magnetism

• what are magnets?

  any material or object that produces a magnetic field
• examples of magnets?

  earth

  bar magnet

  horse-shoe magnet
• what happens when the current is switched off?

  the magnetic field disappears
• how are magnetic fields drawn?

  from north to south
• what happens when the field is more dense?

  the stronger the magnetic field in that area
• what are permanent magnets?

  produce their own magnetic field all the time
• what are induced/temporary magnets?


  only have magnetic field temporarily
• what is magnetic material

  any object that can be influenced by magnetic fields,and has the potential to become a magnet
• examples of magnetic material

  Nickel

  Cobalt

  Iron

  Steel

  (NCIS)
• what happens in electromagnetism?

  electric currents produce their own magnetic field

  direction depends on the direction of the current(e.g right hand rule)

  circular coils(add more turns)

  one piece of wire makes a solenoid
• 4 ways to increase electromagnet strength

  -increase the current that flows through the solenoid

  -increase number of turns(keep the same length)

  -decrease length(keeping the number of turns the same)

  -iron core-induced magnets
• fleming's left hand rule

  rule:

  
• what is the motor effect:

  a current carrying wire in the presence of the magnetic field will experience of a force.
• how to experience a full force in the motor effect?

  wire has to be 90°
• what happens if the wire is not 90°

  it will experience less force
• what happens if the wire is in the same direction?

  no force
• what is the generator effect?

  a pd is generated when a wire experiences a change in magnetic field
• what can a change in magnetic field do?

  can induce a current in a wire
• what does the induce current do?

  oppose the change that made it
• how to change the size of the induced pd

  -increasing the speed of movement-cutting more magnetic fields in given time

  -increasing the strength of the magnetic field
• how the electric motor works?

  1)basic dc motor-forces act on the two side arms of a coil of wire that is carrying a current.

  2)the coil is on an axle and the forces act one up and one down,it rotates

  3)split-ring commutator:swapping the contacts every half turn to keep the motor rotating in the same direction.

  4)the direction of the motor can be reversed either by swapping the polarity of the dc supply or swapping the magnetic poles over.
• how to increase the speed rotation an electric motor?

  -increase turns

  -increase magnetic flux density

  -increase the current
• what are soft materials?

  gain and lose magnetism quickly
• what are hard materials?

  gain or lose magnetism slowly are called magnetically hard materials
• how loudspeakers and headphones use electromagnets?

  1)ac sent through a coil wire(attached to the base of a paper cone)

  2)coil surrounds one pole of a permanent magnet,surrounded by the other pole(causes a force)

  3)current reverses,force acts in opposite directions

  4)variations in current,make the cone vibrate.
• what does a variation in pressure cause?

  a sound wave
• how dynamos generate direct current?

  1)dynamos work in the same way as alternators work

  2)they have split-ring commutators instead of slip rings

  3)which swap the connection every half turn,to keep the current following in the same direction
• how alternators generate alternating current?

  1)alternators rotate a coil in a magnetic field.

  2)as coil/magnet spins,a current is induced in the coil

  -current changes direction every half turn

  3)instead of a split ring commutator,alternators have slip ring and brushes,so the contacts swap every half turn.

  4)producing an alternating pd
• how oscillopes work?

  -show how the pd generates in the coil change over time

  -ac goes up and down,crossing horizontal axis

  -dc isn't straight,still direct current.

  -increasing the frequency of revolutions increase overall pd,creates more peaks
• how microphones work?

  -sound waves have a flexible diaphragm,attached to a coil wire wrapped around a magnet

  -causes the coil of wire to move in the magnetic field,generates current

  -movement of the coil,depends on the sound wave(louder sounds,makes the diaphragm move further)

  -microphones can convert the pressure variations of a sound wave into variations in current and electric current.
• what do transformers have?

  -iron core

  -primary coil+secondary coil

  -magnetic field
• transformers

  -change the pd(only for alternating current)

  -transformers have 2 coils of wire,primary and secondary joined with an iron core

  -when the alternating pd is applied across primary coil.

  -the iron core magnetises and demagnetises

  -if the second coil is complete circuit,the current becomes induced
• what is the ratio of primary coil vs secondary coil?(in terms of pd)

  the ratio of primary pd :secondary pd
• what is the ratio of primary coil vs secondary coil(in terms of number of turns)??


  primary coil:secondary coil
• what is a step down transformer?

  pd decreases

  more primary coils than secondary

  current increases
• what is step up transformer?

  pd increases

  have more secondary coils than primary

  current decrease