Topic 7 – Magnetism

• Permanent Magnet


  Always has a magnetic field
• Induced Magnet


  Easily magnetised when in a magnetic field
• Bar Magnet


  • More lines = Stronger Field
  • Always points from N → S

  
• Materials that can be magnetised

  • Ni
  • Co
  • Fe
• Loses magnetism if moved away from field
• Earth's core (Iron) = Giant Magnetic field
• Compasses point to Geographic North pole


  Magnetic South pole

  
• Moving compass around bar magnet can show you which pole is North/South

  
• Passing a current through a wire to create a Magnetic Field

  1. STRAIGHT WIRE

  2. SOLENOID (Coiled wire)

  
• Magnetic Field from current-carrying wire

  • Concentric circles
  • Field gets stronger closer to wire
  • Similar to bar magnet
  • Current IN (+) = North pole
  • Current OUT (-) = South pole
  • Field strongest inside coil (uniform)

  
• Increasing strength of magnetic field

  1. More Current

  2. More Coils

  3. Soft Iron core (induced magnet)
• Electromagnets


  Magnetic field can be turned ON/OFF by varying current
• Uses of Electromagnets


  • Scrapyards to pick up cars/metal
  • Act as switches in other circuits
• Motor effect


  When a current carrying wire inside a magnetic field experiences a force
• Orientation of wire in relation to magnetic field

  • Parallel
  • Angles in between
  • Perpendicular

  
• Left hand rule


  Work out direction of movement of wire
• D.C Electric Motor


  1. Coil is in a magnetic field

  2. Forces act on each arm of the coil

  3. Current is flowing in opposite directions in each arm

  4. Each arm feels opposite force, one goes up other goes down

  5. Split-ring commutator swaps the contacts every half turn

  6. Swaps the current in each arm to ensure motor turns in same direction
• Reversing a D.C motor


  Reverse Current or Magnetic Poles around coil
• Electromagnetic Induction


  Induction of a potential difference/current (if a complete circuit) in a wire that is experiencing a changing magnetic field
• Examples of Electromagnetic Induction


  • Moving a magnet through a coil
  • Spinning a magnet inside a coil
• Electromagnetic Induction Process


  1. Moving/spinning magnet creates a changing magnetic field

  2. Moves the electrons inside the coil → p.d

  3. If the coil is connect at both end (complete circuit) → current

  4. If magnet stops moving at ANY point = No p.d/current
• Reversing direction of p.d/current in electromagnetic induction

  • By changing direction/orientation of magnet
  • Every half turn of spin the magnetic field flips
• Increasing size of current in electromagnetic induction

  1. More Coils

  2. Stronger Magnet

  3. Move Magnet Faster
• Alternators (A.C)


  Rotate a coil inside a magnetic field to induce a current
• Dynamos (D.C)


  Rotate a coil inside a magnetic field to induce a current
• Alternating Current Generation


  1. Coil is attached to a turbine e.g. wind

  2. Coil rotates inside M.Field and experiences a "changing M.Field"

  3. Current is induced inside coil

  4. Current swaps direction every half turn

  5. Slip ring and brushes ensure contacts don't swap
• Direct Current Generation


  1. Coil is attached to a turbine e.g. wind

  2. Coil rotates inside M.Field and experiences a "changing M.Field"

  3. Current is induced inside coil

  4. Split ring commutator swaps contacts every half turn
• Increasing generator speed


  Larger p.d and more frequent peaks
• Loudspeakers


  Use Electromagnetism (Motor Effect)
• Microphones


  Use Electromagnetic Induction (Generator Effect)
• Loudspeaker Operation


  1. A.C travels into coil at base of cone

  2. Creates magnetic field in coil

  3. M.field of coil interacts with circular magnet

  4. Coil experiences a force (Motor Effect)

  5. Moves paper cone ! vibrations = sound

  6. Freq of A.C = Freq of vibration
• Microphone Operation


  1. Sound vibrations hit diaphragm

  2. Coil begins to move inside M.Field

  3. Induces a current in the coil via EMI

  4. Freq of sound = Freq of A.C produced
• Generators


  Will Increase/Decrease Voltage (only works for A.C) so NOT batteries (D.C)
• Transformer


  • Iron core = easily magnetised material
  • Alternating p.d in primary coil creates alternating magnetic field in iron core
  • Alternating magnetic field passes through secondary coil
  • Induces a p.d in secondary coil
• Transformer Voltage Ratio


  𝑉!

  𝑉"

  = 𝑛!

  𝑛"
• Power (Energy) is same in first coil and second coil