Magnetism

• Magnets


  Any material or object that produces a magnetic field

  have a north and south poles

  field lines represent magnetic field
• Field lines


  Always north to south

  need arrows to show direction

  the closer they are the stronger the magnetic field
• How to find poles

  Use a compass at end of magnets and draw arrow based on the direction of compass

  move it around magnet to get field lines
• Magnets attraction/repulsion


  To same poles = repulsion

  to opposite poles = attraction
• Magnet vs magnetic material

  Magnets produce a magnetic field

  while

  magnetic materials are influenced by magnetic field
• Magnetic materials 


  Cobalt

  iron

  nickel

  steel
• Permanent magnets


  Produce their own magnetic field all the time
• Induced magnets


  Only have a magnetic field temporarily

  made when a magnetic material is placed in a magnetic field
• Electromagnetism


  When an electric current produces its own magnetic field

  direction of magnetic field depends on current

  right hand rule: thumb points direction of current then the direction you fingers curl in is the direction of magnetic field
• Solenoid 


  Magnetic field inside is strong and uniform

  doesn’t have a soft iron core

  to increase strength: increase current, increase turns of coil, decrease length and add an Iron core (becomes induced magnet)
• Motor effect


  A current carrying wire in presence of a magnetic field will experience a force
• Flemings left hand rule

  Thumb = movement

  first finger = poles (field)

  second finger = current

  helps find direction of force (thumb)
• Calc strength of force

  F = BIL

  force =magnetic flux density x current x length of wire

  as long as wire is at a right angle
• Electric motors 


  Current direction changes every half turn so coil will continue to rotate in same direction by useing a split ring commutator (swaps positive and negative connections every half turn) = coil rotates 360 degrees
• How to speed up rotations of motor

  Increase current

  add more turns of coil

  use more powerful magnets (increase flux density)
• Generator effect/electromagnetic induction


  When a piece of wire is moved (up +down) between two magnets (magnetic field) it induces a potential difference in wire

  when wire stops moving potential difference disappears

  when wire changes direction so does the potential difference

  when it’s a circuit the induced potential difference would produce a current
• Changing size of induced PD

  -Change strength of magnetic field w stronger magnets = larger pd

  -move wire faster so faster the field will change = larger pd

  -shape wire into proper coil w more turns = larger of
• Alternators + dynamos


  Are generators that generate electricity from rotational motion

  key difference: dynamo has split ring commutator so make direct current but alternators has slip rings and brushes produce alternating current
• Alternators


  1-coil of wire rotates relative to magnets; induces a magnetic field which induces voltage and current in coil

  2-the slip rings and brushes means contact won’t swap at half turns

  3-this means alternating current and p.d is produced

  4-as coil rotates faster the peaks of oscillation get larger and more frequent (on oscilloscope)
• Dynamos


  1-as coil of wire spins relative to magnets a magnetic field and voltage and current is induced in coil

  2-the split ring commutator means current swaps every half turn

  3-means direct o.d and current are produced so it’s always positive on the oscilloscope

  4-as coil rotates faster the peaks of oscillations gets larger and more frequent
• Loudspeakers + microphones


  Both convert between sound waves and electrical signals
• Loudspeakers


  Take electrical signals and use motor effect to convert them into vibrations and then generate sound waves

  -consists of a coil wrapped around a pole of a permanent magnet and connected to a cone but coil + cone can slide along magnet
• How do loudspeakers work

  -An alternating current flows and creates a magnetic field in coil

  -the magnetic field interacts with field of permanent magnet

  -this will exert a force on the coil causing it to move back and fourth along magnet

  -causes cone to change shape

  -as current is alternating the magnetic field and force will also alternate meaning coil and cone move rapidly back and fourth so vibrate

  -these vibrations are so fast it causes pressure vibrations in the air = sound waves
• Microphones


  Detect vibrations caused by sound waves use generator effect to convert them into electrical signals

  -sound waves hit diaphragm

  -diaphragm and coil wire move

  -as wire is moving in the magnetic field of permanent magnet generating a current

  -the frequency and amplitude of sound waves will determine how much the diaphragm vibrates so determines frequency and amplitude of current
• Transformers


  Used in national grid as step up/down transformers to minimise energy loss and keep it safe

  increases or decreases voltage of electricity
• Transformers how they work:

  Iron core with primary coil in one side and secondary coil on other

  primary coil has an alternating p.d flowing through, this causes current to flow and a magnetic field around coil (also alternating)

  thus induces an alternating magnetic field in iron core as it’s a magnetic material

  Secondary coil induced a p.d and causes current to flow through that coil
• changing size of p.d

  In step up transformers: secondary coil has 2x turns of coil than the primary so voltage is doubled

  in step down transformers: secondary coil has half the turns of coil than the primary so voltage halves
• Transformer calc


  Vp divided by Vs = Np divided by Ns

  VpIp = VsIs

  V = potential difference

  n = turns of coil

  I = current

  s = secondary coil

  p = primary coil