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• Magnetism


  • Acts at a distance
  • Magnets were first used for navigation by the Chinese in the 12th century
• Magnet


  A material that attracts iron and has a north and south pole
• Magnetism and electricity developed almost independently of each other until 1820 when Oersted discovered that an electric current affects a magnetic compass
• Magnetic poles

  Like poles repel, opposite poles attract
• Magnetic poles cannot be isolated, a north pole always has a south pole and vice versa
• Magnetic field


  The space around a magnet that contains magnetic forces
• The direction of a magnetic field is from the north pole to the south pole
• Magnetism


  Produced by the motion of electric charges, specifically the spin and revolution of electrons in atoms
• Magnetic domains


  Clusters of aligned atoms in a magnetic material
• Making a permanent magnet


  1. Placing iron/iron alloy in a strong magnetic field

  2. Stroking iron with a magnet to align domains
• Electromagnet


  A current-carrying coil of wire that produces a magnetic field
• The strength of an electromagnet increases with more turns in the coil
• Maglev trains


  • Use electromagnets without iron cores to levitate and propel the train
  • Eliminate friction experienced by conventional trains
• How an electric motor works


  1. Current is made to change direction every half rotation of the coil

  2. This produces continuous rotation
• Electromagnetic induction


  Producing electric current by moving a magnet in and out of a coil of wire
• Electromagnetic induction ushered in the age of electricity
• The greater the number of loops of wire in a coil, the greater the induced voltage when a magnet moves in and out
• The faster the magnetic field lines enter or leave the coil, the greater the induced voltage
• Electromagnetic induction

  The induction of voltage when a magnetic field changes with time. It is about creating an electrical current using magnetism.
• Electromagnetic induction

  1. Motion of a magnet in a loop of wire

  2. Changing the magnetic field in loops of wire
• Electromagnetic induction


  • Voltage is caused, or induced, by the relative motion between a wire and a magnetic field
  • The greater the number of loops of wire that move in a magnetic field, the greater is the induced voltage
  • Quick motion induces a greater voltage
• There are three ways in which voltage can be induced in a loop of wire: by moving a loop near a magnet, by moving a magnet near the loop, and by changing a current in a nearby loop
• Faraday's Law


  The induced voltage in a coil is proportional to the product of its number of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within those loops
• Generators and Alternating Current

  1. Magnet plunged into and back out of a coil of wire

  2. Rotating the coil in a stationary magnetic field
• Generator


  An electromagnetic induction device that produces electric current by rotating a coil within a stationary magnetic field. It converts mechanical energy to electrical energy.
• Transformer


  A device for transferring electric power from one coil of wire to another, by means of electromagnetic induction, for the purpose of transforming one value of voltage to another
• Primary coil


  Connected to the power source
• Secondary coil


  Not directly connected to the power source, but has a current induced in it
• If the primary and secondary have equal numbers of wire loops, the input and output alternating voltages will be equal
• If the secondary coil has more turns than the primary, the alternating voltage produced in the secondary coil will be greater than that produced in the primary. The voltage is said to be stepped up.
• If the secondary has fewer turns than the primary, the alternating voltage produced in the secondary will be lower than that produced in the primary. The voltage is said to be stepped down.
• Voltage primary


  Voltage secondary = Turns primary / Turns secondary
• Almost all electric energy sold today is in the form of AC, traditionally because of the ease with which it can be transformed from one voltage to another
• Power is transmitted great distances at high voltages and correspondingly low currents (power = voltage x current)
• Magnetic force


  • Between magnets, it is the attraction of unlike magnetic poles for each other and the repulsion of like magnetic poles
  • Between a magnetic field and a moving charged particle, it is a deflecting force due to the motion of the particle

  
• Magnetic field

  The region of magnetic influence around a magnetic pole or a moving charged particle

  
• Magnetic domains


  Clustered regions of aligned magnetic atoms. When these regions themselves are aligned with one another, the substance containing them is a magnet.
• Electromagnet


  A magnet whose field is produced by an electric current. It is usually in the form of a wire coil with a piece of iron inside the coil.
• Electromagnetism


  Making magnetism using electrical current
• Faraday's law states that an electric field is created in any region of space in which a magnetic field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the magnetic field changes. The direction of the induced field is at right angles to the changing magnetic field.