Magnetism & Electromagnetism

The motor effect

• A current-carrying wire placed in a magnetic field experiences a force
• This is the motor effect: the field of the magnets and the field around the current-carrying wire interact, and the resulting force pushes the wire sideways out of the field
• Three requirements for there to be a force:
  • there must be a current flowing in the wire
  • the wire must be in a magnetic field
  • the wire must lie at an angle to the field. The force is largest when the wire is at 90° to the field, and zero when the wire is parallel to the field

Fleming's left-hand rule

• The directions of the three quantities (current, field and force) are mutually perpendicular. To work out which way the force pushes the wire, use Fleming's left-hand rule:
  • First finger → direction of the magnetic Field (N to S)
  • seCond finger → direction of the conventional Current (+ to −)
  • THumb → direction of the THrust (the force on the wire)
• Hold your left hand with the three digits all at right angles to one another like the corner of a box. Rotate your hand until the first finger lines up with the field and the second finger lines up with the current, and the thumb then automatically points along the force

Factors that change the size of the force

• The force on a current-carrying wire in a magnetic field is larger when:
  • the current is bigger (more charge per second crossing the field)
  • the magnetic field is stronger (use a stronger magnet)
  • the angle between the wire and the field is closer to 90° (90° gives maximum; parallel gives zero)
  • in extended geometry, the length of wire inside the field is longer

The d.c. motor (a practical application)

• A simple d.c. motor uses the motor effect to spin a coil of wire continuously between the poles of two magnets:
  • The coil is wired into a circuit through a split-ring commutator at one end. The two halves of the split ring are pressed against two carbon brushes that feed current in
  • With the coil horizontal, current flows one way along the front side of the coil and the opposite way along the back side. Each side sits in the same magnetic field, but their currents are opposite, so Fleming's left-hand rule predicts opposite forces: one side is pushed up, the other pushed down. The coil starts to rotate
  • When the coil has rotated a quarter turn (vertical), the two halves of the split ring lose contact with the brushes, no current flows, and no force acts; the coil's own momentum carries it past the vertical
  • As the coil continues past the vertical, the split-ring swap means that the side now on the right is connected to the same brush as the side that was previously on the right, so the current in each side of the coil reverses relative to the magnets. The forces remain pushing each side the same way they were before, and the coil keeps rotating in the same sense
• The motor's speed can be increased by increasing the current, or by using a stronger magnet, or by adding more turns to the coil
• The direction of rotation can be reversed by reversing the current or by swapping the magnets round
• A loudspeaker runs on the same physics in miniature: an alternating current flows through a small coil sitting in a permanent magnet's field. The coil oscillates back and forth in step with the current, pushing a paper cone that vibrates the air and produces a sound wave