Electromagnetic induction
When a conductor moves across a magnetic field, or the field around a coil changes, an electric current is produced. This effect is called electromagnetic induction. No current flows if the conductor stays still inside a steady field, because the magnetic field lines must be cut for a current to appear.
Key idea
A changing magnetic field through a coil produces an induced current. Faster change means a larger current.
The generator
A generator is a machine that turns kinetic energy into electrical energy using electromagnetic induction. Inside it, a coil spins between the poles of a magnet. As the coil rotates, it keeps cutting field lines, so a current is induced in it. In a power station the coil is turned by a turbine, which may be driven by steam, falling water or wind.
Example
In a hydroelectric dam, falling water spins the turbine. The turbine turns the generator coil, which induces a current that flows out along the cables.
A bigger induced current
The induced current (and voltage) can be increased by using a stronger magnet, winding more turns on the coil, or spinning the coil faster. A simple generator uses two slip rings and brushes to carry the current out. Because the coil sides swap between the north and south poles as they turn, the current keeps reversing direction. This produces alternating current (AC).
Remember
- Field lines must be cut for a current to be induced.
- An AC generator naturally produces alternating current.
- Stronger magnet, more turns, faster spin → larger current.