motion of Q in uniform B field • Force is perpendicular to B,v – B does no work! (W=F d cos q ) x x x x x x x – Speed is constant (W=D K.E. ) x x x x x x x x x x x x x x – Circular motion • Solve for R: x x x x x x x x x x x x x x x x x x x x x Uniform B into page Physics 102: Lecture 8, Slide 23 Motion of Q in uniform B field • Force is perpendicular to B,v – B does no work! (W=F d cos q ) x x x – Speed is constant (W=D K.E. ) x x x x x x v – Circular motion v2 F m R v2 qvBsin(q ) m R Physics 102: Lecture 8, Slide 24 F x x x x x x x x x x x x x x x x x F x v • Solve for R: x x x x R x x x x x x x x x x x Uniform B into page mv R qB 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 v = 75 m/s q = +25 mC What is the speed of the particle when it leaves chamber 2? 1) v2 < v1 2) v2 = v1 3) v2 > v1 Physics 102: Lecture 8, Slide 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 v = 75 m/s q = +25 mC What is the speed of the particle when it leaves chamber 2? 1) v2 < v1 2) v2 = v1 3) v2 > v1 Magnetic force is always perpendicular to velocity, 43 so it changes direction, not speed of particle. Physics 102: Lecture 8, Slide 26 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 v = 75 m/s q = +25 mC Compare the magnitude of the magnetic field in chambers 1 and 2 1) B1 > B2 2) B1 = B2. 3) B1 < B2 Physics 102: Lecture 8, Slide 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 v = 75 m/s q = +25 mC Compare the magnitude of the magnetic field in chambers 1 and 2 1) B1 > B2 2) B1 = B2. 3) B1 < B2 Physics 102: Lecture 8, Slide 28 mv R qB Larger B, greater force, smaller R 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 v = 75 m/s q = ?? mC 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 Physics 102: Lecture 8, Slide 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 v = 75 m/s q = ?? mC 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 Physics 102: Lecture 8, Slide 30 mv R qB See you later! • Read Sections 19.6-8 Physics 102: Lecture 8, Slide 31