Unit 6

• Magnets


  • Produce Magnetic Fields
  • Have two poles - north and south
• Magnetic field


  A region where magnetic materials (e.g. iron) experience a force
• Magnetic field lines (or "lines of force")

  • Used to show the size and direction of magnetic fields
  • Always point from north to south
• Placing the north and south poles of two permanent bar magnets near each other

  • Creates a uniform field between the two magnets
• Using compasses and iron filings to see magnetic field patterns

  1. Compasses and iron filings align themselves with magnetic fields

  2. Use multiple compasses to see the magnetic field lines coming out of a bar magnet or between two bar magnets

  3. Use one compass and move it around (trace its position on paper before each move)

  4. Put the magnets under a piece of paper, scatter the iron filings on top, and tap the paper until the iron filings form a clear pattern
• Magnetism


  • Magnets affect magnetic materials and other magnets
  • Like poles repel each other and opposite poles attract
  • Both poles attract magnetic materials (that aren't magnets)
  • When magnetic materials are brought near a magnet, that material acts as a magnet
  • The closer the magnet and the magnetic material get, the stronger the induced magnetism will be
• Electromagnetism


  When electric current flows it produces a magnetic field
• A current-carrying wire creates a magnetic field

  1. The magnetic field is made up of concentric circles with the wire in the centre

  2. The larger the electric current, the stronger the magnetic field

  3. The direction of the magnetic field depends on the direction of the current
• Motor effect


  When a current-carrying wire is put between magnetic poles, the two magnetic fields affect one another, resulting in a force on the wire that can cause it to move
• Determining the direction of the force on a current-carrying wire in a magnetic field

  Use Fleming's Left-Hand Rule: point first finger in direction of field, second finger in direction of current, thumb points in direction of force (motion)
• Magnetic materials


  • Can be 'soft' (lose induced magnetism quickly) or 'hard' (keep induced magnetism permanently)
  • Iron is used in transformers because it needs to magnetise and demagnetise quickly (50 times per second for UK mains electricity)
  • Adding a magnetically "soft" iron core through the middle of a solenoid can increase the strength of the magnetic field
• D.C. electric motor


  • Uses the motor effect to make a coil rotate
  • The split-ring commutator keeps the motor rotating in the same direction
  • Speed can be increased by more current, more coil turns, stronger magnetic field, or adding a soft iron core
• Loudspeaker


  Uses the motor effect - an a.c. signal in a coil surrounded by a permanent magnet makes the coil and attached cone vibrate, creating sound
• Electromagnetic induction is the creation of a voltage (and maybe current) in a wire by movement of a magnet
• Loudspeakers Work Because of the Motor Effect

  1. The ac. electrical signals from an amplifier are fed to a coil of wire

  2. The coil is surrounded by a permanent magnet, so the a.c. signals cause a force on the coil and make it move back and forth

  3. These movements make the cone vibrate and this creates sounds
• The motor effect has a lot of important applications
• Applications of the motor effect

  • Food mixers
  • DVD players
  • Electric drills
• Electromagnetic induction


  The creation of a voltage (and maybe current) in a wire which is experiencing a change in magnetic field
• Electromagnetic induction


  1. An electrical conductor (a coil of wire is often used) moves through a magnetic field

  2. The magnetic field through an electrical conductor changes (gets bigger or smaller or reverses)
• You can test electromagnetic induction by connecting an ammeter to a conductor and moving the conductor through a magnetic field (or moving a magnet through the conductor)
• If the direction of movement is reversed, then the induced voltage/current will be reversed too
• To get a bigger voltage from electromagnetic induction


  • Increase the strength of the magnet
  • Increase the number of turns on the coil
  • Increase the speed of movement
• Current in the wire and a magnetic field

  Causes movement
• Magnetic field and movement

  Induces a current
• Electromagnetic induction transfers energy from kinetic energy stores to electrical energy stores
• A.C. Generators


  Turn a coil in a magnetic field to produce a current
• A.C. Generators


  • Their construction is pretty much like a motor
  • They have slip rings and brushes so the contacts don't swap every half turn
• A.C. generators produce AC voltage as shown by CRO displays
• Power stations use A.C. generators to produce electricity
• Transformers


  Change the size of the voltage of an alternating current
• How transformers work


  1. They have two coils, the primary and the secondary, joined with an iron core

  2. When an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly

  3. This induces an alternating voltage in the secondary coil
• Turns ratio


  The number of turns on the secondary coil divided by the number of turns on the primary coil
• Transformers only work with an alternating current, not DC
• The ratio of the primary and secondary voltages


  Is the same as the ratio of the number of turns on the primary and secondary coils
• Step-up transformers


  • Have more turns on the secondary coil than the primary coil
• Step-down transformers


  • Have more turns on the primary coil than the secondary coil
• Transformers are nearly 100% efficient, so power in = power out
• Step-up and step-down transformers are used when transmitting electricity across the country
• The voltage produced by power stations is too low to be transmitted efficiently
• Step-up transformers are used to boost the voltage before it is transmitted