Summary and Quick Revision
Magnetic field and field lines
- A magnetic field is the region around a magnet where its force is felt; it has magnitude and direction.
- Field lines: N → S outside the magnet, S → N inside (closed loops); direction = where a compass N pole points; crowded = stronger field; never cross.
Field due to a current
- Oersted: a current produces a magnetic field.
- Straight wire: concentric circles; field ∝ current, weaker with distance. Direction by the right-hand thumb rule (thumb = current, fingers = field).
- Circular loop: field at centre ⟂ to the loop; n turns → n × field.
- Solenoid: like a bar magnet; uniform field inside. A soft-iron core makes an electromagnet (temporary magnet).
Force, motor, induction, generator
- A current-carrying conductor in a magnetic field feels a force; largest when current ⟂ field. Direction by Fleming's left-hand rule (Fore=Field, ceNtre=curreNt, thuMb=Motion).
- Electric motor: electrical → mechanical energy; coil + magnet + split-ring commutator (reverses current each half turn) + brushes.
- Electromagnetic induction (Faraday): a changing magnetic field induces a current. Direction by Fleming's right-hand rule (thumb = motion, fore = field, middle = induced current).
- Electric generator: mechanical → electrical energy. Slip rings → AC; split-ring commutator → DC.
| Motor | Generator | |
|---|---|---|
| Energy | electrical → mechanical | mechanical → electrical |
| Rule | left hand | right hand |
| Rings | split-ring | slip rings (AC)/split-ring (DC) |
Domestic circuits
- Supply: AC, 220 V, 50 Hz. Three wires: Live = red, Neutral = black, Earth = green.
- Appliances are in parallel; each has a switch and fuse on the live wire.
- Fuse: melts on over-current, protecting the circuit and appliances (in series on live wire).
- Short circuit: live and neutral touch → current rises sharply.
- Overloading: too many appliances on one socket, or a voltage rise.
- Earthing: metal body → earth wire → ground; low-resistance path that protects the user from shock.
One-line recall
- Field around wire → right-hand thumb; Force on wire → left hand; Induced current → right hand.
- Motor = split-ring; AC generator = slip rings; earth = green; fuse on live wire.