Magnetism
Physics · चुंबकत्व
📋Quick Overview
Magnetism is the force of attraction or repulsion between magnetic materials. A magnet has two poles: North (N) and South (S). Like poles repel (N-N, S-S) and unlike poles attract (N-S). Magnets were first discovered as natural stones called 'Lodestone' (magnetite ore, Fe₃O₄) in Magnesia, Greece — that's where the name 'magnet' comes from.
📖Properties of Magnets & Magnetic Field
- •Every magnet has N and S pole — you CANNOT have a monopole. If you break a magnet, each piece becomes a complete magnet with its own N and S.
- •Magnetic field lines go from N to S outside the magnet, and S to N inside. They never cross each other.
- •Magnetic field lines are CLOSEST at poles (strongest field) and SPREAD OUT in the middle.
- •Earth is a giant magnet! Earth's geographic North is near its magnetic South (that's why compass N points north — opposite poles attract).
- •Magnetic compass: Freely suspended magnet that aligns N-S. Used for navigation since ancient times. Chinese first used it.
📖Types of Magnetic Materials
| Type | Behavior in Magnetic Field | Examples |
|---|---|---|
| Ferromagnetic | STRONGLY attracted to magnets, can become permanent magnets | Iron, Nickel, Cobalt, Steel |
| Paramagnetic | WEAKLY attracted to magnets | Aluminum, Platinum, Oxygen, Manganese |
| Diamagnetic | WEAKLY repelled by magnets | Copper, Gold, Silver, Bismuth, Water |
📝Electromagnet & Applications
- •Electromagnet: A coil of wire wound around an iron core. When current flows, it becomes a magnet. When current stops, magnetism disappears. It is a TEMPORARY magnet.
- •Electromagnet strength increases by: (1) More turns of wire, (2) More current, (3) Using soft iron core.
- •Uses of electromagnet: Electric bell, telephone, loudspeaker, MRI machine, crane for lifting iron/steel, maglev trains, electric motor.
- •Electric bell: Uses electromagnet to attract and release a hammer repeatedly → produces ringing sound.
- •Permanent magnets vs Electromagnets: Permanent are always magnetic (steel, Alnico). Electromagnets can be turned ON/OFF (more useful).