Prozorov_10 - PHYSICS 222 Introduction to Classical Physics...

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Unformatted text preview: PHYSICS 222 Introduction to Classical Physics II Prof. Ruslan Prozorov Iowa State University Fall 2011 LECTURE 10 Magnetic field Sources of magnetic field Magnetic forces magnetism o Magnets exert forces on each other just like charges. In fact, you can draw magnetic field lines just like you drew electric field lines. o We will soon discover is that electrostatics, electrodynamics, and magnetism are deeply interwoven. Here you see an MRI scans. The magnetic field interacts with molecules in the body to orient spin before radio frequencies are used to make the spectroscopic map. The different shades are a result of the range of responses from different types of tissue in the body. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 2 Magnetic north and south poles’ behavior is not unlike electric charges. For magnets, like poles repel and opposite poles attract. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 3 magnetism and (some) metals A permanent magnet will attract a metal like iron, cobalt, nickel with either the north or south pole. Other metals, like aluminum, gold, copper, silver are not attracted. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 4 magnetism of our planet PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 5 how compasses work PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 6 magnetosphere shield PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 7 magnetic field variation PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 8 units of magnetic field • H - magnetic field strength: [A/m] • B - magnetic induction: [T] (Tesla) • in the simplest case: B 0 H • magnetic permeability of free space N / A2 0 4 10 7 • - relative magnetic permeability 0 PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 9 relative magnetic permeability Magnetic susceptibility and permeability data for selected materials Medium Susceptibilit y χm (volumetric SI) Relative Permeability μ/μ0 2.5×10−2 Mu-metal Permeability μ [H/m] 20,000[6] Magnetic field Frequency max. at 0.002 T 50,000[7] Mu-metal sometimes, this form is used: Permalloy 1.0×10−2 8,000[6] at 0.002 T Electrical steel 5.0×10−3 4,000[6] at 0.002 T Ferrite (nick el zinc) 2.0×10−5 – 8.0×10−4 16–640 100 kHz ~ 1 MHz Ferrite (manganese zinc) >8.0×10−4 640 (or more) 100 kHz ~ 1 MHz Steel 8.75×10−4 100[6] at 0.002 T Nickel 1.25×10−4 100[6] – 600 B H at 0.002 T Neodymium magnet 1.05[8] 1.2569701×1 1.000265 0−6 Platinum Aluminum 2.22×10−5[9] 1.2566650×1 1.000022 0−6 1.00000043[9 Wood 1.00000037 [ Air 10] 1[11] Concrete Vacuum 0 1.2566371×1 [12] 1 0−6 (μ0) Hydrogen −2.2×10−9[9] 1.2566371×1 1.0000000 0−6 1.2567×10−6[ Teflon 6] 1.0000 1.2566368×1 0.99999976 0−6 Sapphire −2.1×10−7 Copper −6.4×10−6 1.2566290×1 or −9.2×10−6[ −6 0.999994 0 9] Water −8.0×10−6 Bismuth −1.66×10−4 Superconduc −1 tors 1.2566270×1 0.999992 0−6 0.999834 0 0 PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 10 magnetic field amplitudes NEUTRON STAR: EARTH X 100 TRILLION While the Earth maintains a magnetic field of about 0.5 gauss, a neutron star's field measures in the trillions. This one is Cassiopeia A, imaged by the Chandra X-Ray observatory. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 11 number names . . . 1,000,000, . . .,000,000 Number of zeros U.S. Other countries 3 thousand thousand 6 million million 9 billion milliard 12 trillion billion 15 quadrillion 1000 billion 18 quintillion trillion PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 12 magnetic pole(s)? o We observed monopoles in electricity. A (+) or (−) alone was stable and field lines could be drawn around it. o Magnets cannot exist as monopoles. If you break a bar magnet between N and S poles, you get two smaller magnets, each with its own N and S pole. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 13 electric current and magnets o In 1820, Hans Oersted ran a series of experiments with conducting wires run near a sensitive compass. The result was dramatic. The orientation of the wire and the direction of the flow both moved the compass needle. o There had to be something magnetic about current flow. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 14 magnetic field and charge o The moving charge interacts with magnetic field. o The force between them is at a maximum when the velocity of the charge is perpendicular to the magnetic field. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 15 the “right-hand rule” o This is for a positive charge moving in a magnetic field. o Place your hand out as if you were getting ready for a handshake. Your fingers represent the velocity vector of a moving charge. o Move the fingers of your hand toward the magnetic field vector. o Your thumb points in the direction of the force between the two vectors. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 16 o Two charges of equal magnitude but opposite signs moving in the same direction in the same field will experience force in opposing directions. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 17 tracing magnetic field lines PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 18 tracing magnetic field lines PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 19 tracing magnetic field lines PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 20 there are other ways to see field credit card barcode PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 21 ferromagnetic material 10 m PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 22 field lines are not lines of force o The lines tracing the magnetic field crossed through the velocity vector of a moving charge will give the direction of force by the right hand rule. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 23 magnetic flux through an area PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 24 charge moving in a magnetic field PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 25 Lorentz force F q E vB PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 26 units PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 27 vector product a a , a , a b b , b , b x y z x y z c a b ab sin ˆ x c a b ax bx ˆ y ay by ˆ z az bz ˆ ˆ ˆ x a y bz az by y az bx axbz z axby a y bx PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 28 F qvB F macentripetal 2 v m R 2 v qvB m R mv R qB PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 29 Auroras PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 30 aurora borealis (or the northern lights) PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 31 aurora australis (or the southern lights) PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 32 particle motion in magnetic fields PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 33 J. J. Thompson was able to characterize the electron PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 34 Bainbridge’s mass spectrometer o Using the same concept as Thompson, Bainbridge was able to construct a device that would only allow one mass in flight to reach the detector. The fields could be “ramped” through an experiment containing standards. PHYS222 - Lecture 10 - Prof. Ruslan Prozorov - Iowa State University 14 September 2011 35 ...
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