Magnetism and Magnetic Fields

This lesson explains Magnetism and Magnetic Fields for Cambridge IGCSE Physics 0625. It covers the Core knowledge required by every candidate. Use the diagrams, method steps and error checks rather than memorising a paragraph. Read the explanation once, then attempt the worked method from a blank page.

What is a magnetic field and how do you draw it?

A magnetic field is the region around a magnet where a magnetic pole experiences a force. Field lines run from the north pole to the south pole outside the magnet. Closer lines mean a stronger field. Around a bar magnet, lines curve from N to S and never cross.

Two rules govern poles. Like poles repel; unlike poles attract. Repulsion is the only test for a magnet. Attraction proves nothing, because a magnet also attracts unmagnetised iron.

Term	Meaning
Permanent magnet	Keeps its magnetism (made from hard magnetic material, e.g. steel)
Induced magnet	Becomes magnetic only inside another field (soft material, e.g. iron)
Hard magnetic material	Hard to magnetise, hard to demagnetise (steel)
Soft magnetic material	Easy to magnetise, loses magnetism quickly (iron)
Magnetic materials	Iron, steel, nickel, cobalt

Why does the iron vs steel choice matter in exams?

Because applications depend on it. An electromagnet core must switch off instantly, so it uses soft iron. A compass needle or fridge magnet must keep its magnetism, so it uses steel. State the material and the reason together.

To plot a field you lay a compass (plotting compass) beside the magnet, mark where the needle points, move the compass to that mark, and repeat. Joining the dots gives one field line. Iron filings show the pattern faster but do not show direction.

Worked example

A student holds an unmarked metal bar near the north pole of a known magnet. The bar is attracted. (a) Explain why this does not prove the bar is a magnet. (2 marks) (b) Describe a test that would prove the bar is a magnet. (2 marks)

Solution. (a) Magnets attract unmagnetised magnetic materials such as iron. So the bar could be a magnet or could simply be made of iron or steel. (b) Bring each end of the bar in turn near the known north pole. If one end is repelled, the bar is a magnet, because only two like poles repel.

Original marking points:

• B1: attraction occurs for both magnets and unmagnetised magnetic material.
• B1: therefore attraction cannot identify a magnet.
• M1: test each end against a known pole and look for repulsion.
• A1: repulsion proves it is a magnet / only magnets repel.

These marking points belong to this original example. They are not an official Cambridge mark scheme.

Common errors and how to correct them

• Using attraction as the test for a magnet. Fix: repulsion is the only valid test. State the distinction clearly.
• Field lines drawn S to N or crossing each other. Fix: arrows go N to S outside the magnet, lines never touch or cross.
• Swapping iron and steel. Fix: iron = soft = temporary (electromagnet cores); steel = hard = permanent (compass needles).
• Calling copper or aluminium magnetic. Fix: only iron, steel, nickel and cobalt are magnetic in 0625. Copper appears in questions as a deliberate trap.
• Vague “stronger field” answers. Fix: say “field lines are closer together, so the field is stronger”.

How to apply this in an exam

Field-drawing questions are assessed through visible features rather than overall appearance. A field-drawing question may credit the line shape, arrow direction, symmetry and spacing separately. Draw four to six smooth lines, add an arrowhead to every line, and keep spacing tighter near the poles. Check every arrow before moving on.

Where this skill matters

Magnetism is Core content, so every candidate sits it. Paper 1 and Paper 2 can test pole rules, materials or field patterns. Paper 3 and Paper 4 can use a structured question to draw the field of a bar magnet, explain induced magnetism, or justify a material choice. It also feeds the electromagnetic effects subtopic, where Extended candidates use field ideas in motors and transformers. Paper 5 and Paper 6 occasionally include compass-plotting as a described practical. Learn the definitions accurately and apply them to the question.