The A.C. Generator (Cambridge (CIE) IGCSE Physics): Revision Note

Simple a.c. generators

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

• 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

• 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

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

• 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