The key idea
Explain magnetic forces using poles, field direction and induced magnetism. These ideas also help you choose materials for permanent magnets and electromagnets, and prepare for the motor and generator effects.
Understand it · 01
Explain attraction without assuming two magnets
A magnet has a north pole and a south pole. Like poles repel and unlike poles attract. An initially unmagnetised piece of a suitable magnetic material can also be attracted: the nearby magnet induces magnetism in it. Its nearer end develops the opposite polarity, so attraction alone is not a reliable proof that the object was already a permanent magnet.
For the usual school test with known poles and freely responding bar magnets, repulsion is the useful evidence that the tested object is magnetised. State the observation rather than assuming every attracted metal must be a magnet. Many familiar metals, including copper and aluminium, are not strongly attracted in the way iron is. Magnetism is not a property of all materials that conduct electricity.
Understand it · 02
Draw a field with direction and relative strength
A magnetic field line shows the direction a small north-seeking compass pole would point. Outside a bar magnet, the direction runs from its north pole towards its south pole. A compass at one position lies tangent to the local field line. Lines that are closer together represent a stronger field in the diagram, so the field is usually drawn more densely near the poles.
Field lines do not cross, because the field at one point cannot have two different directions. They are a representation of a continuous field, not physical threads that exist only where you drew ink. A complete magnetic field forms loops; the familiar outside north-to-south pattern is only part of that loop. In an experiment, small compass readings can build a map of directions around the magnet.
Understand it · 03
Choose soft or hard magnetic materials for the job
A magnetically soft material such as soft iron magnetises readily and loses most of its induced magnetism when the applied field is removed. That makes it suitable for an electromagnet that should release an object after the current is switched off. A magnetically hard material retains magnetism more effectively and is suitable for a permanent magnet. “Hard” here describes magnetic behaviour rather than simply resistance to scratching.
An electromagnet combines a current-carrying coil with a suitable core. Increasing the current or using more turns in the same coil length can strengthen the field; a soft-iron core can strengthen it further. Reversing the current reverses the poles. The design still has practical limits: the wire heats as current rises, and the core does not keep strengthening the field without limit.
Understand it · 04
Measure strength with a fair comparison
To investigate the effect of current, keep the coil geometry, core and test objects unchanged. Change the current in controlled steps and record a measure such as the mass supported before the load detaches. Repeat each reading because contact between the core and load can vary. Counting paperclips can provide a rough comparison, but clip arrangement and their own induced magnetism make it less precise than it first appears.
Keep the test current on only as long as needed and monitor heating, which can change the coil resistance and current. When investigating the number of turns, avoid changing the current unintentionally as well. A graph should label exactly what was measured: “number of clips lifted” is an operational measure of the setup’s lifting performance, not a direct measurement of magnetic flux density in tesla.
Key definitions
- Magnetic field
- A region in which a magnetic pole experiences a force. Outside a bar magnet, field lines point from its north pole towards its south pole.
- Induced magnetism
- Magnetism produced in a magnetic material by an applied magnetic field. In the usual bar-magnet experiment, the nearer end of an initially unmagnetised iron object develops the opposite pole, producing attraction.
- Soft magnetic material
- A material (e.g. iron) that magnetises and demagnetises easily — used for electromagnet cores.
- Hard magnetic material
- A material (e.g. steel) that is difficult to magnetise but keeps its magnetism — used for permanent magnets.
Worked examples, step by step
Example 1
An initially unmagnetised iron bar is brought close to either pole of a magnet. Explain why it is attracted in both cases, and why attraction alone is not a test for a permanent magnet.
Follow the method
- The applied field induces magnetism in the iron bar.
- Whichever magnet pole approaches, the nearer end of the iron develops the opposite polarity.
- The resulting attraction can therefore occur without the iron being a permanent magnet beforehand.
Induced magnetism produces attraction in both arrangements. Use repulsion from a known pole as evidence of magnetisation in the usual bar-magnet test.
Practice questions with worked solutions
Try each question on paper before opening the solution. These are original academy questions; the suggested marks are a self-checking guide, not an official exam-board mark scheme. Difficulty labels describe this practice set, not predicted grades.
Starter · 3 suggested marks
1. Interpret field direction
A compass is placed just outside the north pole of a bar magnet. State which way its north-seeking end points, how it would align along a curved field line, and what closely spaced lines represent.
Show solution and self-check: question 1
- Outside the magnet, field direction is away from north and towards south.
- A compass aligns tangentially to the field line at its position.
- Closer line spacing represents a stronger field.
Away from the north pole; tangent to the local field line; a stronger field.
Self-check · one mark per point
- Give the outward direction near north.
- Describe tangential compass alignment.
- Link dense lines to greater strength.
Build · 4 suggested marks
2. Control an electromagnet experiment
A student claims that adding turns made an electromagnet stronger, but also changed the current from 0.5 A to 0.9 A. Explain why the conclusion is not secure and how to improve the investigation.
Show solution and self-check: question 2
- Both turn number and current affect field strength.
- The test changed two variables, so the effect cannot be assigned to turns alone.
- Keep current constant while changing turns, using the same core and lifting test.
- Repeat each measurement to assess variation.
Control current and the rest of the apparatus, change turns alone, and repeat the lifting measurements.
Self-check · one mark per point
- Identify current as an additional influencing variable.
- Explain why two changes prevent a clear causal conclusion.
- Propose constant current with otherwise comparable apparatus.
- Propose repeated measurements.
Common mistakes & how to avoid them
- Using attraction alone to identify a permanent magnet. In the usual test with known poles, repulsion is evidence that the tested object is magnetised; attraction can also result from induced magnetism.
- Drawing field lines that cross. The field has one direction at a point; outside a bar magnet the arrows run from north to south.
- Choosing a material that strongly retains magnetism for an electromagnet that must release its load. Soft iron loses most of its magnetism when the current stops, while steel retains more.
- Saying stronger field where lines are far apart. Closer field lines mean a stronger field — the spacing is the strength.
Understand the why
Questions, explained
Start with the short answer, then follow the reasoning. Each explanation has a permanent link you can share with a classmate or return to when revising.
How do you test whether a bar is magnetised?
In the usual school test, bring a known magnet pole near a particular end of the bar and look for repulsion. Repulsion shows that the end is magnetised with the same polarity. Attraction alone is inconclusive because an unmagnetised magnetic material can also be attracted.
An iron object may be attracted to a north pole because that pole induces south polarity at the nearer end. The same object can also be attracted when the known magnet is reversed. That does not require it to have been a permanent magnet beforehand. In a careful test, keep track of which end is tested and avoid letting the bar turn freely into an attracting orientation before making the observation. State the pole used, the observed force and the conclusion supported by that observation.
Interpret repulsion
A known north pole repels the labelled end A of a bar. What can be concluded?
- Like poles repel in the standard bar-magnet test.
- End A must behave as a north pole.
The bar is magnetised, with north polarity at end A.
Why is soft iron used in electromagnets?
Soft iron magnetises readily in an applied field and loses most of that magnetism when the field is removed. This suits an electromagnet that must switch its attraction on and off. A material that strongly retains magnetism is better suited to a permanent magnet.
A lifting magnet needs a strong field while current flows but should release its load after switching off. The coil’s current supplies the applied field, while the core increases the magnetic effect. “Soft” describes how readily the magnetic state changes, not merely how easily the material can be bent or scratched. Some residual magnetism may remain, so “instantly loses every trace of magnetism” is too absolute. Connect the material’s behaviour to the device’s purpose instead of stating that one material is always better.
How can you increase the strength of an electromagnet?
Increase current, increase the number of turns in the same coil length, or add a suitable soft-iron core to strengthen a comparable electromagnet. Keep the other variables controlled when testing one change. More current also causes more heating and cannot be increased without practical limits.
A larger number of turns is most directly comparable when the coil’s dimensions are held fixed. If adding wire also raises resistance and reduces current, two effects may compete. Record the actual current rather than assuming the same supply setting produces it. A lifting test should use the same contact surface and load arrangement each time. Repeat measurements because paperclips or other objects can arrange differently and alter the result. The conclusion should describe the tested range rather than suggesting unlimited proportional improvement under all conditions.
Why can magnetic field lines never cross?
Magnetic field lines cannot cross because the field has one direction at a given point. A compass aligns with that local direction. Crossing lines would assign two different compass directions to the same position, making the representation inconsistent.
The lines are a drawing convention for a continuous field, so the spaces between them are not regions with no magnetic field. Where two magnets contribute, their fields combine to give one resultant field at each point. At a neutral point the resultant may be zero and a compass may not have a well-defined preferred direction from that field, but this is not a crossing of two valid resultant-field lines. Sketch the final pattern smoothly and use arrows to show direction outside the magnets.
Are all metals magnetic because they conduct electricity?
Electrical conductivity and strong magnetic attraction are different properties. Copper conducts electricity well but is not strongly attracted like iron in an ordinary bar-magnet test. A material being metallic or conducting does not by itself prove that it is a suitable electromagnet core.
A conductor has mobile charge carriers that can support an electric current. Strong ferromagnetic behaviour involves how magnetic effects within a material respond and align. School questions often contrast iron or steel with copper, aluminium or plastic, so select the property the device actually needs. Copper wire is useful in a coil because current can pass through it; soft iron is useful as a core because of its magnetic response. Swapping these explanations confuses the separate jobs performed by two materials within the same device.
How do I map a magnetic field using a plotting compass?
Place a small compass near a magnet, mark the direction of its north-seeking end, move it a short distance along that direction and repeat. Join the marked directions into smooth field lines, adding arrows. Use several starting positions to reveal the overall pattern.
Keep the magnet fixed so every reading belongs to the same field arrangement. Nearby magnets, electrical currents or steel objects can distort the pattern, especially where the field being mapped is weak. Small steps help a line follow the changing direction rather than cutting corners. Never force the compass to follow a line already imagined on the paper. Repeating the map around both poles also helps identify mistaken arrow directions: outside a bar magnet, the arrows should lead from north towards south.
Plan a field map
Give a short method for recording the field beside a bar magnet.
- Fix the magnet and mark its outline and poles.
- Mark both ends of the compass needle at one position.
- Move the compass forward along the indicated direction and repeat.
- Join the marks smoothly and add the north-seeking arrow direction.
Repeated local direction measurements build a field line.
Your learning checklist
Use these goals to check your understanding. Requirements vary by specification, tier and exam year; the official references below define your full course.
- Describe magnetic poles, attraction and repulsion, and induced magnetism
- Distinguish magnetic hard (steel) and soft (iron) materials
- Draw the magnetic field pattern of a bar magnet; know field direction is north to south
- Describe the field patterns around a straight wire and a solenoid
- Describe how to magnetise and demagnetise materials
Sources & how to cite these notes
Original explanations, illustrations and practice by Dr Desouky Physics Academy. Official specifications guide the course scope; questions and suggested marks on this page are our own revision material.
- Cambridge IGCSE Physics 0625 — 2026–2028 syllabus
- Pearson Edexcel International GCSE Physics 4PH1 — specification
Reference this page
Dr Desouky Physics Academy. IGCSE Physics: Magnetism. Updated 2026-09-25. https://www.drdesouky.com/revision/igcse-physics/magnetism
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