Magnetism and inductionHow does a spinning coil make electricity?

• Like poles repel, unlike poles attract. With two N poles together the flux lines push outwards away from each other. With N and S together the flux lines run straight between them, and they attract.
• The lines of magnetic flux run from the North pole round the outside into the South pole.
• Flux density is the amount of flux in a given area. Lines close together mean a strong field, and lines far apart mean a weak one.
• When a conductor carries a current, a magnetic field forms around it. The field is circular and runs the whole length of the conductor. It goes clockwise or anticlockwise depending on the direction of the current. Motors and transformers both rely on this.
• Flux lines are invisible. Sprinkle iron filings over a magnet (Faraday's school-physics experiment) and you can see their shape.

Electromagnetism is how UK electricity is made: in a power station, a coil rotates inside magnetic fields, generating at about 25,000 V.

Magnetic induction: how a generator makes AC

Magnetic induction is one of the main sources of EMF. A loop of wire is mechanically rotated between the N and S poles of a magnet. As it cuts the lines of flux, a voltage is induced, which is the electrical output.

• One full turn gives one cycle. At 50 Hz the loop turns 50 times a second, giving 50 cycles and 100 peaks (one each way per cycle).
• That's why lights sometimes flicker when you look at them through a phone camera. The camera is catching the 50 Hz cycle.
• The peaks at 90° and 270° are current going one way, then back the other way. It isn't "positive then negative electricity".