Magnets and Magnetic Fields
Know the rules for attraction/repulsion, how to draw field lines, and what makes a material magnetic.
Properties of magnets:
- Every magnet has two poles: North (N) and South (S)
- Like poles REPEL; unlike poles ATTRACT
- The force between magnets is strongest at the poles
- A freely suspended magnet aligns with Earth's magnetic field (N pole points roughly geographic north)
Magnetic and non-magnetic materials:
- Magnetic (ferromagnetic): iron, steel, nickel, cobalt → attracted to magnets
- Non-magnetic: copper, aluminium, brass, wood, plastic, glass → NOT attracted to magnets
- Testing: bring a magnet close — attraction does NOT prove material is a magnet; repulsion DOES prove it is a magnet
Magnetic field:
- Region of space where a magnetic force is experienced
- Represented by field lines (flux lines)
- Field lines: exit from N pole, enter S pole; never cross; closer = stronger field; arrows show direction from N to S
Field patterns:
- Between unlike poles: field lines curve from N to S (attraction between poles)
- Between like poles: field lines repel; a neutral point exists between them where field = 0
Induced magnetism:
- A magnetic material placed in a magnetic field becomes temporarily magnetised
- The end closest to the N pole of a magnet becomes an S pole (explaining attraction)
- Soft iron: easily magnetised and easily demagnetised → used in electromagnets, transformer cores
- Steel: hard to magnetise; but once magnetised, retains magnetism → used in permanent magnets
Demagnetisation:
- Heating above Curie temperature
- Repeated hammering
- Placing in a coil carrying alternating current (AC)
- Like poles repel; unlike poles attract. Repulsion proves both are magnets.
- Magnetic: iron, steel, cobalt, nickel. Non-magnetic: copper, aluminium.
- Induced magnetism: nearest end to N pole becomes S pole.