Ch32 - Physics 4B Lecture Notes Chapter 32 - Magnetism of...

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Physics 4B Lecture Notes 32-1 Chapter 32 - Magnetism of Matter; Maxwell’s Equations Problem Set #11 - due: Ch 32 - 3, 7, 11, 18, 20, 21, 22, 30, 31, 34, 38, 45 Lecture Outline 1. Magnets as Dipoles 2. Gauss’ Law for Magnetism 3. Magnetism in Matter 4. Three Types of Magnetic Behavior 5. Induced Magnetic Fields 6. Maxwell's Equations At this point we know the laws that describe electric fields. They are, Gauss's Law for Electricity r E d r A = q ε o [Charge creates diverging E fields] Faraday's Law of Induction r E d r s = - d Φ B dt [Changing B's create circulating E's] The laws that explain the properties of magnetic fields aren’t complete. As far as we know, the only way to create a magnetic field is with a current and this relationship is given by, Ampere's Law r B d r s = μ o dq dt [Currents make circulating B fields] In this chapter we will complete the laws of magnetism by examining magnets and by finding a way to induce magnetic fields. The complete laws of electricity and magnetism are known as “Maxwell’s Equations.” 1. Magnets as Dipoles By looking at the laws for electric fields you begin to wonder about r B d r A = ?. In other words, are there ways to make diverging magnetic fields. Is there something analogous to electric charges that produce diverging magnetic fields. The obvious guess would be a magnet. Magnetic Filing Demo w/ wire, coil, magnet and power supply The magnetic field of a magnet isn’t diverging. A magnet produces a field that is the same shape as the field due to a coil. Therefor a magnet is a dipole. That is why breaking a magnet in half doesn’t separate the north pole from the south pole. Instead it creates two new magnets. There is no known way to create a single magnetic pole. coil The magnetic field lines due to a coil. N S magnet The magnetic field lines due to a magnet.
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Physics 4B Lecture Notes 32-2 2. Gauss' Law for Magnetism Beyond the Mechanical Universe (vol. 34 Ch 18,19,20,21,22) Gauss's Law for electric fields states that the electric flux through a closed surface is proportional to the enclosed charges. The same statement can be made for the magnetic flux, r B d r A = μ o q enclosed (m) where q enclosed (m) is the magnetic charge of a "magnetic monopole." Since magnetism is always caused by currents there are only magnetic dipoles, therefor, q enclosed (m) = 0 and Gauss's Law for Magnetic Fields is, r B d r A = 0 Gauss's Law for Magnetism Theories of the interaction of fundamental particles often predict the existence of magnetic monopoles. These theories are always suspect because no magnetic monopole has ever been found. 3. Magnetism in Matter Beyond the Mechanical Universe (vol. 35 Ch 14,15,16) The macroscopic properties of matter are a manifestation of the microscopic properties of the atoms of which it is composed. The magnetic dipole moments of moving electrons, protons, and neutrons create the magnetic fields of bulk materials. The motions of these particles can be broken down into orbital
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This note was uploaded on 03/11/2012 for the course PHYSICS 204B taught by Professor Staff during the Fall '09 term at CSU Chico.

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Ch32 - Physics 4B Lecture Notes Chapter 32 - Magnetism of...

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