The motor effect

• When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.
• Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.
• The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid . An electromagnet is a solenoid with an iron core.
• When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.
• Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.
• For a conductor at right angles to a magnetic field and carrying a current:

  force =magnetic flux density ×current ×length 

  [  F =B I l⁢  ]
• force, F , in newtons, N
• magnetic flux density, B , in tesla, T

  current, I , in amperes, A (amp is acceptable for ampere)

  length, l , in metres, m
• A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
• Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.