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Physics 212 Lecture 12 Today's Concept: Magnetic Force on moving charges F qv B Physics 212 Lecture 12, Slide 1 Music Who is the Artist? A) B) C) D) E) The Meters The Neville Brothers Trombone Shorty Michael Franti Radiators Tribute to New Orleans and Mardi Gras Your Comments “How can a magnetic field point in a direction? What about the wire example in the prelecture where is A little at a time! shown to move in a counter-clockwise/clockwise direction?” “This stuff is really neat... It is fun to actually see the calculations for magnetism. However, since this is the first time I’ve really seen it, it is still a bit confusing. If you could go through different examples and go over the actual concepts more, that would be great.” “Magnets. How do they work?” Good questions! “Please explain what causes magnetic fields, and why But very deep! they exist. Unless we learn that later or not at all, I just don’t understand.” “What are we supposed to do if we have two left hands?” “Could you go over this right hand rule again? There are so many of them...” RHR confused many “I refuse to make an attempt at a bad pun.” 05 “We most definitely need examples. Also, every time you post an awful pun, a puppy dies. Just so you know.” Physics 212 Lecture 12, Slide 3 Key Concepts: 1) The force on moving charges due to a magnetic field. 2) The cross product. Today’s Plan: 1) Review of magnetism 2) Review of cross product 3) Example problem 05 Physics 212 Lecture 12, Slide 4 Magnetic Observations • Bar Magnets N S N S S N N S • Compass Needles N S Magnetic Charge? N 07 S cut in half N S N S Physics 212 Lecture 12, Slide 5 Magnetic Observations • Compass needle deflected by electric current I • Magnetic fields created by electric currents • Direction? Right thumb along I, fingers curl in direction of B • Magnetic fields exert forces on electric currents (charges in motion) V F F V F I I I I F V 12 Physics 212 Lecture 12, Slide 6 Magnetic Observations P P I (out of the screen) I (into of the screen) Case I Case II The magnetic field at P points A. Case I: left, Case II: right B. Case I: left, Case II: left C. Case I: right, Case II: left D. Case I: right, Case II: right WHY? Direction of B: right thumb in direction of I, fingers curl in the direction of B Physics 212 Lecture 12, Slide 7 Magnetism & Moving Charges All observations are explained by two equations: F qv B 0 I ds rˆ dB 2 4 r 14 Today Next Week Physics 212 Lecture 12, Slide 8 Cross Product Review • Cross Product different from Dot Product – A●B is a scalar; A x B is a vector – A●B proportional to the component of B parallel to A – A x B proportional to the component of B perpendicular to A • Definition of A x B – Magnitude: ABsinq – Direction: perpendicular to plane defined by A and B with sense given by right-hand-rule 16 Physics 212 Lecture 12, Slide 9 Remembering Directions: The Right Hand Rule y F qv B x F B qv 17 z Physics 212 Lecture 12, Slide 10 Checkpoint 1a Three points are arranged in a uniform magnetic field. The B field points into the screen. 1) A positively charged particle located atstationary. point A and stationary. A positively charged particle is located at is point A and is The is direction of the magnetic force on the is The direction of particle the magnetic force on the particle is: A. right E. zero B. left C. into the screen D. out of the screen “all are into the screen” “Right hand rule says its pointing out of the screen” “qvXB=0 if v=0.” 18 Physics 212 Lecture 12, Slide 11 Checkpoint 1a Three points are arranged in a uniform magnetic field. The B field points into the screen. 1) A positively charged particle located atstationary. point A and stationary. A positively charged particle is located at is point A and is The is direction of the magnetic force on the is The direction of particle the magnetic force on the particle is: A. right E. zero B. left C. into the screen D. out of the screen F qv B The particle’s velocity is zero. There can be no magnetic force. 18 Physics 212 Lecture 12, Slide 12 Checkpoint 1b Three points are arranged in a uniform magnetic field. The B field points into the screen. 1)positive A positively charged at point A magnetic and is stationary. The charge moves fromparticle A towardisB.located The direction of the force on the particle The isdirection of the magnetic force on the particle is: A. right E. zero B. left C. into the screen D. out of the screen “it should be diagonally outward” “v is up, b is into screen, right hand rule says it should be left.” “Because the magnetic force goes into the screen, so will the direction” 21 Physics 212 Lecture 12, Slide 13 Checkpoint 1b Three points are arranged in a uniform magnetic field. The B field points into the screen. 1)positive A positively charged at point A magnetic and is stationary. The charge moves fromparticle A towardisB.located The direction of the force on the particle The isdirection of the magnetic force on the particle is: A. right E. zero B. left C. into the screen D. out of the screen F qv B qv F 21 X B Physics 212 Lecture 12, Slide 14 Cross Product Practice F qv B • protons (positive charge) coming out of screen (+z direction) • Magnetic field pointing down • What is direction of force on POSITIVE charge? A) Left -x B) Right +x C) UP +y D) Down -y E) Zero y x B 24 z Physics 212 Lecture 12, Slide 15 Motion of Charge q in Uniform B Field • Force is perpendicular to v – Speed does not change – Uniform Circular Motion x vx x x x x x q • Solve for R: x R x x x x vx x F qv B F qvB v a R 30 q F F q x x xF x x x x 2 v2 qvB m R x x x Fx x x x xv x x x x x x q mv R qB x x x x xv x x Uniform B into page Physics 212 Lecture 12, Slide 16 R LHC mv p qB qB 17 miles diameter Physics 212 Lecture 12, Slide 17 Checkpoint 2a The drawing below shows the top view of two interconnected chambers. Each chamber has a unique magnetic field. A positively charged particle is fired into chamber 1, and observed to follow the dashed path shown in the figure. What is the direction of the magnetic field in chamber 1? A. up B. down C. into the page D. out of the page “Since the direction of the magnetic field has to be perpendicular to the charged particle’s direction, it will point up..” “Having a magnetic field going into the screen indicates counterclockwise motion.” “Thumb points to the left because particle is deflected to the left and fingers point up because that is velocity vector. Fingers curl out of the screen.” 34 Physics 212 Lecture 12, Slide 18 Checkpoint 2a The drawing below shows the top view of two interconnected chambers. Each chamber has a unique magnetic field. A positively charged particle is fired into chamber 1, and observed to follow the dashed path shown in the figure. What is the direction of the magnetic field in chamber 1? A. up B. down C. into the page D. out of the page F qv B qv . B 34 F Physics 212 Lecture 12, Slide 19 Checkpoint 2b The drawing below shows the top view of two interconnected chambers. Each chamber has a unique magnetic field. A positively charged particle is fired into chamber 1, and observed to follow the dashed path shown in the figure. Compare the magnitude of the magnetic field in chamber 1 to the magnitude of the magnetic field in chamber 2 A. |B1| > |B2| B. |B1| = |B2| C. |B1| < |B2| “because the curve is much sharper inside box 1” “It equals out.” “Because the particle changed direction more slowly in the second chamber.” 36 Physics 212 Lecture 12, Slide 20 Checkpoint 2b The drawing below shows the top view of two interconnected chambers. Each chamber has a unique magnetic field. A positively charged particle is fired into chamber 1, and observed to follow the dashed path shown in the figure. Compare the magnitude of the magnetic field in chamber 1 to the magnitude of the magnetic field in chamber 2 A. |B1| > |B2| B. |B1| = |B2| C. |B1| < |B2| Observation: R2 > R1 R 36 mv qB B1 B2 Physics 212 Lecture 12, Slide 21 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. x0/2 E d enters here What is B? exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Conceptual Analysis – What do we need to know to solve this problem? (A) Lorentz ForceLaw ( F qv B qE ) – – (B) E field definition (C) V definition (D) Conservation of Energy/Newton’s Laws (E) All of the above Absolutely ! We need to use the definitions of V and E and either conservation of energy or Newton’s Laws to understand the motion of the particle before it enters the B field. We need to use the Lorentz Force Law (and Newton’s Laws) to determine what happens in the magnetic field. Physics 212 Lecture 12, Slide 22 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. x0/2 E d enters here What is B? exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Strategic Analysis – – – Calculate v, the velocity of the particle as it enters the magnetic field Use Lorentz Force equation to determine the path in the field as a function of B Apply the entrance-exit information to determine B Let’s Do It !! Physics 212 Lecture 12, Slide 23 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. E d enters here What is B? exits here x0/2 XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • What is v0, the speed of the particle as it enters the magnetic field ? 2E m (A) vo • Why?? – – 2qEd vo m (B) vo 2 ad 2qE md (D) vo (C) vo qEd m (E) Conservation of Energy • • Initial: Energy = U = qV = qEd Final: Energy = KE = ½ mv02 • • a = F/m = qE/m v02 = 2ad Newton’s Laws vo2 2 1 mv 2 o 2 qE d m qEd vo vo 2qEd m 2qEd m Physics 212 Lecture 12, Slide 24 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? x0/2 E d enters here 2qEd vo m exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • What is the path of the particle as it moves through the magnetic field? XXXXXXXXX XXXXXXXXX XXXXXXXXX (A) X X X X X X X X X • Why?? – (B) XXXXXXXXX XXXXXXXXX XXXXXXXXX XXXXXXXXX (C) XXXXXXXXX XXXXXXXXX XXXXXXXXX XXXXXXXXX Path is circle ! • • • Force is perpendicular to the velocity Force produces centripetal acceleration Particle moves with uniform circular motion Physics 212 Lecture 12, Slide 25 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. E d enters here 2qEd vo m What is B? x0/2 exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • What is the radius of path of particle? R xo R 2 xo (A) (B) 1 R xo 2 (C) mvo R qB (D) vo2 R a (E) • Why?? R xo/2 XXXXXXXXX XXXXXXXXX XXXXXXXXX XXXXXXXXX Physics 212 Lecture 12, Slide 26 Calculation A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo 2 mEd q (A) vo 2qEd m E d enters here R 1 x0 2 E B v m BE 2qEd (B) (C) 1 B xo exits here x0/2 XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B 2 mEd q B (D) mvo qxo (E) • Why?? F ma vo2 qvo B m R m vo B q R m 2 B q xo B 2 xo 2 qEd m 2 mEd q Physics 212 Lecture 12, Slide 27 Follow-Up A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo x0/2 E d enters here 2 mEd q exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Suppose the charge of the particle is doubled (Q = 2q),while keeping the mass constant. How does the path of the particle change? (A) X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X No slam dunk.. As Expected ! Several things going on here (B) (C) 1. q changes v changes 2. q & v change F changes 3. v & F change R changes Physics 212 Lecture 12, Slide 28 Follow-Up A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo x0/2 E d enters here 2 mEd q exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Suppose the charge of the particle is doubled (Q = 2q),while keeping the mass constant. How does the path of the particle change? How does v, the new velocity at the entrance, compare to the original velocity v0? v (A) v o 2 • Why?? 1 mv 2 2 v (B) v o (C) v vo 2 QEd 2 qEd 2 1 mvo2 2 (D) v 2vo v 2 2vo2 (E) v 2 vo v 2 vo Physics 212 Lecture 12, Slide 29 Follow-Up A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo x0/2 E d enters here 2 mEd q exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Suppose the charge of the particle is doubled (Q = 2q),while keeping the mass constant. How does the path of the particle change? v 2 vo How does F, the magnitude of the new force at the entrance, compare to F0, the magnitude of the original force? (A) F Fo 2 (B) F Fo (C) F 2 Fo (D) F 2 Fo (E) F 2 2 Fo • Why?? F QvB 2 q 2 vo B F 2 2 Fo Physics 212 Lecture 12, Slide 30 Follow-Up A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo x0/2 E d enters here 2 mEd q exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Suppose the charge of the particle is doubled (Q = 2q),while keeping the mass constant. How does the path of the particle change? v 2vo F 2 2 Fo How does R, the radius of curvature of the path, compare to R0, the radius of curvature of the original path? (A) R Ro 2 (B) R Ro 2 (C) R Ro (D) R 2 Ro (E) R 2 Ro • Why?? v2 F m R v2 Rm F 2vo2 vo2 R Rm m o 2 2 Fo 2 Fo 2 Physics 212 Lecture 12, Slide 31 Follow-Up A particle of charge q and mass m is accelerated from rest by an electric field E through a distance d and enters and exits a region q,m containing a constant magnetic field B at the points shown. Assume q,m,E,d, and x0 are known. What is B? 2 B xo x0/2 E d enters here 2 mEd q exits here XXXXXXXXX X X X X X X X X X x0 XXXXXXXXX XXXXXXXXX B B • Suppose the charge of the particle is doubled (Q = 2q),while keeping the mass constant. How does the path of the particle change? (A) X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X X X X X X X X XX X X X X X X X XX X X X X X X X X (B) Ro R 2 (C) A Check: (Exercise for Student) Given our result for B (above), can you show: R 1 B 2mEd Q R R o 2 Physics 212 Lecture 12, Slide 32