Study Guide · Chapter 24
23. Frequently Asked Facts — SSC/RRB Exam Highlights
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- SI unit of magnetic flux is the weber (Wb); SI unit of magnetic flux density (field strength) is the tesla (T).
- Electromagnetic induction was discovered by Michael Faraday in 1831 (Joseph Henry made an independent, near-simultaneous discovery in the USA).
- Oersted's experiment (1820) first showed that electric current produces a magnetic field, establishing electromagnetism.
- Ohm's law was given by Georg Simon Ohm; the ohm is the SI unit of resistance.
- Coulomb's law was given by Charles-Augustin de Coulomb; it is mathematically analogous to Newton's law of gravitation.
- A transformer works only on AC, never on DC, because it depends on a continuously changing magnetic flux to induce EMF (a steady DC current produces no changing flux).
- Fleming's Left-Hand Rule gives the direction of force on a current-carrying conductor in a magnetic field, applicable to electric motors.
- Fleming's Right-Hand Rule gives the direction of induced current in a conductor moving through a magnetic field, applicable to electric generators.
- Lenz's law (given by Heinrich Lenz) states the induced current always opposes the change in flux that produced it, consistent with the law of conservation of energy.
- An electric motor converts electrical energy into mechanical energy; an electric generator (dynamo) converts mechanical energy into electrical energy — they are functional opposites of each other.
- The fuse wire in household circuits is always connected to the live wire, and is made of a low-melting-point alloy (commonly tin-lead) so that it melts and breaks the circuit under excess current.
- Standard household AC supply frequency in India is 50 Hz, at 220–240 V.
- Copper is preferred for household wiring due to its low resistivity, high conductivity, ductility, and comparatively low cost; silver is a better conductor but far too expensive for everyday wiring.
- Tungsten is used as the filament in incandescent bulbs because of its very high melting point (~3380°C) and low rate of evaporation at high temperature.
- Nichrome (nickel-chromium alloy) is used in heating elements (heaters, toasters, irons) because of its high resistivity and resistance to oxidation at high temperature.
- Soft iron is used for the cores of electromagnets and transformers because it is easily magnetised and easily demagnetised (low retentivity); steel is used for permanent magnets because it retains magnetism well (high retentivity).
- Magnetic monopoles do not exist in nature — breaking a magnet always produces two new complete magnets, never an isolated north or south pole.
- A voltmeter is always connected in parallel (has very high resistance); an ammeter is always connected in series (has very low, ideally zero, resistance) in a circuit.
- 1 kWh (one 'unit' of electricity, as billed by power companies) equals 3.6 × 10⁶ joules of energy.
- Earthing (grounding) protects users from electric shock by providing a low-resistance path for fault current to flow safely into the ground rather than through the human body.
- Alessandro Volta invented the first chemical battery (voltaic pile); the volt is named after him. André-Marie Ampère is regarded as the father of electrodynamics; the ampere is named after him.
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