Magnetism and Electromagnetism

Magnetism and Electromagnetism

1. Magnetic Fields

   • Magnets produce magnetic fields
     • North and South
     • Magnetic field is where magnetic material experiences a force
     • Magnetic field lines
       • to show size and direction of magnetic field
       • always North ---> South
   • Magnetism can be induced
     • Magnets a affect magnetic material of other magnets
     • Both pole attract magnetic material (non magnets)
       • The material then acts as a magnet- has been 'induced'
     • Examples:

2. Electomagnetism

   • Current carrying wire creates a magnetic field
     • larger the electrical current the stonger the field
     • Magnetic field around a straight wire
       • field is made up of concentric circles
       • Right hand rule
     • Magnetic field around a flat circular coil
       • magnetic field similar to a bar magnet
     • Magnetic field around a straight line
       • made up of concentric circles
       • right hand rule
     • Magnetic field around a flat circular coil
       • similar to a bar magnet
       • stretched circles around the coil
   • Magnetic field around a solenoid
     • uniform and strong
     • field like a bar magnet
       • ends act like north and south pole- ELECTROMAGNET SWAG
   • 'Hard or Soft'
     • Soft- loses induced magnetism quickly (eg iron)
       • used in transformers
       • Increase strength of magnetic field around solenoid by adding 'soft' iron core
     • Hard- keeps magnetism permanently (eg steel)

3. Motor Effect

   • charged particl;es moving through a magnetic field will experience a force
     • Force acts in same direction relative to magnetic field of magnets and direction of current
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4. Electric Motors and Loudspeakers

   • 4 factors
     • More CURRENT
     • More COILS
     • STRONGER MAGNETIC FIELD
     • SOFT IRON CORE
   • Split ring communtator
     • swapping contacts every half turn- keeps rotating in the same direction
       • direction can be reversed by swapping polarity of d.c. supply of swapping magnetic poles
         • left hand rule
   • Loudspeakers work with motor effect
     • a.c. signal from amp fed to coil in speaker
       • coil surrounded by permanent magnet
         • a.c. signals cause coil to move back and forth
           • movements make cone vibrate and create sound

5. Electromagnetic Induction

   • Definition: the creation of voltage (maybe current) in a which which is experiencing a change in magnetic field
   • Dynamo Effect
     • Uses electromagnetic induction to transform kinetic energy into electrical energy
       • Get an EM induction in:
         • an electrical conductor moves through a magnetic field
         • magnetic field through and electrical conductor changes
     • Test by
       • connecting ammeter to conductor and moving conductor through a magnetic field
         • ammeter will show magnitude and direction of current
           • direction of movement reversed- voltage/current reversed too
     • Get a bigger voltage by...
       • increasing strength of magnet
       • increasing no. of coils
       • increasing speed of movement
   • A.C Generators
     • rotate a coil in a magnetic field
     • kinda like a motor
       • coil spins, current induced in coil
         • current changes direction every half turn
           • has slip ring brushes, so contacts don't swap every half turn
             • produce a.c. current
               • used in power stations

6. Transformers

   • Change voltage- but only alternating voltage
     • 2 coils, primary and secondary joined with iron core
       • Alternating voltage applied across primary coil- 'soft' iron core magnetises and demagnetises quickly
         • induces alternating current in secondary coil
       • ratio of pri and sec voltage = ratio as turns on pri and sec coil
       • STEP UP TRANSFORMERS step voltage up.
         • more turns on secondary coil than primary
       • STEP DOWN TRANSFORMERS step voltage down
         • more turns on primary coil than secondary coil
   • Input (primary) voltage / Output secondary voltage = Number of turns on primary/ number of turns on secondary
     • Power = voltage x current
     • V(p) I(p) = V(s) I(s)

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