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Chapter 22

Chapter 22. Magnetism. MAGNETISM. Arguable the oldest subject in Physics : ancient Greeks (near the City of Magnesia) and Chinese realized certain strange stones attracted iron. Around 1600, William Gilbert proposed that the Earth itself is A gigantic magnet.

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Chapter 22

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  1. Chapter 22 Magnetism

  2. MAGNETISM Arguable the oldest subject in Physics: ancient Greeks (near the City of Magnesia) and Chinese realized certain strange stones attracted iron. Around 1600, William Gilbert proposed that the Earth itself is A gigantic magnet. For a long time, people knew only one source of magnetism from Iron. In 1821, a Danish physicist, Oersted noticed that an electrical wire carrying current made the near-by compass reorient.  First clue of inter-relation between electricity and Magnetism. Ampere, Faraday established the nature of electricity and magnetism (all from their experimental observations).

  3. Magnets • Poles of a magnet are the ends where objects are most strongly attracted • Two poles, called north and south • Like poles repel each other and unlike poles attract each other • Similar to electric charges • Magnetic poles cannot be isolated • If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole • This differs from electric charges • There is some theoretical basis for monopoles, but none have been detected

  4. More About Magnetism • An unmagnetized piece of iron can be magnetized by stroking it with a magnet • Somewhat like stroking an object to charge an object • Magnetism can be induced • If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized

  5. S S S S N N N N S N

  6. N N S S Magnets exist in pairs of N-S poles. A theoretical prediction says that it is possible to have magnetic mono-poles but they have not been observed!! Magnetic field cannot be defined as E-field, FE = qE FB = qBB

  7. Magnetic Fields • A vector quantity • Symbolized by B • Direction is given by the direction a north pole of a compass needle points in that location • Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look

  8. Magnetic Field Lines, sketch • A compass can be used to show the direction of the magnetic field lines (a) • A sketch of the magnetic field lines (b)

  9. Magnetic Field Lines, Bar Magnet • Iron filings are used to show the pattern of the magnetic field lines • The direction of the field is the direction a north pole would point

  10. Magnetic Field Lines, Unlike Poles • Iron filings are used to show the pattern of the magnetic field lines • The direction of the field is the direction a north pole would point • Compare to the electric field produced by an electric dipole

  11. Magnetic Field Lines, Like Poles • Iron filings are used to show the pattern of the magnetic field lines • The direction of the field is the direction a north pole would point • Compare to the electric field produced by like charges

  12. Magnetic and Electric Fields • An electric field surrounds any stationary electric charge • A magnetic field surrounds any moving electric charge • A magnetic field surrounds any magnetic material

  13. Earth’s Magnetic Field • The Earth’s geographic north pole corresponds to a magnetic south pole • The Earth’s geographic south pole corresponds to a magnetic north pole • Strictly speaking, a north pole should be a “north-seeking” pole and a south pole a “south-seeking” pole

  14. Earth’s Magnetic Field • The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior

  15. S N In unifrom field, no force only torque!!!

  16. Magnetic field can make a closed loop (not with E-field). I X Right-handed cork-screw rule

  17. Magnetic Fields • In a magnetic field, a current carrying wire experiences a magnetic force • This force has a maximum value when the wire is perpendicularly to the magnetic field lines • This force is zero when the wire is along the field lines

  18. Magnetic Fields, cont • One can define a magnetic field in terms of the magnetic force exerted on current carrying wire • Similar to the way electric fields are defined

  19. Units of Magnetic Field • The SI unit of magnetic field is the Tesla (T) • Wb is a Weber • The cgs unit is a Gauss (G) • 1 T = 104 G

  20. Magnetic induction Magnetic flux density Magnetic field (strength) [B] = [F/IL] = Ns/Cm = Tesla *1 Tesla = 104 gauss F Length of the section in B-field B I FB = ILB

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