Understanding Magnetism: Magnetic Fields in Astronomy and Forces on Charges, Study notes of Physics

Download Understanding Magnetism: Magnetic Fields in Astronomy and Forces on Charges and more Study notes Physics in PDF only on Docsity! Chapter 19: Magnetic Fields a ~~ ° a . 7 we eo @ B \ f 8 © 8 8 Xe eo / + t etect eo fe © © © 4e ff 1 8 t | | | ——_——*: 4 = 8t¢ a Highest Lowest energy energy PHY 2054: Chapter 19 PHY2054: Chapter 19 2 Magnetic Fields Magnetic field (units, field lines) Magnetic field of the earth and other astronomical objects Effects of magnetic fields on charges and currents Force on a moving charge Force on a current Torque on a current loop Path followed by particle in magnetic field Generating magnetic fields Long wire Current loop Solenoid Instruments Mass spectrometers Cyclotrons and synchrotrons PHY2054: Chapter 19 5 Reading Quiz When I cut a magnet into two pieces I get: An isolated north and south magnetic pole Two smaller magnets The two pieces are no longer magnets PHY2054: Chapter 19 6 Bar Magnets Two poles: “north” and “south” Like poles repel Unlike poles attract Magnetic poles cannot be isolated NS Similar to dipole field from electrostatics PHY2054: Chapter 19 7 Magnetic Monopoles? Can any isolated magnetic charge exist? We would call this a “magnetic monopole” It would have a + or – magnetic charge How can we isolate this magnetic charge? Cut a bar magnet in half? NO! Magnetic monopoles have never been seen! What you get is a bunch of little magnets! PHY2054: Chapter 19 10 What Causes Magnetism? What is the origin of magnetic fields? Electric charge in motion! For example, a current in a wire loop produces a field very similar to that of a bar magnet (as we shall see). Understanding the source of bar magnet field lies in understanding currents at the atomic level within matter Orbits of electrons about nuclei Intrinsic “spin” of electrons (more important effect) PHY2054: Chapter 19 11 Magnetic Field Units From the expression for force on a current-carrying wire: B = Fmax / I L Units: Newtons/A⋅m ≡ Tesla (SI unit) Another unit: 1 gauss = 10-4 Tesla Some sample magnetic field strengths: Earth: B = 0.5 gauss = 0.5 x 10-4 T Galaxy: B ∼ 10-6 gauss = 10-10 T Bar magnet: B ∼ 100 – 200 gauss Strong electromagnet: B = 2 T Superconducting magnet: B = 5 – 10 T Pulse magnet: B ∼ 100 T Neutron star: B ∼ 108 – 109 T Magnetar: B ∼ 1011 T PHY2054: Chapter 19 12 Pulsars Rapidly Rotating Neutron Stars Enormous Magnetic Fields Beam off Beam on Crab Pulsar R = 10 km M = 1.4 solar mass B ≈ 108 T Period = 1/30 sec PHY2054: Chapter 19 15 Example Particle with m = 1.5 g, q = −2μC moves with velocity 2,000 m/s through a magnetic field of 2.5 T at an angle of 30° to the field. Magnitude of force Direction of force: up out of the page. Use RHR and take opposite direction because of −q ( )( )( )( )6sin 2 10 2.5 2000 0.5 0.005NF qBv φ −= = × = B −q v F is up out of page PHY2054: Chapter 19 16 Quiz A charged particle moves in a straight line through some region of space. Can you conclude that B = 0 here? (1) Yes (2) No A B field can exist since if v || B there is no magnetic force PHY2054: Chapter 19 17 A negative particle enters a magnetic field region. What path will it follow? (1) A (2) B (3) C (4) D (5) E 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 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 Magnetic Force A B C D E (1) RHR says it bends down (− charge) (2) But force cannot instantaneously change v and the velocity vector bends continuously (3) So the answer is D, not E