magnets

• Magnets


  Have a north and south pole. Two like poles repel, two opposite poles attract.
• Permanent magnets


  • Always magnetic and always have poles
• Induced magnets


  • Can be produced from materials that are magnetic but do not have fixed poles. Can be made into temporary magnets by 'stroking' them with a permanent magnet. Iron, Nickel and Cobalt are common induced magnetic materials.
• Magnetic fields


  Shown using field lines that point from north to south. Strength of the field decreases as distance from the magnet increases. The greater the concentration of field lines in an area, the greater the strength of the field in that area.
• Plotting magnetic fields


  Small plotting compasses can be used to plot the shape of a magnetic field around a small magnetised object.
• Earth's core


  • Magnetic and currents within it create a large magnetic field around the Earth. A freely suspended magnetic compass will align itself with the earth's field lines and point North, but the magnetic north pole is over North Canada.
• Current in a wire

  Produces a magnetic field around the wire. The direction of this field is dictated by the 'right hand grip rule'.
• Current in a wire

  The greater the current flowing, the stronger the magnetic field. The greater the distance from the wire, the weaker the magnetic field becomes.
• Solenoid


  • Formed from a coil of wire with current flowing through. The shape of the resulting magnetic field is similar to that of a bar magnet. Inserting an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air.
• Factors affecting strength of solenoid magnetic field

  • Size of current
  • Length of coil
  • Cross sectional area
  • Number of turns (coils)
  • Type of core used
• Magnetic force


  Two magnets will interact, feeling a magnetic force of attraction or repulsion. 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.
• Fleming's left-hand rule


  Used to determine the direction of the force on a current-carrying wire in a magnetic field. Each component (force, field, current) is at 90° to one another.
• Electric motors


  Use permanent magnets in fixed positions and a coil of current-carrying wire that rotates between them. The induced force on one side moves upwards and the induced force of the opposite side moves downwards, causing rotation.
• Electromagnetic induction


  A conductor (such as a wire) forms a potential difference as electrons within it move to one side as the field changes. If the conductor is connected to a circuit, a current will flow. This current will produce its own magnetic field in the opposite direction to the first field.
• Electric generators (dynamos)


  Use a coil of wire able to rotate between two permanent magnets. The movement of the wire causes it to cut through the magnetic field, generating a potential difference. If the coil is connected to a complete circuit, an alternating current will flow.
• Alternating current


  Current that is constantly changing direction, therefore the magnetic field it produces also constantly changes.
• Transformers


  The alternating current in the primary coil creates a changing magnetic field. This changing magnetic field cuts through the secondary coil, inducing a current in the secondary coil which is also alternating.
• Dynamic microphones


  Produce a current which is proportional to the sound signal. A fixed magnet is at the centre, and the coil of wire around the magnet is free to move. Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil as it cuts the magnetic field.