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 degrees 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
• 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 pd is induced in the coil (because it cuts the magnetic field), this then induces a current if the circuit is complete
  • 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
• Electromagnetic induction --> links to the generator effect

  when a conductor "cuts" the magnetic field lines of the magnet , it induces a potential difference . If there is a complete circuit, the induced potential difference will result in a current flowing
• The size of the current induced can be increased by

  • Increasing the speed at which the conductor is moved through the magnetic field, as more field lines are cut in a given time
  • Increasing the strength of the magnetic field , more field lines to be cut
  • Using a coil and increasing the number of turns and/or the surface area of the coil
• A split ring commutator

  • reverses the direction of the current around the coil every half turn.
  • Otherwise it will stop in a vertical position
  • So that the force is always acting in the same direction
  • Because the sides swap every half-turn the coil is always pushed in the same direction
• Dynamos


  A dynamo is very similar to an alternator, however it is used to induce a direct current

  It uses a split ring commutator to achieve direct current. This ensures that the connection swaps every half-turn and hence the current flows in the same direction
• Alternators 


  Alternators rotate a coil in a magnetic field

  As the coil spins, a current is induced in the coil. This current changes direction every half turn

  Instead of a split ring commutator alternators have slip rings and brushes so the contacts don't swap every half turn.

  This means they produce an alternating pd
• Transformers

  • An AC supply produces an alternating current in the primary coil
  • This creates a changing magnetic field in the primary coil, linked and amplified by the iron core
  • Which cuts through the secondary coil inducing an alternating pd
  • Which then induces an alternating potential difference in the secondary coil
  • Which therefore induces an alternating current (which has the same frequency as that supplied)
• In transformers, the size of the pd induced is affected by the number of turns in the primary coil and the secondary coil as well as the size of the pd induced across the primary coil
• secondary voltage/primary voltage = Number of turns on secondary/ Number of turns on primary