Magnetism and Magnetic Effect of Electric Current
NIOS · Class 12 · Physics
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Magnets, Their Properties, and Magnetic Field Lines
- Every magnet has two poles — North (N) and South (S) — which are always inseparable. Even if you break a magnet, each piece has both poles. This is a fundamental difference from electric charges.
- Directive Property: A freely suspended bar magnet always aligns itself in the geographical North-South direction. This is because Earth itself acts as a giant magnet.
- Attractive Property: A magnet attracts magnetic materials (iron, nickel, cobalt). Attraction is strongest at the poles.
Earth's Magnetic Field and Its Elements
- Earth behaves as if a large bar magnet is placed inside it. The South pole of this imaginary magnet is near the geographical North Pole and the North pole is near the geographical South Pole.
- The magnetic axis of Earth does NOT coincide with its geographical (rotation) axis — they are tilted by about 11.5°.
- The magnetic North Pole (where Earth's South magnetic pole is) is located near Hudson Bay in Canada, about 650 km from the geographical North Pole.
Oersted's Experiment and Biot-Savart's Law
- Oersted's Discovery (1820): Hans Christian Oersted showed that a current-carrying conductor deflects a nearby compass needle, proving that electric current produces a magnetic field around it.
- When current flows north in a wire above a compass needle: N pole deflects west. When current is reversed: N pole deflects east. When no current: no deflection.
- Magnetic field lines around a straight current-carrying conductor are concentric circles, with the wire at the centre.
Ampere's Circuital Law and Its Applications
- Ampere's Circuital Law: The line integral of the magnetic field B around any closed loop (Amperian loop) is equal to μ₀ times the total current enclosed by the loop.
- Mathematically: ∮ B·dl = μ₀I. This is independent of the shape or size of the closed loop chosen.
- APPLICATION 1 — Straight Infinite Wire: B = μ₀I / 2πr. The field decreases with increasing distance r from the wire.
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