Magnetism

• Current-carrying wires?

  when current flows through a wire, a magnetic field is generated around it

  • the strength of the field depends on the magnitude of the current & distance from the wire
• electric motor?

  a current-carrying coil of wire in a magnetic field

  • the two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions, causing rotation
• fleming’s left hand rule?

  a rule used to determine the orientation of the force (thumb), current (2nd finger) and magnetic field (first finger), when a current-carrying wire is placed in a magnetic field (motor effect)
• generator effect?

  when there is a relative motion between an electrical donut or and a magnetic field, a voltage will be induced across the ends of the conductor

  • a current will flow if this conductor is part of a complete circuit
• induced magnet?

  a material that becomes a magnet when it is placed in an existing magnetic field, but loses its magnetism quickly once it is removed

  • induced magnetism always produces attractive forces
• like magnetic poles?

  when matching poles of a magnet are brought near each other they repel each other
• magnetic field lines?

  lines that show the strength and direction of a magnetic field

  • the lines point from north to south and their concentration represents the magnitude of the field
• magnetic field?

  the region around a magnet in which another magnet or magnetic material will experience a non-contact force
• magnetic materials?

  iron, steel, cobalt and nickel
• magnetic poles?

  the regions of a magnet where the magnetic forces are at their strongest
• magnetically hard?

  a material property that means the material can be permanently magnetised

  • examples are iron & alloys containing large amounts of iron
• magnetically soft?

  a material property that means the material can be temporarily magnetised

  • examples include alloys with lower amounts of iron
• permanent magnet?

  a magnet that produces its own magnetic field
• A magnet is an object which produces a magnetic field
• Magnets repel & attract other magnets

  • magnets only attract magnetic materials
• Non-magnetic materials are materials not attracted to magnets & cannot be magnetised

  • eg glass or plastic
• magnetic materials are materials attracted to magnets & can be magnetised

  • eg iron, steel, cobalt, nickel
• magnetism can be induced in magnetic materials by placing them in a magnetic field

  • materials that can be permanently magnetised = magnetically hard
  • materials that are only temporarily magnetised = magnetically soft
• magnetically hard materials can be permanently magnetised

  • they are difficult to magnetise & demagnetise
  • eg steel
• Magnetically soft materials can be only temporarily magnetised

  • they are easily magnetised & demagnetised
  • eg soft iron
• Magnetic field lines represent the magnetic force on a North Pole at any given point
• magnetic field lines show the strength & direction of a magnetic field

  • closer the lines = stronger the magnetic field
  • the magnetic field is strongest at the poles
  • the field lines always point from north to south

  
• Uniform magnetic field lines are when the magnetic field strength is the same in an area of a magnetic field

  • a uniform field is created when both magnets are adjacent to each other between a north & South Pole
  • the field lines are evenly spaced & parallel

  
• investigating field lines using a compass?

  place magnet on paper so N pole is facing north & S pole facing south

  • starting from pole N, mark the positions of the ends of the compass needle at S & N with pencil dots at different angles around magnet
  • repeat pencil process unit all dots completed
  • join dots & will give plot of field lines
• Magnetic field lines between opposite poles will move from north to south

  
• between like poles, the magnetic field lines will divert from each other

  
• an electric current passing through a conductor produces a magnetic field around it
• the motor effect?

  a current-carrying wire experiences a maximum force when at 90 degrees to a magnetic field

  • the direction of the force is at 90 degrees to the wire & magnetic field
• the motor effect is when a force acts on a current-carrying conductor in a magnetic field

  • this is at 90 degrees to the wire & magnetic field
  • shown using fleming's left hand rule
• the motor effect is caused by:

  • a conventional current flows through a wire, creating a magnetic field around it
  • this interacts with the permanent magnetic field, causing a resultant force to act on the wire
• if the current or magnetic field is reversed = the force will also be reversed
• if magnitude of the current or magnetic field is increased = magnitude of the force will also increase
• fleming's left hand rule is used to predict the direction of force acting on a current-carrying wire at 90 degrees to a magnetic field

  • thumb = direction of force
  • pointer finger = magnetic field
  • 2nd finger = direction of current

  
• DC motors consist of a coil of wire between two permanent magnets
• DC current?

  • direct current flows through wire & it experiences a turning effect due to forces exerted on it in the magnetic field
  • as current flows in opposite directions on each side of the coil, the forces on each side are in opposite directions - making it turn
• DC current is current that flows in one direction
• turning effect can be increased by:

  • increasing current
  • using stronger magnetic field
  • increasing number of turns on the coil
• a split ring commutator is used to ensure that the current flows in the coil reverses every half turn

  • this ensures the coil keeps rotating in the same direction
• how does an electric motor work?

  DC current flows through coil & creates magnetic field

  • magnetic field around coil interacts with permanent magnetic field
  • resultant force acts on opposite sides of coil where current flows at 90 degrees to magnetic field = due to motor effect
  • this causes the coil to rotate
  • split ring commutator reverses the current through the coil every half turn, to ensure coil keeps rotating in same direction
• AC current = the current continuously alternates between positive & negative