Topic 12

• Magnetic forces


  Strongest at the poles of a magnet
• Two magnets brought close together

  Exert a force on each other
• Force between two like poles

  Repulsive, non-contact force
• Force between two unlike poles


  Attractive, non-contact force
• Magnetic field


  The region surrounding a magnet where another magnet or magnetic material experiences a 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 materials


  • Iron
  • Steel
  • Cobalt
  • Nickel
• The force between a magnet and a magnetic material is always attractive
• Strength of a magnetic field

  Decreases the further you move away from the magnet producing it
• Direction of a magnetic field

  • In the direction of the force that a north pole would experience 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
• When current flows through a conducting wire, a magnetic field is produced around the wire
• Strength of magnetic field around a current-carrying wire

  Determined by the magnitude of the current flowing through the wire
• A high concentration of magnetic field lines means the field is strong
• Magnetic field lines never cross each other
• Using a plotting compass to map out a magnetic field

  1. Place a compass (containing a needle magnet) on a piece of paper near the field

  2. Draw an arrow in the direction the compass points

  3. Repeat at different points on the paper

  4. Join the arrows to make a complete field pattern
• Solenoid


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

  • Strong and uniform
• Magnetic field outside a solenoid

  Weak, because the fields from each coil cancel out
• Electromagnet


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


  The rule used to determine the force produced by the motor effect
• 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
  • Thumb points in the direction of the force produced by the motor effect
• Factors affecting the size of the 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
• 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
• 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
• The criteria for the equation linking force, magnetic flux density, current and length to hold is that the conductor must be at right-angles to the magnetic field it is placed in
• Magnetic flux density


  Tesla, T
• If the current and the magnetic field are parallel to each other, no force will act
• 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 electric motors are kept rotating

  A split ring commutator is used. This switches the current direction every half turn, which ensures that the coil keeps spinning