7.2 The Motor Effect

• What is produced when current flows through a conducting wire?


  A magnetic field is produced around the wire
• Magnetic field around a current-carrying wire

  • The magnitude of the current flowing through the wire
  • The distance from the wire
• Solenoid


  A coil of wire which when current passes through creates a strong magnetic field
• Magnetic field inside a solenoid

  • Strong and uniform
• Electromagnet


  • A solenoid with an added iron core
  • Adding the iron core increases the strength of the magnetic field
• Motor effect


  When a force is exerted between a magnetic field and a current-carrying conductor placed in that field
• Fleming's Left-Hand Rule


  Used to determine the force experienced due to the motor effect
• Fleming's left hand rule

  • Forefinger points in the direction of the magnetic field
  • Second finger points in the direction of current flow in the conductor
• Force on a current-carrying wire in a magnetic field

  • The magnitude of the current flowing through the conductor
  • The strength of the magnetic field that the conductor is placed in
• If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed


  The direction of the force is reversed
• If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased


  The strength of the force is increased
• The conductor must be at right-angles to the magnetic field it is placed in for the equation linking force, magnetic flux density, current and length to hold
• Magnetic flux density


  Tesla, T
• How an electric motor works

  1. A coil of wire, carrying a current, is placed in a magnetic field

  2. The forces on the two sides perpendicular to the field experience forces in opposite directions

  3. This causes a rotational effect
• How loudspeakers make use of the motor effect

  The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound
• How a loudspeaker works

  1. A cone with a wire wrapped around it is connected to an a.c power supply and is placed in a permanent magnetic field

  2. When current flows through the wire, it creates a second magnetic field, which interacts with the permanent field

  3. This produces a force which causes the cone to vibrate
• How the pitch of the sound from a loudspeaker is changed

  • The frequency of the a.c current is altered
  • This creates a different frequency of vibration in the cone