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In 1820 a Danish teacher named Hans Christian Oersted was showing a circuit to his class when the compass needle lying on the table twitched. That twitch ended two thousand years of treating electricity and magnetism as two separate subjects. Eleven years later Michael Faraday ran the experiment backwards: he moved a magnet inside a coil and got a current out of nothing but motion.
Those two sentences are the entire chapter, and between them they generate almost every unit of electricity in India. A current makes a magnetic field — that is the electromagnet, the electric bell, the motor. A changing magnetic field makes a current — that is the generator in every thermal and hydel plant, and the transformer on the pole outside your house.
Examiners love this pair precisely because the two directions are so easy to swap. Get the direction right and forty scattered facts collapse into four devices.
1. Oersted's 1820 experiment established that:
2. In Fleming's left-hand rule, the centre (middle) finger points along:
3. The core of an electromagnet is made of soft iron rather than steel because soft iron:
4. In an electric motor, the split-ring commutator serves to:
5. A transformer has 100 turns in the primary and 500 turns in the secondary. If 220 V AC is applied across the primary, the secondary voltage is:
6. Inside a bar magnet, the magnetic field lines run:
7. In a domestic electric circuit, the fuse is always connected in the:
8. An AC generator differs from a DC generator chiefly in that the AC generator uses: