magnetism

• Magnets

  Have two poles - north (or north seeking) and south (or south seeking)
• Magnets

  • Produce a magnetic field - a region where other magnets or magnetic materials experience a force
• Magnetic field is a non-contact force, similar to the force on changes in an electric field
• Showing a magnetic field

  1. Drawing magnetic field lines

  2. Lines go from north to south

  3. Show the direction of force on a north pole
• Magnetic field

  • Stronger when field lines are closer together
  • Weaker the further away from the magnet
• Magnetic field is strongest at the poles of a magnet
• The force between a magnet and a magnetic material is always attractive, no matter the pole
• When the two poles of a magnet are put near each other

  They will exert a force on each other, which can be attractive or repulsive
• Like poles


  • Repel each other
• Unlike poles


  • Attract each other
• Compasses show the direction of magnetic fields

  1. Compass needle points in the direction of the magnetic field it is in

  2. You can move a compass around a magnet and trace the needle's position to build up a picture of the magnetic field
• Compass needles always point north when not near a magnet, because the Earth generates its own magnetic field
• Permanent magnets


  Produce their own magnetic field
• Induced magnets


  Magnetic materials that turn into a magnet when put into a magnetic field
• The force between permanent and induced magnets is always attractive
• Induced magnets quickly lose most or all of their magnetism when the magnetic field is removed
• Solenoid


  • A coil of wire that can increase the strength of the magnetic field produced by a current-carrying wire
• How a solenoid works

  1. Field lines around each loop of wire line up, resulting in a strong, uniform magnetic field inside the coil

  2. Putting an iron core inside the coil increases the field strength even more
• Electromagnet

  A magnet whose magnetic field can be turned on and off with an electric current
• Uses of electromagnets

  • Cranes to pick up magnetic materials
  • Switches in circuits
• Current-carrying wire in a magnetic field


  • Experiences a force due to the interaction of the magnetic fields
  • Force acts at right angles to both the magnetic field and the current direction
• Motor effect


  The force experienced by a current-carrying wire in a magnetic field, which can cause the wire to move
• Finding the size and direction of the force

  1. Force depends on magnetic flux density, current size, and length of conductor in field

  2. Use Fleming's left-hand rule to find the direction of the force
• Current-carrying coil in a magnetic field

  • Forces act on the sides of the coil, causing it to rotate
  • Split-ring commutator swaps contacts to keep the motor rotating in the same direction
• Direct current (DC) is current that only flows in one direction
• What are the 4 magnetic materials?

  Iron, nickel, cobalt, steel
• The generator effect


  The induction of a potential difference (and current if there's a complete circuit) in a wire which is experiencing a change in magnetic field
• Inducing a potential difference

  1. Moving a magnet in a coil of wire

  2. Moving a conductor (wire) in a magnetic field ("cutting" magnetic field lines)
• Shifting the magnet from side to side creates a little "blip" of current in the conductor if it's part of a complete circuit (the current can be shown on an ammeter in the circuit)
• If you move the magnet (or conductor) in the opposite direction, then the potential difference/current will be reversed. Likewise if the polarity of the magnet is reversed, then the potential difference/current will be reversed too
• If you keep the magnet (or the coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction, which produces an alternating current
• Rotation can also cause the generator effect


  Turning a magnet end to end in a coil, or turning a coil inside a magnetic field
• How generators work to produce alternating current (ac) or direct current (dc)


  1. As the coil (or magnet) spins, a change in the magnetic field through the coil induces a potential difference, which can make a current flow in the wire

  2. Every time the magnet moves through half a turn, the direction of the magnetic field through the coil reverses. When this happens, the potential difference reverses, so the current flows in the opposite direction around the coil of wire

  3. If you keep turning the magnet in the same direction, the potential difference will keep on reversing every half turn and you'll get an alternating current
• Alternators


  • Rotate a coil in a magnetic field (or a magnet in a coil)
  • Have slip rings and brushes so the contacts don't swap every half turn, producing an alternating potential difference (pd)
• Dynamos

  • Have a split-ring commutator instead of slip rings, swapping the connection every half turn to keep the current flowing in the same direction
• The induced current always opposes the change that made it
• Induced potential difference (and so induced current)

  Can be increased by increasing the speed of the movement (cutting more magnetic field lines in a given time) or increasing the strength of the magnetic field
• Oscilloscope


  Shows how the potential difference generated in the coil changes over time
• For ac, the oscilloscope trace goes up and down, crossing the horizontal axis. For dc, the line stays above the axis (the pd is always positive) so it's still direct current
• Increasing the frequency of revolutions increases the overall pd, but it also creates more peaks too