9.1 Theory of Magnetism & Magnetic Materials
Key Takeaways
- A magnet has two poles (N and S); like poles repel and unlike poles attract; flux lines leave the north pole externally and enter the south pole
- Earth behaves as a huge magnet; the geographic North Pole is near a magnetic south pole, so a compass north-seeking end points geographic north
- Magnetisation aligns magnetic domains; demagnetisation disorders them by heat, shock, or an alternating field that is gradually reduced to zero
- Magnetic shielding uses high-permeability soft materials to divert flux around a sensitive region rather than “blocking” magnetism with an insulator
- Hard magnetic materials retain magnetism (permanent magnets); soft magnetic materials magnetise and demagnetise easily (cores, shields); store magnets with keepers and away from instruments
9.1 Theory of Magnetism & Magnetic Materials
Quick Answer: Magnets have north and south poles; like poles repel, unlike poles attract. External flux leaves N and enters S. Earth is a weak but global magnetic field. Hard materials keep magnetism (permanent magnets); soft materials take and lose magnetism easily (cores, shields). Heat, shock, or a decaying AC field can demagnetise; high-μ soft iron shields by diverting flux.
CAAS SAR-66 Module 3 topic 3.10 Magnetism is the bridge between DC circuit theory and electromagnetism, inductance, and rotating machines. Before you write Fleming’s rules or talk about flux density in amperes per metre and teslas, you need a clear picture of what a magnet is, how the Earth field behaves, how materials become magnets or lose magnetism, and how technicians store and shield magnetic devices around aircraft instruments.
Properties of a Magnet
A magnet is a body that produces a magnetic field and exerts force on ferromagnetic materials and on other magnets. Every magnet has two poles:
| Property | Statement (exam language) |
|---|---|
| Poles | Always in pairs — north-seeking (N) and south-seeking (S) |
| Force law | Like poles repel; unlike poles attract |
| External flux | Lines leave N, enter S, form closed loops |
| Internal path | Inside the magnet, flux returns from S toward N |
| Strength | Strongest near the poles; weakens with distance |
| Indivisibility | Breaking a bar magnet yields two smaller magnets, each with N and S |
Magnetic flux lines (lines of force) are a visualisation aid:
- They never cross.
- They form closed loops.
- Density of lines represents field strength (later formalised as flux density B).
- They take the path of least reluctance through soft magnetic materials.
Worked concept check. Two bar magnets face N-to-N. They repel. Turn one so N faces S: they attract. A soft-iron nail placed near either pole becomes a temporary magnet (induced poles) and is attracted — that is magnetic induction, not “electric charge attraction.”
Domain picture (Level 2 depth)
In ferromagnetic materials (iron, nickel, cobalt, and many alloys), groups of atomic magnets form domains. In an unmagnetised piece, domains point randomly and cancel. In a magnetised piece, domains align preferentially toward one direction, producing external N and S poles. This mental model supports later ideas of saturation (almost all domains aligned) and hysteresis (domains lag the magnetising field).
Earth’s Magnetic Field
The Earth behaves like a huge bar magnet. Important Module 3 / navigation-adjacent facts:
| Idea | Practical meaning |
|---|---|
| Geographic vs magnetic | Geographic North Pole ≠ magnetic pole location |
| Compass “north” | The north-seeking end of a compass is attracted toward the Earth’s magnetic south region near geographic north |
| Declination | Angle between true north and magnetic north — charts and aircraft systems account for it |
| Dip / inclination | Field lines are not horizontal everywhere; they dip into the Earth |
| Weak field | Earth’s field is weak compared with a workshop permanent magnet or an electromagnet, but it is enough to orient compasses and to bias sensitive instruments if magnets are stored carelessly nearby |
For Module 3 electrical fundamentals you are not examining IFR navigation procedures; you are expected to know that Earth has a magnetic field with poles, that a freely suspended magnet aligns with it, and that the naming of poles follows the north-seeking / south-seeking convention.
Magnetisation and Demagnetisation
Magnetisation (making a magnet)
Common training methods:
| Method | How it works | Typical use |
|---|---|---|
| Stroking with a permanent magnet | Repeated unidirectional strokes align domains | Simple demos; weak magnets |
| Direct current through a coil (electromagnet) | Strong H-field aligns domains in a core or workpiece | Electromagnets, magnetisers |
| Placing in a strong field while cooling (special processes) | Domains “freeze” into alignment | Industrial permanent-magnet manufacture |
Retentivity (covered in depth in §9.3) decides whether the piece stays magnetised after the field is removed — hard materials retain; soft materials largely lose magnetism when H is removed.
Demagnetisation (degaussing)
Domains must be disordered again:
| Method | Mechanism | Hangar / shop note |
|---|---|---|
| Heating above Curie temperature | Thermal agitation destroys alignment | Permanent damage risk to coatings/insulation — controlled process |
| Severe mechanical shock | Jars domains out of alignment | Accidental drop can weaken a magnet or tool |
| Alternating field reduced to zero | AC coil or degausser; amplitude decreased gradually so residual domains cancel | Preferred controlled method for tools, watches, some instruments |
Worked scenario. A steel tool becomes lightly magnetised and attracts filings. Passing it slowly through (or near) a degaussing coil while withdrawing it as the AC field decays leaves domains randomised → tool is demagnetised.
Magnetic Shielding
There is no magnetic insulator analogous to rubber for electricity. Shielding works by providing a low-reluctance path that diverts flux around a protected volume.
| Shielding idea | Detail |
|---|---|
| Material | Soft, high-permeability alloys (soft iron, mu-metal class materials in precision work) |
| Geometry | Closed or nearly closed magnetic path around the sensitive item |
| Effect | Flux prefers the shield; interior field is reduced |
| Wrong idea | “Thick plastic stops magnetism” — false; non-magnetic materials do not provide a preferred flux path |
Aircraft and avionics context: keep strong magnets and magnetised tools away from magnetic compasses and magnetically sensitive sensors; where shielding is designed in, it is soft magnetic enclosures or intentional soft-iron paths — not ordinary insulation tape.
Types of Magnetic Material
Module 3 classifies materials by how they respond to magnetic fields:
| Class | Behaviour | Examples / use |
|---|---|---|
| Ferromagnetic | Strongly attracted; can be magnetised | Iron, steel, nickel, cobalt, ferrites |
| Paramagnetic | Weakly attracted | Aluminium, platinum (weak effects) |
| Diamagnetic | Weakly repelled | Copper, bismuth, many non-magnetics |
| Hard magnetic | High remanence / high coercivity — permanent magnets | Alnico, hard steel, ceramic/rare-earth magnet materials |
| Soft magnetic | Low coercivity — easy to magnetise and demagnetise | Soft iron, silicon steel laminations, soft ferrites |
Hard vs soft (exam gold):
- Hard → permanent magnets, speakers, some sensors, latch magnets.
- Soft → transformer and inductor cores, relay armatures, magnetic shields, motor laminations (also reduces eddy-current issues when laminated — §9.3).
Non-magnetic materials (brass, most aluminium alloys, copper, many plastics, austenitic stainless grades used as “non-magnetic”) are chosen where you must not distort local fields or create unwanted attraction.
Storage Precautions
Permanent magnets and magnetised assemblies need disciplined storage — especially near aircraft magnetic compasses and workshop instruments.
| Precaution | Why |
|---|---|
| Use keepers (soft-iron bars bridging N–S on horseshoe or across bar ends) | Provides a closed flux path; reduces self-demagnetising open-circuit field and preserves strength |
| Store magnets together with opposite poles adjacent when keepers are used as designed | Maintains closed magnetic circuit |
| Keep away from compasses, CRTs/legacy instruments, magnetic media, and sensitive avionics | Stray fields cause errors or damage |
| Avoid heat, hammering, and dropping | Demagnetises or cracks brittle magnet materials |
| Do not store strong magnets against soft-iron tools unintentionally | Tools become magnetised; filings and FOD risk |
| Label and segregate magnetised tooling | Prevents accidental approach to aircraft during compass swing / maintenance |
Aircraft maintenance link. Before work near a magnetic compass or flux valve, control magnetised tools and permanent-magnet devices. A “harmless” magnetic base or magnetised screwdriver left in the cockpit can throw compass readings — exactly the practical reason Module 3 stresses storage and shielding alongside theory.
Section Synthesis Table
| Topic | One-line exam takeaway |
|---|---|
| Poles & force | Unlike attract, like repel; flux out N, in S |
| Earth | Global field; compass north-seeking end points geographic north |
| Magnetise | Align domains (stroke, DC coil, process fields) |
| Demagnetise | Heat, shock, or decaying AC field |
| Shield | High-μ soft path diverts flux |
| Hard / soft | Retain vs easy magnetise–demagnetise |
| Storage | Keepers, segregation, no heat/shock near instruments |
Master these qualitative rules before §9.2 turns magnetism into electromagnets and hand rules, and §9.3 attaches the quantitative language of MMF, B, permeability, hysteresis, and eddy currents.
Which statement correctly describes the external magnetic field of a bar magnet?
Why is soft iron preferred for magnetic shielding around a sensitive instrument?
Which demagnetisation method gradually reduces residual magnetism by disordering domains in a controlled way?
For permanent magnets used as field sources, which material class is most appropriate?