Sec 4 Physics Magnetism notes

A magnetic material is one a magnet attracts. The ferromagnetic club is famously short: iron, nickel and cobalt, plus alloys containing them — most importantly steel (iron + carbon). Copper, aluminium, zinc, tin, brass, wood, plastic and glass are non-magnetic: a magnet ignores them completely. Exam trap: "all metals are magnetic" is false, and mixing it up costs marks in both directions — a copper rod can never be magnetised, and an aluminium can feels no force near any magnet.

Hold a magnet near an unmagnetised iron nail and the nail suddenly attracts paperclips: it has become a magnet by induction. The mechanism: the magnet’s field lines up regions inside the nail. The pole nearest the magnet becomes an unlike pole (magnet’s N near → near end S), and the far end becomes a like pole (N). That is exactly why an unmagnetised rod is always attracted, whichever end faces the magnet — the near end is always opposite.

Magnetic materials split by how well they remember. Soft iron (as in "soft" = easily changed, not physically soft) magnetises instantly and completely inside a field and loses almost all of it when the field is removed: a temporary magnet. Steel magnetises slowly but keeps its magnetism for years: a permanent magnet. The difference is microstructural — carbon atoms in steel pin the aligned regions in place.

This single fact decides every "choose the material" question. Electromagnet cores, relay armatures, electric-bell hammers: soft iron, because they must switch on and off hundreds of times a day. Compass needles, bar magnets, fridge magnets, loudspeaker magnets: steel (or specialised alloys), because the magnetism must survive years of handling. Getting this backwards gives either a relay that never releases or a compass needle that forgets which way north is within a week.

The magnetic force is concentrated at the poles, found at the ends of a freely-suspended magnet: the end pointing geographically north is the north-seeking (N) pole. The law of magnetism: like poles repel, unlike poles attract. From it comes the sharpest exam idea in this chapter: hanging an unknown rod near a magnet and seeing attraction proves nothing — the rod could be a magnet (unlike poles) or just unmagnetised iron (induced attraction). But seeing repulsion is conclusive: only like poles repel, and only a magnet has poles. Repulsion is the only sure test that a body is already magnetised.

The two-identical-rods problem is the classic application. Test both ends of both rods with a known magnet. All-round attraction → unmagnetised iron. Attraction at one end and repulsion at the other → a magnet, and the repelled end is a like pole of the known one. The trick examiners use: neither rod may be assumed magnetised just because it sticks.

The space around a magnet where magnetic materials feel a force is a magnetic field, mapped with field lines: the line’s direction at any point is the direction a north pole would be pushed (by convention, the way a plotting compass’s N-needle points). Rules for drawing: outside the magnet, lines leave the N pole and enter the S pole; they never cross; they are closest together where the field is strongest — crammed at the poles, sparse far away. Equal spacing with even arrows means a uniform field, such as between opposite poles of two facing magnets.

Three patterns to know cold. Single bar magnet: symmetrical curves from N looping to S. Two unlike poles facing: lines run nearly straight from N to S across the gap — a near-uniform field, the attract case. Two like poles facing (N and N): lines from each pole bend away from the gap, and midway there is a neutral point where the two fields cancel — a plotting compass placed there points geographic north only, undisturbed by the magnets.

Two magnetising methods. Stroking: drag a magnet along the steel object from end to end in one direction only, lifting clear between strokes; the object gains poles at its ends (the end where the stroking magnet leaves is an opposite pole to the stroking pole). Slow and imperfect, but needs no equipment. Electrical method: place the steel inside a solenoid carrying direct current for a few seconds — strong, fast and uniform, and the standard way to make strong permanent magnets.

Three demagnetising methods, one idea each. Heating to a high temperature and cooling in an east–west direction: thermal agitation scrambles the alignment. Hammering: mechanical shocks do the same. Alternating current: put the object inside a solenoid carrying a.c. and withdraw it slowly to a distance before switching off — the field reverses direction hundreds of times per second while shrinking to zero, leaving the magnetism randomised. Note the trap: the electrical magnetising method uses d.c.; the electrical demagnetising method uses a.c. Mixing these two up is the single most common mark lost in this topic.

Everyday permanent magnets (steel): compass needles for navigation, fridge and whiteboard magnets, magnetic catches on cupboard doors, loudspeaker magnets, and magnetic separators that pull iron and steel out of mixed scrap on a conveyor. In each case the requirement is the same: magnetism that survives switching off, heat and knocks.

Soft iron appears wherever magnetism must switch: electromagnet cores (next chapter), relay armatures that snap across when a small control current flows, electric-bell hammers, and magnetic guards on machinery. In every use the design requirement is fully magnetised when on, fully demagnetised when off. If an answer names a device, name the material and give exactly this reason — that is the full mark scheme for material-choice questions.