Magnetism

• Uniform field

  Placing north and south poles of two permanent bar magnets near each other
• Practical: Magnetic Field Patterns

  1. Compasses and iron filings align themselves with magnetic fields
  2. Iron filings. Put magnet under a piece of paper, sprinkle the iron filings on top and tap the paper until the iron filings form a clear pattern
• Inducing

  When magnetic materials are brought near to a magnetic field the material acts as a magnet meaning it has been induced

  The closer the magnet and the magnetic material get, the stronger the induced magnetism will be
• Current carrying wire creates a magnetic field

  1. An electric current in a conductor produces a magnetic field around it
  2. The larger the electric current, the stronger the magnetic field
  3. The direction of the magnetic field depends on the direction of the current
• Soft magnetic materials

  If a magnetic material loses its magnetism quickly

  e.g iron

  You can increase the strength of a magnetic field around a solenoid by adding a soft iron core through the middle of the coil
• Hard magnetic materials

  When a magnetic material keeps its magnetism permanently after being induced

  e.g steel
• The Motor Effect

  1. Wire has to be at 90 degrees to the magnetic field because if it runs along the magnetic field it won't experience any force at all
  2. Force acts in same direction relative to magnetic field and direction of current
  3. A good way of showing the direction of current is to apply a current to a set of rails inside a horseshoe magnet
  4. The magnitude of the force increases with the strength of the magnetic field
  5. The force increases with the amount of current passing through the conductor
• Flemings left hand rule

  Thumb = direction of force (motion)

  First finger = direction of field

  Second finger = direction of current
• Speeding up an electric motor

  1. more current
  2. more turns on the coil
  3. stronger magnetic field
  4. a soft core in the iron coil
• Electric motors

  • Split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction
  • Direction of motor can be reversed by swapping the polarity of the d.c supply or swapping the magnetic poles over
• Loudspeakers work because of the motor effect

  1. A.c electrical signals from an amplifier are fed to a coil of wire in the speaker which is wrapped around the base of a cone
  2. The coil is surrounded by a permanent magnet so the a.c signals cause a force on the coil and make it move back and forth
  3. This makes the cone vibrate and creates sound
• Electromagnetic induction

  the creation of a voltage in a wire which is experiencing a change in magnetic field
• The dynamo effect

  When electromagnetic induction is used to generate electricity using energy from kinetic energy stores
• EM induction used:

  1. Electrical conductor moves through a magnetic field
  2. When a magnetic field moves through an electrical conductor

  Test this by connecting an ammeter to a conductor and moving the conductor through a magnetic field. Shows the magnitude and direction of the induced current
• To get a bigger voltage increase:

  The strength of the magnet

  The number of turns on the coil

  The speed of movement
• A.C generator

  when a magnetic field induces a current

  1. Generators rotate a coil in a magnetic field
  2. As the coil spins a current is induced in the coil. Current changes direction every half turn.
  3. Have split rings and brushes so the contacts don't swap every half turn
  4. Means they produce a.c voltage
  5. Power stations use a.c generators to produce electricity