Physics Notes

Magnetism

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Concepts:

(A detailed list of terms and objectives is available.)

• Magnetism
  • Magnetism Concept Summary (PowerPoint)
  • Magnetic Poles
    • Like poles repel, unlike poles attract
    • called north & south
    • Unlike charges, poles never exist alone (monopoles)
      • Broken magnets retain N & S poles
  • Magnetic Fields
    • Magnetic field lines
      • From N to S poles
      • Unbroken loops
    • Produced by the motion of an electric charge
      • Electrons revolve about nucleus and spin on their axis
      • Spin generally predominates
        • electron spins align or are opposite in atom
        • Spins cancel in most meterials
        • Spins do not always cancel in some materials (iron)
        • Spinning electron is a tiny magnet
    • Current produces a magnetic field
      • Field "circulates" around the wire
  • Electromagnets
    • Many coils (turns) of wire increase field inside the coil
    • Iron core intensifies magnetic field inside the coil
    • Superconducting Electromagnets
      • Iron core can "saturate"
      • Superconducting core generates powerful field with little or no power
  • Magnetic domains
    • Large clusters of atoms (iron) align
    • Domains randomly oriented in unmagnetized iron
    • External magnetic field causes:
      • Magnetic domains aligned with the field to grow
      • Domains rotate to align with the field
  • Creating Permanent magnets
    • Place Iron or iron alloy in strong magnetic field
    • Soft iron is easier to magnetize
    • Tapping the iron can help align the domains
    • Stroke iron or iron alloy with a magnet
  • Destroying a Permanent Magnet
    • Dropping or heating
  • Magnetic Force on a Particle
    • Only on moving charges
    • Force perpendicular to charge's velocity
    • Force perpendicular to the magnetic field
    • F=qvBsinQ
      • B measured in Tesla
      • Tesla = field that produces a maximum force of 1 N on a charge of 1 C moving at 1 m/s
      • Old unit is Gauss = 10-4 Tesla
      • Earth's magnetic force at surface about 1 Gauss = 10^-4 Tesla
      • Right hand rule
        • Maximum force when v perpendicular to B
        • Zero force when v parallel to B
  • Magnetic Force on a Wire
    • Reversing current reverses force
    • Due to forces on moving charges in the wire
  • Meters
    • Current through loop generates magnetic force
    • Ammeter
      • measures current
      • low resistance
      • Galvanometer
        • very sensitive ammeter
    • Voltmeter
      • Measures potential difference
      • needs high resistance
  • Motors
    • Similar to galvanometer in design
    • Permanent magnet turns current-carrying coil
    • Coil current direction reversed every half turn
      • Brushes
  • Earth's Magnetic Field
    • Earth's magnetic poles not aligned to geographic poles
      • Magnetic declination
    • Cause not entirely known
    • May be due to circulation of iron core
    • Very small velocity would account for the field
      • Convection currents
    • Not stable
      • More than 20 reversals in past 5 million years most recent 700 000 years ago

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Links:

• CPSurf - Magnetism - a nice set of annotated links keyed to the sections in your textbook.
• Magnetic Field Lines - an applet that simulates the old "magnet and iron filings" activity
• Attraction & Replusion by Magnetic Poles - neat!
• Seat Experiments

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last update April 10, 2007 by JL Stanbrough