Magnetism and Electromagnetic Effects

Magnets

• A magnet attracts iron, nickel, cobalt. Magnetic substances are attracted; non-magnetic ones (wood, plastic, copper, aluminium) are not.
• Every magnet has two poles, north and south; they cannot be separated (cut a magnet and each piece has two poles). Like poles repel, unlike attract. Repulsion is the sure test of a magnet.
• A freely suspended magnet points north-south: the principle of the compass. The Earth behaves like a giant bar magnet; its geographic south is near the magnetic north.
• Natural magnet: lodestone (magnetite). Artificial magnets: bar, horseshoe, needle, ring, cylindrical.
• Permanent magnets use steel, alnico; electromagnets use soft iron.
• Magnetic field: the region around a magnet where its force acts. Field lines go from north to south outside the magnet, and never intersect; closer lines mean a stronger field.
• Magnetic field strength is measured in tesla (T).

Magnetic effect of current

• Oersted (1820) found that a current-carrying wire deflects a compass needle.
• Right-hand thumb rule: hold the wire with the thumb along the current; the curled fingers show the field direction.
• Solenoid: a coil of many turns; it acts like a bar magnet. A soft-iron core inside makes an electromagnet.
• Uses of electromagnets: electric bells, cranes lifting scrap, MRI, loudspeakers, relays.

Force on a current-carrying conductor

• A conductor in a magnetic field feels a force. Fleming's left-hand rule: thumb = force, forefinger = field, middle finger = current (all mutually perpendicular).
• Electric motor converts electrical to mechanical energy; it uses this force. Parts: armature coil, commutator (split ring), brushes, magnet.

Electromagnetic induction

• Faraday (1831) showed that a changing magnetic field around a coil induces a current.
• Fleming's right-hand rule: thumb = motion, forefinger = field, middle finger = induced current.
• Generator (dynamo) converts mechanical to electrical energy. It uses slip rings and gives AC.
• AC (alternating current) reverses direction regularly (50 Hz in India, so 100 reversals per second); DC flows one way. AC is easier to transmit over long distances.
• Transformer changes AC voltage using mutual induction: step-up raises voltage, step-down lowers it; it does not work on DC.