Electromagnetic Induction, Generator and Domestic Electric Circuits

12.8 Electromagnetic Induction

In 1831 Michael Faraday asked a natural question: if a current produces a magnetic field, can a magnetic field produce a current? After years of careful experiments he showed that the answer is yes — but only when the magnetic field changes with time.

Faraday's careful observations from this simple setup:

• Induced current flows only while the magnet is in motion.
• Faster motion → larger induced current.
• Reversing the direction of motion reverses the direction of the induced current.
• The effect is the same whether the magnet moves and coil is fixed, or the coil moves and magnet is fixed. What matters is the relative motion.

Induced current also appears in a coil (secondary) kept near another coil (primary) at the instant the primary current is switched on or off — because switching the current on or off changes the magnetic field through the secondary coil.

12.8.1 Fleming's Right-Hand Rule (for generators)

The direction of the induced current in a straight conductor moving in a magnetic field is given by another three-finger rule.

12.9 Electric Generator

An electric generator works on the reverse principle of a motor. Mechanical rotation of a coil inside a magnetic field induces an EMF in the coil; this EMF drives a current through an external circuit. Generators are the backbone of every power station in the world.

12.9.1 AC Generator (Alternator)

Working. The coil ABCD is rotated (by a turbine) in the magnetic field. In the first half rotation arm AB moves up and CD moves down; applying Fleming's right-hand rule, the induced current flows A → B → C → D — say, from left brush to the external circuit back to the right brush. In the second half rotation AB moves down and CD moves up; the induced current reverses its direction in the coil (now flows D → C → B → A). Because slip rings are continuous, this reversal reaches the external circuit as well — the current in the external circuit also reverses every half cycle. Thus the generator produces alternating current (AC).

In India, household AC has frequency \(50\,\text{Hz}\) — the direction of the current reverses 100 times a second.

12.9.2 DC Generator

A DC generator is identical to an AC generator except for one change — the slip rings are replaced by a split-ring commutator, just like the one in a motor. The commutator acts as a brush-swapping device: every time the coil rotates through half a turn, the brush that had been positive now touches what was the negative half-ring and vice versa. The current in the external circuit therefore always flows in the same direction, even though inside the coil it reverses each half cycle.

AC generator vs DC generator — at a glance

12.10 Domestic Electric Circuits

The electricity that enters your home arrives through three wires:

• Live (phase) wire — insulated with red (or brown) plastic. It is at a high potential of about +220 V.
• Neutral wire — insulated with black (or blue) plastic. It is at (nearly) zero potential.
• Earth wire — insulated with green (or green-yellow striped) plastic. It is connected to a thick metal plate buried deep in the ground near the house.

The potential difference between live and neutral is 220 V at 50 Hz.

Each room is connected in parallel so that every appliance receives the full 220 V and can be switched on independently. There are usually two separate circuits in a home:

• Lighting circuit — with a 5 A fuse — for bulbs and fans (low current).
• Power circuit — with a 15 A fuse — for high-current sockets that feed geysers, ACs, microwaves.

12.10.1 Earthing — The Safety Wire

Earthing is a life-saver. If the insulation of an appliance fails and the live wire accidentally touches the metal case of (say) a geyser, the case would go to 220 V — any person who touches it could be killed. The earth wire provides a low-resistance path from the case to the ground. As soon as the live wire touches the case, a huge current rushes to earth, the fuse melts (or the MCB trips) and the supply is cut.

12.10.2 Fuse and MCB

12.10.3 Short Circuit and Overloading

• Short circuit: The live and neutral wires come in direct contact (damaged insulation, wet walls). The resistance of the path becomes nearly zero, so a very large current flows — sparks fly, wires burn and fire can start.
• Overloading: Too many high-power appliances are plugged into the same socket (or the voltage rises above 220 V). The wires carry more current than they are designed for, heat up, and may ignite.

In both cases, a correctly rated fuse or MCB protects the circuit by breaking it before damage is done.

12.10.4 Safety precautions

• Always use good-quality, well-insulated wires.
• Check that the earth wire is connected to every three-pin socket and appliance with a metal body.
• Never use more appliances than the circuit is rated for (avoid multi-plug boards on one socket).
• Never touch a switch or socket with wet hands.
• Always switch off the mains before replacing a bulb, a fuse or repairing any appliance.
• Use MCBs / ELCBs rather than rewirable fuses where possible.