4.1 Simple Phenomena of Magnetism
Learning objective
Students will be able to describe the pattern and direction of magnetic field lines around a bar magnet and to apply this knowledge to typical Cambridge IGCSE 0625 (Section 4.1 Core) exam tasks.
1. Magnetic field – definition
- A magnetic field is the region of space in which a magnetic pole would experience a force.
- Field‑lines are a convenient visual representation of this region. They show the direction a north‑pole‑seeking needle would point.
2. Basic magnetic forces & materials
- Attraction and repulsion – unlike poles repel, unlike poles attract.
- Every magnetic material can be classified as:
- Magnetic* (ferromagnetic) – e.g. iron, steel, nickel, cobalt. They are attracted strongly to a magnet.
- Non‑magnetic* – e.g. wood, plastic, copper. They feel no noticeable force.
- Every magnet has a **north (N)** and a **south (S)** pole. The force on a pole is strongest near the ends of the magnet and weakest at the centre.
3. Induced (temporary) magnetism
When a piece of ferromagnetic material (e.g. an iron nail) is placed in the magnetic field of a permanent magnet, the domains inside the material align and the object becomes a **temporary magnet**. It will retain magnetisation only while it remains in the external field and loses it when the field is removed.
4. Permanent vs. temporary magnets
| Feature | Permanent magnet (hard‑steel) | Temporary magnet (soft‑iron) |
|---|
| Retention of magnetisation |
Retains magnetisation for a long time (months‑years) |
Magnetised only while in an external field; loses magnetisation quickly when removed |
| Typical examples |
Bar magnet, fridge‑door magnet |
Paper‑clip attracted to a strong magnet, iron nail placed near a magnet |
| Use in syllabus terminology |
“Permanent (hard‑steel) magnet” |
“Temporary (soft‑iron) magnet” |
5. Magnetic field lines – pattern
- Lines **emerge from the north pole**, curve round the sides of the magnet and **enter the south pole**.
- Inside the magnet the lines continue from **south → north**, completing closed loops.
- Key properties:
- Never cross.
- Density of lines = qualitative indication of field strength (more lines = stronger field).
- Outside the magnet the field is strongest near the poles and weakest in the central region.
6. Checklist for drawing field‑line patterns (exam tip)
- Start each line at the **north pole** (draw an arrow away from N).
- Make the line curve smoothly round the magnet and end at the **south pole** (arrow into S).
- Inside the magnet draw lines from **south → north** to close the loops.
- Ensure that **no two lines cross**.
- Show a greater concentration of lines near the poles to indicate stronger field.
- Label the direction of the field with arrows on every line.
7. Direction of the field at any point
- The direction is given by the **tangent** to the field line at that point, following the arrow.
- Examples:
- Just outside the north pole – tangent points **away from the pole** (perpendicular to the surface).
- Mid‑way between the poles – tangent is almost **parallel to the length of the magnet**, pointing from N toward S.
8. Right‑hand rule for a bar magnet (magnetic version)
Grip the magnet with your right hand so that the thumb points toward the north pole**. Your fingers naturally curl around the magnet; the direction in which they curl shows the **direction of the magnetic field lines outside the magnet** (from N to S). Inside the magnet the field runs from S to N.
Check question: If you grip the magnet with the thumb pointing toward the **south pole**, which way do the field lines go outside the magnet?
Answer: They run from S to N (the opposite of the usual N→S direction).
9. Practical demonstration – visualising field lines
- Materials: bar magnet, white sheet of paper, fine iron filings, small compass.
- Procedure:
- Place the magnet in the centre of the paper.
- Evenly sprinkle a thin layer of iron filings over the paper.
- Gently tap the paper to help the filings settle into the field pattern.
- Observe the pattern and note where the filings are most densely packed (near the poles).
- Use the compass at a few points to confirm that the needle aligns with the tangent to the nearest line.
- What to observe:
- Lines emerge from N and enter S.
- Higher line density (more filings) near the poles → stronger field.
- Inside the magnet (if a thin slice is used) the direction reverses (S → N).
- Safety & clean‑up: Keep iron filings away from electronic devices, phones, and computers. Collect filings with a stiff piece of paper and dispose of them safely.
10. Uses of permanent magnets (linked to AO2/AO3 skills)
- Fridge doors – explain how the magnetic attraction holds the door shut (AO2: describe the magnetic force).
- Speakers & headphones – describe how a changing magnetic field interacts with a coil to produce sound (AO3: explain the principle of electromagnetic induction).
- Electric motors & generators – discuss the role of permanent magnets in converting electrical energy to mechanical energy and vice‑versa (AO3).
- Magnetic clasps – explain the simple attraction that keeps bags or jewellery closed (AO2).
11. Supplement – field lines of a solenoid (higher‑ability learners)
A long current‑carrying solenoid produces a magnetic field that is:
- Parallel and uniform inside the coil, directed from the solenoid’s **south face → north face**.
- Looping round the ends in the same way as a bar magnet, giving a pattern that closely resembles that of a permanent magnet.
This provides a useful bridge to the study of electromagnets later in the syllabus.
12. Summary table
| Feature |
Outside the magnet |
Inside the magnet |
| Origin of lines |
North pole (N) |
South pole (S) |
| Termination of lines |
South pole (S) |
North pole (N) |
| Direction of arrows |
From N → S |
From S → N |
| Line density (relative field strength) |
Highest near the poles, lower in the central region |
Essentially uniform (qualitative only) |
13. Key points to remember
- Magnetic field lines always form closed loops.
- Outside a magnet they run **N → S**; inside they run **S → N**.
- The direction of the field at any point is given by the tangent to the line, following the arrow.
- Higher line density = stronger magnetic field (qualitative only).
- Right‑hand rule: thumb = north pole; fingers curl in the direction of the field outside the magnet.
- When drawing, remember the checklist in section 6.
14. Typical examination question
“A bar magnet is placed on a sheet of paper. Sketch the magnetic field lines around the magnet and label the direction of the field at points A (near the north pole) and B (mid‑way between the poles).”
Answer outline:
- Draw several lines emerging from the north pole, curving round the sides and entering the south pole; include a few interior lines from S back to N.
- Place arrows on every line pointing away from N and toward S.
- At point A the arrow points outward, perpendicular to the surface of the magnet.
- At point B the arrow is almost parallel to the length of the magnet, pointing from N toward S.
15. Note on further modules
This note covers the core requirements of Section 4.1. Separate modules will be provided for the remaining IGCSE 0625 topics: 4.2 Electrical quantities, 4.3 Electric circuits, 4.4 Electrical safety, and 4.5 Electromagnetic effects. Each will follow the same clear, syllabus‑aligned format.