Magnetism and Electromagnetism

• Magnets: A magnet is a material that can attract or repel other magnets.
• Like poles repel, and opposite poles attract.
• Permanent Magnets are made from materials that have a permanent magnetism. Always magnetic, always have poles.
• Induced Magnets: A magnet that is induced to have a magnetic field by a current. They do not have fixed poles.
• Iron, Nickel and Cobalt are all materials that can create a temporary magnet.
• Magnetic Fields:

  • Field lines point from North to South
  • Strength decreases with distance from the magnet
  • Direction always points to south pole and away from the north pole, at any point
• Plotting Compasses are small compasses which shows the direction of the magnetic field at a certain point.
• Earth's Core:

  • The core is magnetic, and creates a large magnetic field around the Earth.
  • We know this because a freely suspended magnetic compass will align itself with earth's fields lines and point North.
  • It doesn't point to the Geographic North pole.
  • The compass is effectively a suspended bar magnet, with its own north pole lining up with Earth's North pole.
• Current: The flow of electric charge through a conductor.

  • Current produces a magnetic field around the wire.
  • The direction is dictated by the right had grip rule
  • Plotting compasses on a piece of paper through which a wire pierced shows this.
• Right Hand Grip Rule: thumb is pointing to the right of the object (the direction of the current), index finger is pointing to the left of the object (the direction of the field).
• Strength of Magnetic Field:

  • Greater current, stronger magnetic field.
  • Greater distance from wire, weaker field.
• Solenoid: A coil of wire that produces a magnetic field around it. It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current. Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air.
• Factors that affect the strength:

  • Size of current
  • Length
  • Cross sectional area
  • Number of turns/coils
  • Using a soft iron core
• The Motor Effect: Two magnets will interact, feeling a magnetic force of attraction/repulsion. So a magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and the current in the wire) will also interact.

  • The magnetic field around a wire is circular, but the magnetic field between two magnets is straight.
  • When the two interact, the wire is pushed away from the field between the poles (at right angles to the wire direction and the field direction).
• Fleming's Left Hand Rule: thumb = force, index finger = field, middle finger = current.
• Force = (magnetic flux density) x (current) x (length)
• Where magnetic flux density is measured in Tesla and it is the number of flux lines per metre squared.
• How Electric Motors Work:

  • Permanent magnets lie in fixed positions
  • In between, a coil a current-carrying wire lies on an axis.
  • Force on one side moves that side up
  • Force on the other side (where current is flowing in opposite direction) moves down.
  • Hence it rotates.
• Electromagnetic Induction:

  • When there is a relative movement between a conductor and a magnetic field, a potential difference is induced across the conductor.
  • This happens if the magnetic field changes as well.
  • A current flows if the conductor forms a complete circuit.
  • This current will produce its own magnetic field, which oppose the change inducing it.
• How Electric Generators (Dynamos) work:

  • Same setup as a motor, with a coil of wire able to rotate between two permanent magnets.
  • A turbine spins turning the coil of wire.
  • The movement of the wire causes the wire to cut through the magnetic field.
  • It experiences a change in magnetic field.
  • This creates a potential difference.
  • If the coil of wire is connected to a complete circuit, an alternating current will flow.