Electromagnetic induction. Faraday discovered that a changing magnetic field can produce an electric current. When a conductor (or coil) moves in a magnetic field, or when the magnetic field through a coil changes, an induced current flows in the coil. This phenomenon is called electromagnetic induction — the reverse of the motor effect. No battery is needed; the energy comes from the motion.
When is current induced? Current is induced only while there is a relative change: moving a magnet into or out of a coil, moving a conductor across a field, or starting/stopping a current in a nearby coil. A faster change gives a larger induced current. If nothing changes, the induced current is zero.
Fleming's Right-Hand Rule. To find the direction of the induced current, stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular:
- Forefinger points along the magnetic field.
- Thumb points along the motion of the conductor.
- Middle finger then gives the direction of the induced current.
Note: the left hand is for the force on a current (motor); the right hand is for induced current (generator).
Electric generator. A generator (dynamo) converts mechanical energy into electrical energy using electromagnetic induction. A rectangular coil ABCD is rotated in a magnetic field. As the coil turns, the magnetic field through it changes, so an induced current flows in the external circuit.
AC generator. In an AC generator the coil ends connect to two separate slip rings that rotate with the coil and touch the brushes. After every half rotation each arm moves in the opposite direction across the field, so the induced current reverses direction every half turn — this gives alternating current (AC). In India the mains supply is AC at $50$ Hz, meaning the current changes direction $100$ times per second.
DC generator. A DC generator uses a split-ring commutator (like a motor) instead of slip rings. The split ring reverses the connection every half turn, so the output current always flows the same way in the external circuit — giving direct current (DC).
Domestic electric circuits. Power reaches homes through three wires: live (red/brown) at high potential, neutral (black/blue) at nearly zero potential, and earth (green) connected to a metal plate buried in the ground. The supply is usually $220$ V AC. Appliances are connected in parallel so each gets the full voltage and can be switched independently.
- Earthing: the metal body of an appliance is connected to the earth wire. If the live wire touches the body, current flows safely to the ground instead of giving a shock.
- Fuse: a thin wire of low melting point placed in the live wire. If the current exceeds a safe value it melts and breaks the circuit, protecting the wiring and appliances.
- Short circuit: when live and neutral wires touch directly, the resistance becomes very small and a huge current flows — the fuse blows.
- Overloading: drawing too much current by running many appliances on one socket; this also blows the fuse and can cause fires.