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Unformatted text preview: Lab 6: The Earths Magnetic Field 1 Introduction The earth just like other planetary bodies has a mag- netic field. The purpose of this experiment is to mea- sure the horizontal component of the earths magnetic field B H using a very simple apparatus. The measure- ment involves combining the results of two separate experiments to obtain B H . The first experiment will involve studies of how a freely suspended bar magnet interacts with the earths magnetic field. The second experiment will involve an investigation of the com- bined effect of the magnetic field of a bar magnet and the earths magnetic field on a compass needle. A magnetic dipole is the fundamental entity in mag- netostatics just like a point charge is the simplest con- figuration in electrostatics. A bar magnet is an exam- ple of a magnetic dipole. The magnetic field lines of the magnet are closed loops that are directed from north to south outside the magnet, and from south to north inside the magnet, as illustrated in figure 1. The magnetic dipole moment (by convention) is a vector drawn pointing from the south pole toward the north pole. At any point in space, the direction of the magnetic field is along the tangent to the field line at that point. The earth is another example of a magnetic dipole, with the north magnetic pole lo- cated at an inclination of 11 . 5 with respect to the rotational axis passing through the north and south geographic poles. In fact, the north magnetic pole should be labeled as the south pole of the earths mag- netic dipole, and the south magnetic pole should be labeled as the north pole of the dipole (figure 2). This can be inferred by considering the behavior of a freely suspended bar magnet or compass needle. The north pole of the magnetic needle is attracted toward the south pole of the earths magnetic dipole. Studies of the evolution of the earths magnetic field have revealed that magnetic poles migrate relative to the geographic pole, and that the direction and the magnitude of the field changes. Most interestingly, the magnetic poles reverse periodically, once every several hundred thousand years! EXERCISES 1, 2, 5 AND 9 PERTAIN TO MT78 MT83 MT68MT105MT114MT101MT99MT116MT105MT111MT110MT32MT111MT102MT32MT116MT104MT101 MT109MT97MT103MT110MT101MT116MT105MT99MT32MT100MT105MT112MT111MT108MT101 MT109MT111MT109MT101MT110MT116MT32MT109 Figure 1: Magnetic field lines of a bar magnet MT78 MT83 MT65MT120MT105MT115MT32MT111MT102MT32MT82MT111MT116MT97MT116MT105MT111MT110 MT80MT108MT97MT110MT101MT32MT111MT102MT32MT116MT104MT101 MT69MT97MT114MT116MT104MT146MT115MT32MT111MT114MT98MT105MT116 MT77MT97MT103MT110MT101MT116MT105MT99MT32MT65MT120MT105MT115 MT49MT49MT46MT53 MT111 MT50MT51MT46MT53 MT111 MT71MT101MT111MT103MT114MT97MT112MT104MT105MT99MT32MT115MT111MT117MT116MT104 MT112MT111MT108MT101 Figure 2: Earths magnetic field THE BACKGROUND CONCEPTS AND EX- ERCISES 3-4, 6-8 AND 10-13 PERTAIN TO THE EXPERIMENTAL SECTIONS....
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- Spring '10