Unit 3

• Magnets


  Have magnetic fields around them
• Magnets


  • Have two opposite poles (N & S)
  • Like poles repel, unlike poles attract
• Magnets


  Exert little or no force on a non-magnetic material
• Magnets


  Attract magnetic materials by inducing magnetism in them
• Iron


  • Loses magnetism - it was only a temporary magnet
• Steel


  • Retains magnetism - it became a permanent magnet
• Magnetizing a piece of steel

  1. Placing it near a magnet

  2. Stroking it with one end of a magnet

  3. Placing it in a long coil of wire and passing a large, direct current through the coil
• The best way of magnetizing is to place the steel bar in a long coil of wire and pass a large, direct (one way) current through the coil
• Unmagnetized material


  The tiny electrons, or atomic magnets point in random directions
• Magnetized material


  More and more of the tiny atomic magnets line up with each other, acting as one BIG magnet
• If a magnet is hit with a hammer, the tiny atomic magnets get thrown out of line again, so the material becomes demagnetised
• A magnet will also become demagnetized if heated to high temperature
• Magnetic field lines


  • Run from the north pole (N) to the south pole (S)
  • The magnetic field is strongest where the field lines are closer together
• Common characteristics of magnetic materials

  • Attracting other magnetic material
  • Response to the magnetic field
  • The polarity of two poles are opposite
  • When magnetizing, some parameter of material is changed (Current I, Magnetic flux)
• Magnetic moment (μm)


  • A vector quantity associated with the magnetic properties of electric current loops or, more generally, magnets
  • Equal to the amount of current flowing through the loop multiplied by the area encompassed by the loop
• Magnetisation (M)


  • A magnetic material acquires magnetism in an applied magnetic field
  • The magnetic dipole moment per unit volume of the material
• Magnetic flux density (B) and Magnetic field Intensity (H)

  • B = μH = μ0μrH
  • μ = magnetic permeability of the material
• Magnetic Susceptibility (χm)


  A dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field
• Total Magnetic field (B) of a material placed in Magnetic field (H)

  • B = μ0(H + M)
  • μr = 1+ χm
• Classification of Magnetic materials

  • Diamagnetic materials
  • Paramagnetic materials
  • Ferromagnetic materials
  • Anti-ferromagnetic materials
  • Ferrimagnetic materials
• Diamagnetic materials


  Susceptibility is negative and does not depend on temperature
• Paramagnetic materials


  Susceptibility is positive and dependent on temperature
• Ferromagnetic materials


  Susceptibility is positive and dependent on temperature
• Curie Temperature


  • The interplay of applied field and thermal randomization leads to temperature dependence described by the Curie Law
  • c = C/T
  • C is a constant known as the Curie constant, and T is in Kelvin
• Curie-Weiss Law


  • c = C/(T-θ)
  • θ, is referred to as the "molecular field constant" or Weiss constant and C is called Curie's constant
  • Ferromagnetic substances have θ > 0, Antiferromagnetic substances have θ < 0, Paramagnetic substances have θ = 0
• Weiss theory of ferromagnetism

  Considers mutual interaction of the elementary magnets or molecular magnetic fields
• Domain theory


  Explains the magnetic properties of ferromagnetic materials
• Hysteresis Curve


  • Shows the relationship between the magnetic field (H) and the magnetization (M) of a ferromagnetic material
  • Retentivity is the ability of a material to retain its magnetism after the magnetizing force is removed
  • Coercivity is the ability of a material to resist demagnetization
• Soft Magnetic materials


  • Iron
  • Iron-silicon alloys
  • Nickel-iron alloys
• Hard Magnetic materials


  • Neodymium-iron-boron
• Superconductor


  An element or metallic alloy which, when cooled to near absolute zero, dramatically lose all electrical resistance
• Types of superconductors


  • Type-I superconductors
  • Type-II superconductors
• Superconductive magnets


  • Do not need to use (dissipate) energy to maintain the magnetic field
  • Use Type-II superconductor materials with high critical magnetic field (Bc)
• BCS Theory of Superconductivity

  • Explains the phenomenon in which a current of electron pairs flows without resistance in certain materials at low temperatures
  • Electrons on their flight through the lattice cause lattice deformation, which results in a trail of positively charged region that attracts another electron and provides for electron-electron coupling
• Josephson effect


  When two superconductors are separated by a thin insulating layer, a DC voltage bias across the junction results in an AC current through the junction that oscillates with a specific frequency
• Applications of Superconductors


  • Particle accelerators
  • SQUIDs (magnetometers)
  • MRI machines
  • Power transmission
  • Electromagnetic impulse devices
  • Maglev trains