7. Magnetism + Electromagnetism

• Magnetic field


  Effect of (a) nonmagnetic and (b) magnetic materials on

  
• Lines of force


  • Unaltered
  • Pass through Fe rather than air
• Magnetic Flux (F)


  Unit: Weber (Wb) 108 lines - usually µWb

  
• Magnetic Flux Density (B)


  • B = F/A
  • Unit Tesla (T) = 1Wb/m2
  • cgs system unit = gauss; 104 Gauss = 1T

  
• Domain Theory of magnetism

  Ferromagnetic materials: Fe, Ni, Co

  
• Magnetic field values


  • 0.1 - 1.0 pT – human brain magnetic field
  • 58 µT (5.8×10−5 T) - strength of Earth's magnetic field at 50° latitude
  • 0.5 mT - the suggested exposure limit for cardiac pacemakers
  • 5mT – strength of a typical refridgerator magnet
  • 1 T to 2.4 T - coil gap of a typical loudspeaker magnet
  • 1.5 T to 3 T - strength of medical magnetic resonance imaging systems in practice, experimentally up to 8 T
  • 16 T - strength used to levitate a frog
  • 45 T - strongest continuous magnetic field yet produced in a laboratory
  • 100 T - strongest (pulsed) magnetic field yet obtained non-destructively in a laboratory
  • 2.8 kT - strongest (pulsed) magnetic field ever obtained (with explosives) in a laboratory
  • 1 to 100 MT (106 T to 108 T) - strength of a neutron star
• Johann Carl Friedrich Gauss: 1777-1855
• Wilhelm Eduard Weber: 1804-1891
• Nikola Tesla: 1856-1943
• Magnetic Switch Operation


  Applications: Perimeter alarm systems, Doors, Windows
• Electromagnetism


  • Current in conductor → Magnetic field
  • Electromagnetic field concentric circles
  • Direction of lines of force → right hand grasp rule

  
• Permeability (µ)


  • Absolute permeability, ease to establish magnetic field (Wb/At.m)
  • Vacuum µ0 = 4Õx10-7 Wb/ampere-turn meter
  • Ferromagenetics –100 x µ0
  • Relative permeability µr = µ/µo

  
• Reluctance (R)


  • Opposition to establish magnetic field (At/Wb)
  • R=l/(µA)

  
• Magnetomotive Force (mmf)


  Fm = NI, Force that produces magnetic field from current (units: At)

  
• Basic magnetic circuit


  Flux F = Fm/R, c.f. Ohm's Law for electrical circuits
• Electromagnet


  • Current reversed then mag field is reversed
• Electromagnetic recording head


  • Recording information on magnetic tape by magnetising the tape as it passes by

  
• Solenoid


  • Moveable iron core, e.g. valves, Car door locks
• Relay


  • Electromagnetic action opens/closes electrical contacts, Single pole – double throw
• Loudspeaker


  • Converts audio signal voltages into sound waves
• Analogue Meter Movement (d'Arsonval meter)


  • Flemings left hand rule determines the field
• Magnetic Hysteresis (B-H curve)


  • Magnetising force (H) = magnetomotic force (Fm)/length of material (At/m)
  • Remanence or retentivity, Coercive force

  
• Electromagnetic Induction


  • Relative Motion of conductor and magnetic field
  • Voltage and speed, Vind
  • Flemings right hand rule for direction of current
• Polarity of Induced Voltage

  Induced current
• Faraday's Law


  Voltage induced Vin = Rate of change of magnetic flux x Number of turns
• Lenz's Law


  Defines polarity of induced voltage, "When the current through a coil changes an induced voltage is created as a result of the changing magnetic field and the direction of the induced voltage is such that it always opposes the change in current"

  
• Applications of electromagnetic induction

  • Automotive crankshaft sensor
  • D.C. Generator
• D.C. Generator: Rotates with loop parallel to magnetic lines of force – does not cut any → Vin = 0, Cuts through maximum number of lines of force →Vin = max
• D.C. Generator: Brushes switch to opposite commutator sections, polarity of Vin is same, d.c. polarities don't change but voltage pulses