The A.C. Generator (Cambridge International IGCSE Physics)

Simple a.c. generators

Spec point — 4.4.3 · Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings and brushes where needed

Extended Tier Only

• An a.c. generator is a device which converts energy from motion into an electrical output

• An alternating e.m.f. is generated which causes an alternating current to flow

• A simple a.c. generator consists of:

  • a rotating coil of wire between the poles of a permanent magnet
  • slip rings and brushes connected to an external circuit

A simple a.c. generator consists of a rotating coil in a magnetic field connected to an external circuit via slip rings and carbon brushes

• The functions of each component are shown in the table:

Operation of an a.c. generator

• A rectangular coil rotates in a uniform magnetic field

• The coil is connected to an external circuit via slip rings and brushes

  • The induced e.m.f. in the coil can be measured by adding a galvanometer (centre-zero meter) to the external circuit

• An e.m.f. is induced in the coil as it cuts the magnetic field

  • The pointer deflects first one way, then the opposite way, and then back again
  • This indicates the size and direction of the e.m.f. is constantly changing

• As a result of the alternating e.m.f., an alternating current is also produced as the coil rotates

  • This continues as long as the coil keeps turning in the same direction

Motion of an a.c. generator

The size and direction of the induced e.m.f. (and current) depend on the orientation of the coil with the field

• A maximum e.m.f. is induced when:

  • the position of the coil is parallel to the field
  • the motion of the coil is perpendicular to the field

• This is because the greatest number of lines are cut when the coil is moving perpendicular to the field

• No e.m.f. is induced when:

  • the position of the coil is perpendicular to the field
  • the motion of the coil is parallel to the field

• This is because no lines are cut when the coil is moving parallel to the field

Graphs for a.c. generators

Spec point — 4.4.3 · Sketch and interpret graphs of e.m.f. against time for simple a.c. generators and relate the position of the generator coil to the peaks, troughs and zeros of the e.m.f.

Extended Tier Only

• The output of an a.c. generator can be seen on a graph of e.m.f. against time, or angle of rotation

• The shape of the graph is a sine or cosine curve, depending on the starting position of the coil

  • When it starts from a horizontal position (e.m.f. is at a maximum), the graph is a cosine curve
  • When it starts from a vertical position (e.m.f. is zero), the graph is a sine curve

Graph of induced e.m.f. with angle for an a.c. generator

Alternating e.m.f. with corresponding positions of the coil relative to the field

• When the coil is vertical at 0°:

  • it is moving parallel to the direction of the magnetic field
  • the size of the induced e.m.f. is zero

• When the coil has rotated by 90°:

  • it is now horizontal and moving perpendicular to the direction of the magnetic field
  • the size of the induced e.m.f. is at a maximum

• When the coil has rotated by 180°:

  • it is vertical again and moving parallel to the direction of the magnetic field
  • the size of the induced e.m.f. is zero

• When the coil has rotated by 270°:

  • it is horizontal again and moving perpendicular to the direction of the magnetic field
  • the size of the induced e.m.f. is at a maximum and in the opposite direction to its position at 90°

• When the coil has completed a full 360° rotation:

  • it is back at its starting point where it is moving parallel to the direction of the magnetic field
  • the size of the induced e.m.f. is zero

Factors affecting a.c. generators

• The magnitude of the induced e.m.f. can be increased by:

  • increasing the frequency of rotation of the coil
  • increasing the number of turns on the coil
  • increasing the strength of the magnet
  • inserting a soft iron core into the coil

Exam Hint For your exam, you need to be aware that an alternating current can be produced by:

• a coil rotating in a magnetic field
• a magnet rotating within a coil

Both will induce an e.m.f. in the coil as they both ensure the coil will experience a changing magnetic field.

Take a look at these notes on trigonometric graphs if you need to brush up on your knowledge of sine and cosine graphs.