(electro)magnetism

• magnetism

  a non contact force

  magnetic fields always go north to south

  the field is strongest by the poles (when the lines are drawn closer together)
• permanent magnetics

  have their own magnetic field
• induced magnets

  materials that become magnetic when placed in a magnetic field so loose magnetism when removed from the field
• electromagnetism

  a moving charge creates a magnetic field

  right hand thumb rule: thumb shows direction of the current; fingers point in the direction of the field

  to strengthen the field:

  • have a larger current
  • be closer to the wire
• solenoid

  a coil of wire

  used to increase the strength of the field, by wrapping the wire into a coil (solenoid)

  they make the field lines line up with each other

  - many field lines going in the same direction, very close together make a stronger field
• the field inside the coil is strong and uniformed

  
• placing an iron core inside a solenoid increases field strength

  it becomes an induced magnet when there is current
• an electromagnet is a solenoid coil and an iron core
• uses of electromagnets

  • within circuits, they act as switches
  • in scarp yards, they can turn magnetism on and off, to move and deposit metal
• the motor effect


  a current carrying wire interacts with a magnetic field and they exert a force, which acts at 90º to the magnetic field, on each other, which causes the wire to move
• Flemmings lefts hand rule

  used with the motor effect

  the thumb shows motion (the force direction)

  the first fingers shows the magnetic field

  the second finger shows current
• split ring commutator


  swaps the current every half turn to keep the motor rotating in the same direction
• current flows positive to negative

  a current carrying coil of wire rotates in a magnetic field
• an ac is sent through the coil, which is wrapped around one pole of a permanent magnet

  the current causes a a force on the coil

  this causes the cone to move, creating vibrations in the air, making a sound wave

  the soundwave has the same frequency as the ac, so changing the frequency of the ac will change the frequency of sound

  
• generator effect

  the induction of a potential difference ( and current if there is a complete circuit) in a wire which is moving relative to a magnetic field or experiencing a change in magnetic field
• to create the generator effect

  move a magnet in a coil of wire

  OR move the wire in a magnetic field, which cuts the magnetic field lines
• to increase the induced current caused by the generator effect

  increase the speed of the movement - this cuts more lines in a given time

  increase the strength of the magnetic field
• microphones work because of the generator effect - like speakers but in reverse
• alternators

  have slip ring and brushes so produce an alternating current because the current does not changer each turn
• dyanmos

  have a split ring commutator so produce a direct current
• transformers

  change the potential difference and only work in an alternating current
• how transformers work

  when the ac is applied to the primary coil, the iron core magnetises and demagnetises quickly, inducing a alternating potential difference in the secondary coil

  If the secondary coil is part of a complete circuit, a current is induced
• step up transformer

  the secondary coil has more turns, which increases the potential difference
• step down transformer


  the primary coil has more turns, which decreases the potential difference