Magnetic Fields and Forces - Lecture Slides | PHYS 102, Study notes of Physics

Download Magnetic Fields and Forces - Lecture Slides | PHYS 102 and more Study notes Physics in PDF only on Docsity! Phys 102 – Lecture 10 Magnetic fields & forces 1 Office Hours: Prof. Yoshi Oono 3 - 4PM 222 Loomis Today we will... • Learn about the magnetism Magnetic field B Magnetic force F on moving charge • Apply these concepts! Charged particle motion in a magnetic field Mass spectrometry Earth’s magnetic field & northern/southern lights Phys. 102, Lecture 10, Slide 2 + Sources of magnetic fields Intrinsic magnetic dipole (“spin”)Moving electric charge (current) Lecture 12 Geodynamo There is no magnetic charge, so where do magnetic fields come from? – Lecture 25 – Phys. 102, Lecture 10, Slide 5 Magnetic force Magnetic field B exerts a force on a moving charge q: M ag ni tu de Di re cti on “Right-hand rule” (RHR) Speed of charge q Magnetic field strength Angle between v and B + F is  to both v and B θ q Only component of v  to B (or B  to v) matters v⊥B⊥ Thumb along Fingers along on + q is out of palm on – q is into palm– Phys. 102, Lecture 10, Slide 6 DEMO sinF q vB θ q v B  B  v q vB  v  B  F  F  F ACT: right hand rule practice A + charge moving to the right in a uniform B field experiences a force F up. Which way does the B field point? +q v F A. Up B. Down C. Into the page D. Out of the page Thumb along Fingers along on + q is out of palm B Phys. 102, Lecture 10, Slide 7  v  B  F What is the direction of the force on the particle just as it enters region 1? A.up B.down C.left D.right 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. Particle is moving straight upwards then veers to the right Checkpoint 1.1 + v Phys. 102, Lecture 10, Slide 10 What is the direction of the magnetic field in region 1? A.up B.down C.into page D.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. ACT: Checkpoint 1.2 + B1 v B F v Phys. 102, Lecture 10, Slide 11 What is the direction of the magnetic field in region 2? A.up B.down C.into page D.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. Checkpoint 1.4 + v B2 F v B F Phys. 102, Lecture 10, Slide 12 Mass spectrometer (z = q/e) Mass spectrometry uses a B field to analyze chemical compounds Compound is vaporized into fragments & ionized, accelerated with a E field into a B field Fragments separate according to mass to charge ratio (m/q) Phys. 102, Lecture 10, Slide 15 Calculation: Mass spectrometer R = 6.8 cm B = 0.2 T 13C+ A mass spectrometer is used to separate different isotopes of carbon. Carbon ions are accelerated to a speed v = 105 m/s; assume all have charge +1e = 1.6×10–19 C. Find which C isotope travels along the green dotted path to the detector. FA13 EX2 Which way does the B field point? Out of the page 13C+v = 105 m/s Phys. 102, Lecture 10, Slide 16 mv R qB  qBR m v  19 26 5 1.6 10 0.2 0.068 2.18 10 kg 10       271amu 1.67 10 kg  13amu ACT: Mass spectrometer I The mass spectrometer isolates three C isotopes a, b, c. They move at a speed v = 105 m/s entering the B field and follow the dashed paths. R a bc Detector C ion beam Magnet v = 105 m/s How do the speeds of the different isotopes a, b, c leaving the B field compare? A. va > vb > vc B. va = vb = vc C. va < vb < vc B fields do NO work, so kinetic energy cannot change! Speed is the same, v = 105 m/s, throughout Phys. 102, Lecture 10, Slide 17 Aurora borealis & australis Earth’s B field protects against stream of ions from sun (“solar wind”) x y z B B field directs ions to atmosphere in north and south hemispheres. Ions collide with particles in atmosphere and emit light: “aurora” Aurora australis “Southern lights” Phys. 102, Lecture 10, Slide 20 Summary of today’s lecture Phys. 102, Lecture 10, Slide 21 Force: Electric Magnetic Source: Charge Moving charge Act on: Charge Moving charge Magnitude: FE = q E FB = q v B sin(θ) Direction: || to E  to v, B Work: WE = qEd cos(θ) WB = 0 Electric vs. magnetic forces Phys. 102, Lecture 10, Slide 22 F v B B F v B F v v B F