magnetism

• A magnetic field is a region in space where a magnetic force can be detected.
• Electric currents create magnetic fields that surround wires carrying them.
• Magnetic fields are created by moving charges or changing electric currents.
• The strength of the magnetic field around a bar magnet decreases as we move away from it, but its direction remains constant.
• A current carrying wire can be used as an electromagnet by wrapping it around a core made from iron or steel, which increases its strength.
• The distance between the magnetic field lines shows the strength of the magnets.
• The direction of the magnetic field around a wire depends on the direction of the electric current flowing through it.
• 3cs:

  Core - needs to be made of a magnetic material


  Coil - number of turns

  Current - more current=stronger magnet
• a magnetic field is produced around a wire when a current is passed through
• electricity + magnetism = movement

  movement +magnetism = electricity
• Electromagnetic Induction - the induction of electricity from magnetism and motion
• if a wire 'cuts' through magnetic field lines, a voltage is induced across the end of the wire
• if the wire is part of a complete circuit, a current is induced IN the wire.
• If the wire moves PARALLEL to the field lines 0 PD is induced.
• Increasing the size of the induced current:

  • move the wire or the magnets faster
  • increase the magnetic field strength
  • coil the wire more so it has more turns
• AC - Alternating current
• AC can go both ways, the direction of the current reverses when the coil turns.
• Step Up Transformer - If there are more turns on the SECONDARY coil , then the output voltage is higher than the input voltage. Increases p.d.
• Step down Transformer - If there are FEWER turns on the SECONDARY coil, then the output voltage is less than the input voltage.
• Transformer - a circuit component that uses two coils and a core which changes the p.d.
• Vs/Vp = Ns/Np
• 3 main parts of a transformer: iron core, primary coil, and secondary coil.
• Transformer:

  if you have a change in the magnetic field, then the wire wrapped around will induce a current.
• Transformer:

  insulated wire so that electricity can't flow from wire to the core.
• Transformer:

  core has to be magnetic
• Why do we use AC current for transformers?

  we need the magnetic field to change to induce a current in the wire wrapped around the core. AC (alternating current) reverses the direction of the flowing current, and therefore the magnetic field changes because of Fleming's Left Hand Rule and can induce a current.
• Speakers:

  1. The coil is in a magnetic field, so the coil will move back and forth at the input motor effect.
  2. the coil is attatched to a core, so the core moves bac and forth too.
  3. the motion of the core causes compressions and sound waves to be produced
  4. the p.d. produces an alternating current in the coil
• Microphones

  1. the pressure from the sound waves causes the diaphragm to move
  2. the coil moves in relation to the magnetic field, inducing a current through the wire
  3. Inside the coil is a permanent magnet so the motion of the coil in the field induces a p.d. in the coil
• Electromagnetism - a current flowing in a wire causes a magnetic field.
• Electromagnetic Induction - a wire moving in a magnetic field induces a voltage
• Lenz's Law - "induced current opposes motion"
• Generators

  • the coil is turned in a magnetic field
  • a p.d. is induced in the coil
  • the coil is connected to a circuit and an electric current flows
• Dynamos

  • use a split ring commutator to produce a dc (direct current)
  • every 180 degrees, the commutator reverses the direction of the p.d. so the current never goes reverse
• Alternators


  They use slip rings and brushes to generate ac (alternating current). each ring is connected to a different side of the coil. Is always in contact with the same side of the circuit.