magnetism and electromagnetism

• Poles of a magnet


  The part of a magnet where the magnetic forces are strongest
• Two magnets are brought close to each other


  They exert a force on each other
• Force exerted when two like poles of a magnet are brought near each other

  Repulsive, non-contact force
• Force exerted when two unlike poles of a magnet are brought near each other

  Attractive, non-contact force
• Permanent magnet


  Produces its own magnetic field
• Induced magnet


  Becomes magnetic when placed in a magnetic field
• Induced magnetism always causes a force of attraction
• When an induced magnet is removed from a magnetic field, it loses most/all of its magnetism
• Magnetic field


  The region surrounding a magnet where another magnet or magnetic material experiences a non-contact force
• Magnetic materials


  • Iron
  • Steel
  • Cobalt
  • Nickel
• The force between a magnet and a magnetic material is always attractive
• Moving further away from the magnet producing the magnetic field

  The magnetic field strength decreases
• Direction of a magnetic field

  • In the direction that a north pole would experience a force if placed in the field
  • From the north seeking pole to the south seeking pole of a magnet
• Magnetic compass


  Contains a small bar magnet that points in the direction of the Earth's magnetic field
• What is produced when current flows through a conducting wire?


  A magnetic field is produced around the wire
• Magnetic field around a current-carrying wire

  • The magnitude of the current flowing through the wire
  • The distance from the wire
• Solenoid


  A coil of wire which when current passes through creates a strong magnetic field
• Magnetic field inside a solenoid


  • Strong and uniform
• Electromagnet


  • A solenoid with an added iron core
  • Adding the iron core increases the strength of the magnetic field
• Induced potential difference


  A potential difference induced across the ends of a conductor when it moves relative to a magnetic field
• Induced potential difference


  A potential difference induced across the ends of a conductor when there is a change to the magnetic field it is placed in
• Requirement for induced potential difference to cause current flow

  • The conductor must form a closed loop or be part of a complete circuit
• Motor effect


  When a force is exerted between a magnetic field and a current-carrying conductor placed in that field
• Fleming's Left-Hand Rule


  Used to determine the force experienced due to the motor effect
• Direction of magnetic field produced by induced current

  • The direction of the field is such that it opposes the change that induced the current (either the movement of the conductor or a change to the field)
• Fleming's left hand rule

  • Forefinger points in the direction of the magnetic field
  • Second finger points in the direction of current flow in the conductor
• Force on a current-carrying wire in a magnetic field

  • The magnitude of the current flowing through the conductor
  • The strength of the magnetic field that the conductor is placed in
• Generator effect used to generate different types of current

  1. In an alternator to produce alternating-current

  2. In a dynamo to produce direct-current
• Electromagnetic effect used by microphone

  The generator effect, converting pressure variations in sound waves into alternating current in a circuit
• Makeup of a basic transformer


  • A primary coil and a secondary coil of wire wrapped around an iron core
• If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed


  The direction of the force is reversed
• Why iron is used as the core for a transformer

  It is easily magnetised
• If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased


  The strength of the force is increased
• How a transformer works

  An alternating current flows through the primary coil, inducing a changing magnetic field in the core, which then induces a current to flow in the secondary coil
• Why the current in the primary coil must be alternating


  For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing, which requires an alternating current in the primary coil
• For a 100% efficient transformer, the electrical power input is equal to the electrical power output
• The conductor must be at right-angles to the magnetic field it is placed in for the equation linking force, magnetic flux density, current and length to hold
• Magnetic flux density


  Tesla, T
• How an electric motor works

  1. A coil of wire, carrying a current, is placed in a magnetic field

  2. The forces on the two sides perpendicular to the field experience forces in opposite directions

  3. This causes a rotational effect
• How loudspeakers make use of the motor effect

  The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound