4.1 Simple Phenomena of Magnetism
Key Terms
- Magnetic substances — examples: iron, steel, nickel, cobalt.
- Ferromagnetism — in iron, steel, nickel, cobalt.
- Magnetic field — direction is the force on a N pole at that point.
Properties of Magnets
- Attract other magnets and unmagnetised magnetic substances.
- Attract iron, steel, nickel, cobalt.
- Ends are poles (north & south) of equal strength.
- A freely suspended magnet points north–south.
- Like poles repel; unlike poles attract.
Magnetically Hard vs Soft Substances
| Type | Behaviour | Examples / Notes |
|---|---|---|
| Magnetically hard | Retain magnetism (good permanent magnets). | Alloys rich in iron/nickel/cobalt. |
| Magnetically soft | Lose magnetism easily (good for temporary cores). | Alloys with less iron/nickel/cobalt → weaker field. |
Types of Magnets
| Type | Definition | Uses |
|---|---|---|
| Permanent | Retain magnetism once magnetised. | Motors, compasses, fridge doors, lodestone. |
| Temporary | Act as magnets only while in a magnetic field. | Electromagnets for lifting scrap, relays. |
| Electromagnets | Coils that become magnets when current flows; strength ↑ with current and iron core. | Scrapyard cranes, bells, relays. |
Core Choice for Electromagnets
Use a soft iron core for switching — it magnetises and demagnetises quickly. Do not use steel where a switchable magnet is required (steel keeps magnetism).
Magnetic Induction
Bringing a magnetic substance near a magnet induces the opposite pole at the near end of the substance.
Magnetic Field Lines — Facts
- Arrows show direction N → S outside the magnet.
- Field is strongest where lines are most concentrated (at poles).
- The direction at a point is the force on a north pole there.
Plotting Field Lines with a Compass
- Place a bar magnet on paper; put a small compass near one pole.
- Mark a dot at the compass needle tip (blackened end = north).
- Move the compass forward to the dot and repeat to the other pole.
- Join the dots to trace a line; repeat for several starting points.
- Where the needle points outward is the N pole; inward is the S pole.
- Lines always run from N to S.
Non-Magnetic Substances
Materials not attracted by magnets include: wood, plastic, copper, paper, aluminium, rubber, stone.
Exam Tips
- State like repel / unlike attract and identify poles accordingly.
- Use soft iron for electromagnet cores; avoid steel when a temporary magnet is needed.
- On diagrams, draw field lines denser at poles and from N to S with arrowheads.
- Induction: near end becomes the opposite pole to the magnet’s nearby pole.
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