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Magnetism

Magnetism. Chapter 24. General Properties of Magnets. Attractive/Repulsive forces between magnets A “North” pole and a “South” pole. General Properties of Magnets. Cannot be broken into “monopoles” ALNICO – aluminum, nickel and cobalt. Important Definitions. Magnetic field

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Magnetism

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  1. Magnetism Chapter 24

  2. General Properties of Magnets • Attractive/Repulsive forces between magnets • A “North” pole and a “South” pole

  3. General Properties of Magnets • Cannot be broken into “monopoles” • ALNICO – aluminum, nickel and cobalt

  4. Important Definitions • Magnetic field • Magnetic forces around a magnet • Example (overhead)

  5. Important Definitions • Magnetic flux • Number of magnetic field lines passing through a surface

  6. Electromagnetism • 1820 – Hans Christian Oersted • Experimented with electric currents in wires over a compass • Thought needle would point to wire or be parallel to wire

  7. Electromagnetism • 1820 – Hans Christian Oersted • Needle points away from wire • Electric current in a wire produces a magnetic field

  8. Electromagnetism I • First Right-Hand Rule • Thumb points in direction of current • Fingers follow magnetic field lines (direction of magnetic field) I

  9. Electromagnetism • What about a coil of wire? • The RHR still applies! I

  10. Electromagnets • Coil has a field like any permanent magnet with N and S poles • Advantage: can be turned off and on

  11. Electromagnets • 2nd Right-Hand Rule • Determine magnetic field of electromagnets • Fingers follow current as it curls in the coil • Thumb points in direction of N pole

  12. Forces caused by Magnetic Fields • Vectors • Perpendicular to magnetic field lines and current

  13. Forces caused by Magnetic Fields • 3rd Right-Hand Rule • Determine direction of Force on a current-carrying wire in a magnetic field I N S

  14. 3rd RHR Thumb points in direction of current Fingers point in direction of magnetic field Palm faces direction of Force Forces caused by Magnetic Fields

  15. Forces caused by Magnetic Fields I S N

  16. Forces caused by Magnetic Fields N S I

  17. Forces caused by Magnetic Fields • F = BIL • B = strength of magnetic field • I = current in the wire • L = length of wire in magnetic field • We know how to measure F, I and L, but not B so instead we use…

  18. Forces caused by Magnetic Fields • B = F / (IL) • Magnetic induction – strength of the magnetic field • Units: Tesla (T) • 1 T is very strong • Most lab magnets are 0.01 T • Earth’s magnetic field is 5 X 10-5 T

  19. Galvanometers • Measures very small currents • Torque on the wire causes it to rotate • What is necessary for loop to rotate 360o? • Current must reverse right as loop flips • Process repeats each half turn

  20. Electric Motors • Several rotating loops or wire • (overhead 35)

  21. Force on a single charged particle • Cathode ray tube – TV! • Electrons deflected by magnetic fields to form pictures

  22. Cathode Ray Tube Electric fields pull electrons off atoms, then more electric fields gather, and focus electrons into a beam. • Magnetic fields deflect electrons side to side and up and down across the screen • Screen coated with phosphorous that glows when struck

  23. Force on a single charged particle • F = BIL • F = B(qv/L)L • F = Bqv • q = charge of electron • v = particle velocity

  24. Van Allen Radiation Belts • Electrons trapped in Earth’s magnetic field • Solar storms send high-energy charged particles toward Earth • They knock electrons off VA belts • The electrons excite nitrogen and oxygen in the atmosphere creating a “halo” • The halo surrounds geomagnetic north

  25. Van Allen Radiation Belts

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