Magnetism

Magnetic materials. At 4PH1 level, only four metals are magnetic:

• Iron (and soft iron — see spec 6.3).
• Steel (an iron-carbon alloy).
• Cobalt.
• Nickel.

All four are sometimes called ferromagnetic. Other common metals (copper, aluminium, gold, silver, lead, zinc) are NOT magnetic at 4PH1 level — useful when distinguishing test scenarios.

Magnet ↔ magnet interaction.

• LIKE poles repel (N-N or S-S).
• UNLIKE poles attract (N-S or S-N).

Magnet ↔ unmagnetised magnetic material interaction.

• Always attractive. NEVER repulsive.
• The magnet INDUCES magnetism (spec 6.5) → the near face becomes the OPPOSITE pole.

How to tell if a piece of iron is magnetised. Bring it near a known magnet:

• If it can both attract AND repel that magnet (depending on orientation), then yes, it's magnetised.
• If it only ever attracts (or feels like nothing), it's unmagnetised.

This is the standard test in spec 6.2 / 6.5 question style — REPULSION is the only certain proof of magnetism.

Hard magnetic materials. Difficult to magnetise but, once magnetised, RETAIN magnetism for a long time.

• Standard example: steel (an iron-carbon alloy).
• Used for: PERMANENT magnets (compass needles, fridge magnets, loudspeaker magnets, hard-disk recording surfaces).
• Why hard? The arrangement of magnetic domains is 'stuck' — they require a strong external field or impact (heating, hammering, ageing) to be disturbed.

Soft magnetic materials. Easy to magnetise, but LOSE their magnetism easily when the magnetising field is removed.

• Standard example: soft iron (pure or near-pure iron).
• Used for: ELECTROMAGNET cores (spec 6.9P, relay/transformer iron core), magnetic recording heads.
• Why soft? Domains can realign quickly with very little resistance.

Why this matters in technology. An electromagnet needs to switch on and off quickly with current. If the core were made of steel (hard), it would stay magnetised after the current is switched off — useless. So electromagnet cores use SOFT iron.

Why steel for compass needles? A compass needle must REMAIN magnetised for years/decades while pointing north. A soft-iron needle would lose its magnetism if knocked or warmed. Steel retains it reliably.

Magnetic field. A region where a magnetic force acts on magnets or magnetic materials.

Field lines (= lines of force). Represent the field visually:

• Direction: outside the magnet, lines run from NORTH pole to SOUTH pole. The arrow on the line shows the direction in which the N pole of a freely-suspended plotting compass would point.
• Density: where lines are CLOSE TOGETHER, the field is STRONG (especially near the poles). Where they are far apart, the field is WEAK (further from the magnet).
• Loops: field lines form CLOSED loops — they continue from S back to N inside the magnet (usually not drawn).

Field pattern around a single bar magnet.

• Lines emerge from N pole, curve out into the surrounding space, and return into the S pole.
• Densest at the poles.
• Symmetric about the magnet's axis.

Field pattern between two same poles (N-N or S-S) facing each other.

• Field lines push out sideways from each pole.
• A NEUTRAL POINT exists between them — where the two fields are equal and opposite → cancel → no net force on a compass placed there.

Field pattern between two opposite poles (N-S) facing each other.

• Straight, parallel lines from N → S → uniform field (spec 6.7).

Earth's magnetic field. The Earth itself acts like a giant bar magnet, but with the south magnetic pole near the geographic NORTH (so that a compass north-end points TOWARDS geographic north). Useful background for spec 6.4.

Induced magnetism. When a magnetic material (iron, steel, cobalt, nickel) is placed in a magnetic field, it itself becomes (temporarily) magnetised.

Pole orientation. The face of the material NEAREST the inducing pole becomes the OPPOSITE pole. For example:

• Iron nail held below the S pole of a magnet → top of nail becomes N → unlike poles attract → nail lifts up.
• Iron nail held below the N pole of a magnet → top of nail becomes S → unlike poles still attract → nail lifts up.

Always attractive. Whichever pole of the magnet you use, the near face of the magnetic material develops the opposite pole. Result: the force between the magnet and the unmagnetised iron is ALWAYS attractive.

Chain of attraction. A magnet can pick up a chain of paper clips:

• Magnet's S pole → top of clip 1 = induced N. → clip 1 attracted upward.
• Bottom of clip 1 → induced S (= the same pole as the magnet's facing pole would have been, since the clip has now polarised).
• Bottom S of clip 1 induces N on top of clip 2 → clip 2 hangs from clip 1.
• And so on, with the field weakening at each step.

Permanent vs temporary magnetisation. Soft iron LOSES its induced magnetism the moment the inducing field is removed. Steel retains some of the induced magnetism. This is why a soft-iron core makes a good electromagnet (switches off cleanly) and steel makes good permanent magnets.

Aim. Investigate the magnetic field pattern around a bar magnet and between two bar magnets.

Method 1 — plotting compass (gives DIRECTION and shape).

1. Place the magnet flat on plain paper. Draw around it.
2. Place a small plotting compass near one end. Mark a dot at the TAIL and TIP of the compass needle.
3. Move the compass so its tail starts where the tip was. Mark the new tip.
4. Continue until you reach the other pole or run out of space.
5. Join the dots with a smooth curve and ADD AN ARROW pointing from N to S (the direction the N-end of the compass was pointing).
6. Repeat starting from different points to build up the field pattern.

Method 2 — iron filings (gives SHAPE only, no direction).

1. Place the magnet on a sheet of plain paper.
2. Sprinkle iron filings over the paper around the magnet.
3. Gently tap the paper. The filings align along the field lines.
4. The shape of the field becomes visible.

Comparison.

• Plotting compass: gives direction (arrows), more time-consuming, gives clearer single lines.
• Iron filings: quick, gives overall pattern visibly, but no arrows.

Two bar magnets — variant. Place magnets with N-N (or N-S) facing each other and repeat either method. Observe the NEUTRAL POINT (N-N case) or the UNIFORM FIELD (N-S case, spec 6.7).

The setup. Two bar magnets with their NORTH and SOUTH poles facing each other across a small gap (or a horseshoe magnet whose N and S faces are close).

The field pattern in the gap. Field lines are:

• STRAIGHT (not curved).
• PARALLEL to each other.
• EVENLY SPACED.
• Pointing from N face to S face (left → right, say).

Why 'uniform'. A uniform field is one where the field strength AND direction are the same at every point. The parallel, evenly-spaced lines represent this.

Why this matters. The uniform field between opposite poles is the standard setup for:

• Demonstrations of the FORCE on a current-carrying wire (spec 6.12).
• The motor effect (spec 6.13, Fleming's left-hand rule).
• The behaviour of charged particles in a uniform field (spec 6.11P, Paper 2).

Edge effects. Right at the edge of the gap, the lines curve outward — the field is NOT perfectly uniform near the edges, but is uniform in the central region.