The ac generator
An alternating current (ac) Device that converts kinetic energy into electrical energy. is a device that produces a The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them.. A simple Alternating current, eg the mains supply of electricity from a plug is alternating current. generator consists of a coil of wire rotating in a magnetic field. Cars use a type of ac generator, called an An electrical generator which produces alternating current, an ac generator. to keep the battery charged and to run the electrical system while the engine is working.
The alternator
The diagram shows a simple alternator.
As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. This means that the alternator produces a current that is constantly changing. This is alternating current or ac.
Alternator output on a graph
The output of an alternator as it rotates can be represented on a potential difference-time graph with potential difference (voltage) on the vertical axis and time on the horizontal axis.
The graph shows an alternating The shape of the graph obtained for y = sin x. The voltage produced by an ac generator follows a sine curve.. The maximum potential difference or current can be increased by:
- increasing the rate of rotation
- increasing the strength of the magnetic field
- increasing the number of turns on the coil
The diagram shows four different positions of the coil in an alternator, and the corresponding potential difference produced.
A - The coil is at 0掳. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
B - The coil is at 90掳. The coil is moving at 90掳 to the direction of the magnetic field, so the induced potential difference is at its maximum.
C - The coil is at 180掳. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
D - The coil is at 270掳. The coil is moving at 90掳 to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the opposite direction to what it did at B.
A - The coil is at 360掳, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.