Motor Effect, generators, transformers

• Motor effect


  The act that happens when you put a conductor in a magnetic field
• A current in a magnetic field experiences a force
• Motor effect


  1. Conductor placed between magnetic poles

  2. Conductor experiences a force
• The force acts at right angles to the magnetic field and the direction of the current in the wire
• Demonstrating the motor effect

  1. Current applied to a set of rails inside a horseshoe magnet

  2. A bar is placed on the rails, completing the circuit

  3. This generates a force that rolls the bar along the rails
• Factors affecting the force on a conductor in a magnetic field

  • Magnetic flux density (strength of magnetic field)
  • Size of current through the conductor
  • Length of the conductor in the magnetic field
• Fleming's left-hand rule


  Used to determine the direction of the force on a current-carrying conductor in a magnetic field
• If either the current or the magnetic field is reversed, the direction of the force will also be reversed
• Electric motors and loudspeakers make use of the motor effect
• How an electric motor works

  1. Current-carrying coil of wire in a magnetic field

  2. Forces act on the coil, causing it to rotate

  3. Commutator reverses the current every half turn to keep the motor rotating in the same direction
• Loudspeakers


  • Use electromagnets - a current-carrying coil of wire around a permanent magnet
  • The force on the coil causes the paper cone to vibrate, creating sound waves
• Generator effect


  The induction of a potential difference (and current if there's a complete circuit) in a conductor that is moving in a magnetic field or experiencing a change in magnetic field
• Generating a potential difference

  1. Moving a magnet in a coil of wire or moving a conductor in a magnetic field

  2. Changing the direction of the magnet or the magnetic field reverses the potential difference/current
• The induced current opposes the change that made it
• Factors affecting the size of the induced potential difference

  • Speed of the movement (cutting more magnetic field lines)
  • Strength of the magnetic field
• Alternators


  Generate alternating current by rotating a coil in a magnetic field
• Dynamos


  Generate direct current by using a split-ring commutator to keep the current flowing in the same direction
• Oscilloscope


  Can be used to see the generated potential difference over time
• Microphones


  Generate current from sound waves by using a coil of wire in a magnetic field, attached to a diaphragm
• Transformer


  Changes the size of the potential difference of an alternating current
• How a transformer works


  1. Has a primary and secondary coil wound around an iron core

  2. Alternating current in the primary coil creates a changing magnetic field

  3. This induces an alternating potential difference in the secondary coil
• Transformer equation


  Relates the input and output potential differences to the number of turns in the primary and secondary coils
• Transformers are almost 100% efficient
• The national grid transmits power at high potential differences to reduce energy losses