7-Magnetism and electromagnetism

• magnets

  • magnets are object that produces a magnetic field
  • all magnets have a north and south pole
  • ISNO - into the south, out of the north
  • magnetic fields are represented by magnetic lines - must go from north to south, must have arrows on the lines
  • the more magnetic lines the stronger the magnetic field in that area
  • magnetic field is stronger nearer the poles

  
• magnets interacting

  • if you push two opposite poles of different magnets then they will repel
  • if you push two same poles of different magnets then they will attract

  A) attract

  
• magnetic material

  • any object or material that can be influenced by a magnetic field and has the potential to become a magnet
  • this includes nickel, cobalt, iron as well as their alloys such as steel
• induced magnets

  • they only have a magnetic field temporarily
  • the force between a permanent and induced magnet will always be attractive
  • magnetically soft - lose magnetism quickly (iron)
  • magnetically hard - lose magnetism slowly (steel)
• how induced magnets are induced

  1. it happens when a piece of magnetic material is put into a permanent magnet's magnetic field
  2. magnetic material will develop it's own magnetic field with poles
  3. when the magnet is removed from the magnetic field it will lose it's magnetism
• electromagnetism

  • electric currents always produce their own magnetic field
  • such as in wires, coils, solenoids, electromagnets
• right hand rule

  • use your right hand to point your thumb in the direction of the current
  • the way the rest of your fingers curve is the direction of the magnetic field lines

  A) current

  B) electromagnetic field

  
• wires

  • the magnetic field lines will be closer together nearer the wire as it's where the magnetic field is strongest

  
• coils

  • as the magnetic field of the two sides of the coil interact the magnetic field lines will stretch out and form elipses
  • as the magnetic fields combine they'll form a single magnetic field running through the centre of the coil

  
• solenoid

  • formed of lots of coils and is an electromagnet
  • the magnetic field within a solenoid is strong and uniform
  • it is like a bar magnet and has poles
  • pros - only magnetic if current goes through it, the direction of the magnetic field can be reversed by reversing the direction of current flowing which will also reverse the poles

  
• increasing strength of electromagnet (solenoid)

  • increase the current
  • increase the number of coils/turns - keep length the same so coils are densely packed
  • decrease the length of the coil - keep the number of coils the same so coils are densely packed
  • add an iron core - it's a soft magnetic material so will become induced, quickly lose magnetic field
• motor effect

  • the motor effect is that a current carrying wire in the presence of a magnetic field will experience a force
• finding the direction of force - motor effect

  • if a current carrying wire is placed between the north and south poles of two magnets, the two magnetic fields will interact which results in a force on the wire
  • to experience the full force the wire must be at exactly 90 degrees (perpendicular)
  • use Fleming's left hand rule to find force

  
• Fleming's left hand rule - finding force

  A) force

  B) magnetic field

  C) current

  
• calculating strength of force - motor effect

  • force = magnetic field strength x current x length
  • F = B x I x L
  • units:
  • force - newtons (N)
  • magnetic field strength - teslas (T)
  • current - amps (A)
  • length - metres (m)
• electric motors

  1. a coil would have current flowing in at a positive terminal and coming out of the negative terminal
  2. two sides of the coil will experience different forces (upwards and downwards) which will cause the coil to spin
  3. once the coil spins 180 degrees, the current is travelling the opposite direction which swaps the sides of the forces, this is repeated
  4. this would be an unuseful motor as it wouldn't spin fully

  
• electric motors - making them useful

  • change the direction of the current every half turn so that the coil continues to rotate in the same direction

  1. use a split-ring commutator - it swaps the negative and positive connections every half turn
  2. the direction of the current will also swap every half turn
  3. the forces acting on the coil will always be acting in the same direction
• increase speed of rotations - electric motor

  • increase the current
  • add more turns to the coil
  • increase the magnetic flux density - using more powerful magnets