pmt notes - magnetism and electromagnetism

• North and South Poles

  Opposite poles attract, same poles repel
• Permanent Magnets

  • Always magnetic, always have poles
• Induced Magnets


  • Materials that are "magnetic" but do not have fixed poles
  • Can be made into temporary magnets by 'stroking' them with a permanent magnet
• Magnetic Fields

  • Field Lines point from North to South
  • Strength decreases with distance from the magnet
  • Direction always points to south pole and away from north pole, at any point
• Plotting Compasses are small compasses which show the direction of the magnetic field at a certain point
• Earth's Core


  • The core is magnetic, and creates a large magnetic field around the Earth
  • A freely suspended magnetic compass will align itself with the earth's field lines and point North
  • The compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth's 'North pole'
  • However, Earth's magnetic pole above Canada is actually a magnetic South Pole
• Current


  • Current produces a magnetic field around the wire
  • The direction is dictated by the "right hand grip rule"

  
• Greater current


  Stronger magnetic field
• Greater distance from wire


  Weaker magnetic field
• Solenoid


  • Magnetic field shape is similar to a bar magnet
  • It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current
  • Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air
  • Factors that affect the strength: size of current, length, cross sectional area, number of turns (coils), using a soft iron core
• Motor Effect

  • Two magnets will interact, feeling a magnetic force of attraction/repulsion
  • A magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and the current in the wire) will also interact
• Fleming's Left Hand Rule

  • Each direction is 90° to each other
  • Use this to work out the unknown factor out of the three (usually the direction of the force felt)
  • Remember current is conventional current, which moves in opposite direction to the electrons

  
• Magnetic Flux Density is measured in Tesla, and it is the number of flux lines per metre squared
• How Electric Motors work

  • Permanent Magnets lie in fixed positions
  • In between, a coil of current-carrying wire lies on an axis
  • Force on one side moves that side up, force on the other side (where current is flowing in opposite direction) moves down
  • Hence it rotates
• Electromagnetic Induction


  • When there is a relative movement between a conductor and a magnetic field, a potential difference is induced across the conductor
  • This happens if the magnetic field changes as well
  • A current flows if the conductor forms a complete circuit
  • This current will produce its own magnetic field, which oppose the change inducing it
• How Electric Generators (dynamos) work

  • Same setup as a motor, with a coil of wire able to rotate between two permanent magnets
  • A turbine spins turning the coil of wire
  • The movement of the wire causes the wire to cut through the magnetic field
  • It experiences a change in magnetic field
  • This creates a potential difference
  • If the coil of wire is connected to a complete circuit, an alternating current (AC) will flow
  • Direct current (DC) current is produced if the ends are connected to a split ring commutator, which reverses the current each half-rotation so current remains positive
• AC is produced by an Alternator, DC is produced by a Dynamo
• Transformers

  • AC in first coil creates a changing magnetic field
  • This changing magnetic field cuts through the secondary coil
  • This induces a current in the secondary coil
  • More coils on secondary: Step up transformer, as voltage will be increased
  • Fewer coils on secondary: Step down transformer, as smaller pd forms on secondary
• How Dynamic Microphones Work

  • Fixed magnet is at the centre, and the coil of wire around the magnet is free to move
  • Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field)
  • This current is then sent to a loudspeaker
• How Loudspeakers Work


  • The setup is identical to a dynamic microphone, working in reverse
  • The current flows into the coil
  • The magnetic field from magnet and from current interact, causing the coil to move
  • The cone therefore moves, producing pressure variations and making sound
• What property does matter often have that allows it to attract or repel other matter?

  Magnetism
• What are the two poles of a magnet called?

  North and south
• What does magnetism depend on?

  Electron movement and spin affecting neutrons
• What materials are permanent magnets made of?

  Ferromagnetic materials
• What are the two main types of magnets?

  • Permanent magnets
  • Electromagnets
• How do induced magnets behave when removed from a permanent magnet?

  They lose their magnetism
• What happens when unlike poles of magnets are brought close together?

  They attract each other
• What happens when like poles of magnets are brought close together?

  They repel each other
• What is necessary for an induced magnet to work correctly?

  Magnets need to be aligned correctly
• What happens to materials when no magnetic field is involved?

  They have no magnetic properties
• What do magnetic field lines represent?

  The strength and direction of a magnet's field
• How does the density of a magnet's field lines vary?

  It is variable
• How can the strength of a magnetic field change?

  By changing the shape of the magnetic material
• What does it indicate if magnetic field lines are close together?

  The strength of the field is strong
• What does it indicate if magnetic field lines are far apart?

  The strength of the field is weak
• Where is the magnetic field strongest?

  At the poles
• What kind of field does a bar magnet create?

  A magnetic field
• How do magnetic field lines behave around a magnet?

  They are stronger at the poles
• What happens to magnetic field lines as they get farther apart?

  They indicate a weaker field
• What exists as an experienced force between magnets' opposite poles?

  An attraction force