MAGNETISM

• unlike magnetic poles and like magnetic poles repel
• what are permanent magnets?

  magnets that produce its own magnetic field
• examples of permanent magnets:

  steel
• what is an induced magnet?

  a material that becomes a magnet when it is placed in a magnetic field
• examples of induced magnets:

  iron, cobalt, nickel
• what is a magnetic field?

  the region around a magnet where it has a magnetic effect
• describe how to find the field lines around a bar magnet:

  • put a magnet on the surface with a piece of a paper on top
  • sprinkle iron filings near the bar magnet
  • tap the paper to evenly spread out the filings
  • look at the pattern of the iron filings to see the magnetic field lines
• what does the uniform magnetic field show?

  • shows there's the same strength everywhere
  • there will be evenly spaced out field lines from North to South
• what does the magnetic field around a bar magnet show?

  • shows the strength of the magnetic field by the spacing of the field lines
  • the field lines will be closest together at the poles (but never touching)
• a current carrying conductor placed near a magnet experiences a force and that an equal and opposite force acts on a magnet
• the strength of the field depends on the size of the current and the distance from the long straight conductor
• magnetic forces are dueto interactions between magnetic fields
• how to plot the magnetic field around a magnet (usingf a compass):

  • place the magnet on a sheet of paper
  • place the compass near the end of the magnet
  • mark the position that the compass needle points to
  • move the compass so the opposite end is at this position and mark the new position where the compass tip settles
  • repeat above and below the magnet and then connect the marks together to condtruct field lines
• how can a compass show the Earth's core is magnetic?

  the compass needle points to the geographical north of the planet, this means that the Earth's north pole is actually magnetic South
• any current flowing through a wire creates a magnetic field
• how to increase the strength of an electro-magnet:

  • increase the current in the wire
  • add an 'iron core' to the centre of the loops of wire
  • add more turns to the coil
• what is a coil of wire called?

  solenoid
• how does a current create a magnetic effect around a long straight conductor?

  • when a current flows through a wire, a magnetic field is created around it
  • the field is made up of concentric circles perpendicular to the wire in the centre
  • changing the current's direction, changes the magnetic field's direction
  • the larger the current through the wire, or the closer you are to the wire, the stronger the magnetic field
• describe a solenoid's magnetic fields:

  • inside the coil, the magnetic fields from both sides of the coil are in the same direction
  • the two fields reinforce each other, outside the coil the fields from each side of the coil are in opposite directions
  • the fields partly cancel each other out, so the field is weaker outside the coil
• what is the right hand grip rule?

  Relationship between current and magnetic field
• how to use the right hand grip rule?

  • using right hand
  • point your thumb in direction of current
  • curl your fingers
  • your fingers show the direction of the field
• what is Fleming's left-hand rule?

  shows the relationship between the directions of current, force and magnetic field (where they are mutually perpendicular)
• how does a force on a conductor in a magnetic field is used to cause rotation in DC motors?

  • when a current flows through a coil it creates a magnetic field
  • the magnetic field interacts with the motor's magnetic field, causing a force
  • there is an equal and opposite force on each side of the coil as current is flowing in opposite directions
  • the force causes the motor to rotate
  • if the current doesnt reverse, then the coil would experience a force in the opposite direction and stop
  • to stop this, a split ring commutator changes the direction of current every half turn
• how to make the motor spin in the opposite direction?

  • reverse the direction of current
  • reverse the direction of the magnetic field lines
• how to make the motor spin faster?

  • increase the current
  • increase the magnetic flux density
  • increase the number of turns on the coil