Magnetism

Every bar magnet has two ends called poles — a north-seeking pole (N) and a south-seeking pole (S). The names come from how a freely-suspended magnet lines up with the Earth.

There are two rules you must know:

• Unlike poles attract (N–S pull together).
• Like poles repel (N–N or S–S push apart).

The force is a non-contact force — it acts through air, paper, glass and most plastics without anything touching.

How can you tell whether something is a magnet or just a magnetic material? Attraction alone is not proof — a piece of unmagnetised iron is attracted to a magnet too. The only reliable test is repulsion: if two objects repel each other, they must both be magnets.

Only a small number of materials are attracted to a magnet. The common magnetic (ferromagnetic) materials are:

• Iron, steel (iron + carbon), and most iron alloys
• Cobalt
• Nickel
• Some special alloys such as alnico (aluminium-nickel-cobalt) and neodymium-iron-boron (NdFeB)

Everyday materials such as copper, aluminium, plastic, wood, water and glass are non-magnetic — a magnet has no effect on them.

Magnetic materials are classified as hard or soft by how easily they magnetise and how long they stay magnetised:

Property	Soft magnetic material	Hard magnetic material
Example	Soft iron	Steel, alnico
Easy to magnetise?	Yes	No (needs strong field)
Easy to demagnetise?	Yes — loses magnetism quickly	No — keeps magnetism
Typical use	Electromagnet core	Permanent magnet

Soft iron is "soft" magnetically, not physically — the word refers to how readily it follows a changing magnetic field. That is why it is used in the cores of electromagnets, relays and transformers: it turns on and off cleanly.

A magnetic field is the region around a magnet where another magnet or magnetic material feels a force. We draw the field with magnetic field lines that obey three rules:

1. Lines come out of the north pole and into the south pole (outside the magnet).
2. Lines never cross — at every point the field has just one direction.
3. Closer lines = stronger field. The field is strongest near the poles.

Two practical ways to plot a magnetic field:

• Iron filings: sprinkle fine iron filings onto a sheet of paper over a magnet and gently tap the paper. The filings line up along the field lines, showing the pattern quickly but without arrows.
• Plotting compass: place a small compass near the magnet. The N-end of the compass needle points along the field line at that point. Move the compass and mark the direction at each location to trace the line. This method does show direction.

The Earth's magnetic field behaves as if a giant bar magnet sat inside the Earth, tilted slightly from the spin axis. Because a compass needle's N end points to geographic north, the Earth's geographic North Pole is actually a magnetic south pole.

Magnetic induction is the process by which a magnetic material becomes a magnet temporarily, simply by being placed in a magnetic field. A steel paper clip held near the N pole of a magnet becomes a small magnet itself — its end nearest the N pole becomes a south pole. This is why a chain of paper clips can hang from a single magnet.

How to magnetise a magnetic material:

• Stroking — drag the end of a strong permanent magnet along an iron bar repeatedly, always in the same direction. The iron's atomic-scale magnets line up.
• Electrical method — put the bar inside a coil (solenoid) carrying a steady direct current. The magnetic field of the coil magnetises the bar; steel keeps the magnetism after the current is switched off.

How to demagnetise a magnet:

• Heat it strongly — above the Curie temperature its atomic magnets jiggle out of alignment.
• Hammer or drop it repeatedly — the impacts shake the alignment apart.
• Place it inside a coil carrying an alternating current that is slowly reduced to zero — the field decays in random directions and washes out the alignment.

Inside the metal: picture millions of tiny atomic magnets ("domains"). In an unmagnetised piece they point in random directions and cancel out. In a magnetised piece they all line up the same way, giving the bar an overall N and S pole. This domain model is a simple but useful picture for MYP level — you don't need to know the quantum details.