Magnetism

• A current carrying wire produces its own magnetic field around it, with the direction determined by Fleming's left hand rule (thumb points along the current, fingers curl round the wire).
• When two magnets are brought close together, they experience a force of attraction or repulsion depending on their poles being like or unlike.
• The strength of the magnetic field produced is proportional to the current.
• If two wires carry currents at right angles to one another, they produce separate fields that are perpendicular to both wires.
• When two parallel wires carry equal currents in opposite directions, their fields cancel out completely.
• Magnetic fields can be represented using lines of flux which show the direction and magnitude of the field at different points.
• Lines of flux always form closed loops and never cross one another.
• The number of lines of flux between any two parallel wires carrying equal currents is directly proportional to the distance between them.
• The SI unit of magnetic flux density is tesla (T) where 1 T = 1 N/Am.
• Where is magnetic flux most dense?

  Magnetic poles (where magnetic lines of force enter and leave the magnet)
• What is a magnetic field?

  Region in which a magnetic material is acted upon by a magnetic force
• The north geographic pole of the earth is a ____ magnetic pole

  south
• What are the characteristics of magnetic lines of force?

  -they possess direction

  -they always form complete loops

  -they represent tension along their length that tends to make them as short as possible

  -they repel each other

  -they cannot intersect or cross, but must always form individual loops

  -they always tend to arrange themselves in such a manner that the maximum number of lines of force are set up
• The ____ pole of the compass always tends to point towards the ____ pole of the magnet

  north, south
• Magnetic lines of force flow out of the north pole and into the south pole
• Magnetic lines of force can flow through any material. You cannot block the lines of force, but you can deflect them
• What deflects lines of force?

  Magnetic screens (can be made of soft iron because it has a low reluctance)
• What is reluctance?


  Property that opposes the setting up of flux lines. Materials with low reluctance tend to concentrate magnetic flux
• What is magnetism by induction?

  Magnetic lines of force align and concentrate within ferromagnetic material, causing poles to be established at the points where the lines of force enter and leave the ferromagnetic material
• Soft iron is easily magnetized but it loses its magnetism as the magnetic field is removed
• Hard iron or steel requires a much stronger magnetic field to be magnetized but when the magnetic field is removed, the iron material retains most of its magnetism
• What are the 3 common methods of producing artificial magnets?

  1. using electric current
  2. stroking with a magnet
  3. aligning with a magnetic field and tapping
• How to create magnet with current?

  Most effective way is using electric current. Anytime a current flows, a magnetic field is set up around the conductor. To increase strength, wind the conductor into a coil. Magnetic flux lines add to each other and produce a much denser magnetic field in the centre of the coil. If a piece of ferromagnetic material is placed in the coil and a current is passed through the coil in 1 direction, the magnetic field of the coil causes the domains to align in the ferromagnetic material
• What is the least effective way of making an artificial magnet?

  Aligning with magnetic field and tapping
• How can we demagnetize ferromagnetic materials?

  1. using electric current (reduce field strength slowly)
  2. using heat
  3. using vibration
• How to demagnetize using current?

  Use source of current that changes direction constantly and rapidly (AC)
• What is the Currie temperature?


  Temperature at which alignment of domains in a ferromagnetic material is lost even after it is cooled
• What is a keeper?

  Usually a soft piece of iron placed around the poles of the magnet to provide an easy path for magnetic flux (protects magnet)
• DC is used to magnetize, AC used to demagnetize
• Magnetic lines of force do not start or end, they simply exist
• Magnetic screens are made of material with a low reluctance
• The number of lines of force per unit area is inversely proportional to the distance from the conductor
• Loops of magnetic flux lines act in a flat surface at right angles to the conductor
• What is the left hand rule?(for conductor)

  Used to determine direction of flux lines around the current carrying conductor. Thumb points in direction of electron flow and fingers wrap around the conductor in the direction of the flux lines. Your thumb points in the direction of the north pole
• What is the dot and cross used for?

  Dot = current travelling out from the page to you

  cross = moving away from you
• If 2 current-carrying conductors are placed parallel to each other, they exert a force on each other. The direction of force will depend on the direction of current flow in one conductor with respect to the other
• Same-direction currents attract, opposite current repel
• The magnitude of force b/w parallel conductors is directly proportional to the product of the currents flowing in the conductors and inversely proportional to the distance between them
• What is a helix?

  A spiral-shaped coil (helically wound)
• What is a solenoid?

  A helically wound coil that produces a strong magnetic field