IP Physics Notes (Upper Secondary, Year 3-4): 14) Magnetism

Quick recap -- Magnetism revolves around poles, field lines, and how materials respond. Distinguish soft vs hard magnets, master field sketches, and know how to magnetise or demagnetise safely.

Laws & Properties of Magnets

• Like poles repel, unlike poles attract; forces strongest near poles.
• Freely suspended magnet aligns roughly north-south (Earth behaves like a giant magnet: geographic north magnetic south).
• Magnetic materials: iron, steel, cobalt, nickel. Non-magnetic: copper, aluminium, plastic, wood.

Magnetic Materials: Soft vs Hard

Magnetisation & Demagnetisation

• Magnetise:
  • Stroking with a permanent magnet (in one direction).
  • Placing in a solenoid carrying direct current (define north/south by right-hand grip).
• Demagnetise:
  • Heating above Curie temperature.
  • Hammering while not aligned with Earth's field.
  • Placing inside an alternating-current solenoid (gradually reduce current).

Magnetic Field Patterns

• Field lines run from north to south outside a magnet.
• Density of lines corresponds to field strength.
• Key diagrams to practise:
  • Single bar magnet (radial-like pattern around each pole).
  • Pair of unlike poles (lines connect across the gap).
  • Pair of like poles (lines bulge apart, showing repulsion).
• Plotting method: move a small compass from point to point, marking successive directions to trace a line.

Worked Example: Drawing Field Between Plates

For two parallel magnetic poles (north opposite south), draw at least four evenly spaced straight lines, parallel through the gap; arrows point from north to south. Add slight spreading near edges to indicate fringe fields.

Magnetic Screening

• Soft iron enclosures provide low-reluctance paths, diverting external fields away from sensitive equipment.
• Describe as "soft iron has high permeability so magnetic flux prefers the shield, leaving interior nearly field-free".

Current-Carrying Conductors (Preview of Chapter 15)

• Right-hand grip rule: thumb indicates conventional current; curled fingers show circular field direction around a wire.
• Solenoid mimics bar magnet (define poles via right-hand grip rule: curled fingers follow current; thumb points to north pole).

Key Takeaways

• Always reference electron alignment when explaining magnetisation/demagnetisation.
• Use neat, evenly spaced arrows for field sketches and respect the "lines never cross" rule.
• Choose iron (soft) for temporary magnets/shields; choose steel (hard) for permanent magnets.
• Remember screening rationale: magnetic flux follows the path of least reluctance provided by soft iron.