Know that the relative strength of a magnetic field is represented by the spacing of the magnetic field lines

4.1 Simple Phenomena of Magnetism

Objective

Understand the basic magnetic phenomena required by the Cambridge IGCSE Physics 0625 syllabus, in particular that the relative strength of a magnetic field is represented by the spacing of the magnetic field lines.

1. Definition of a Magnetic Field

• A magnetic field is the region around a magnet (or a current‑carrying conductor) in which a magnetic pole would experience a force.
• The field at any point is represented by the vector B (tesla, T).
• Field lines are a visual tool:

  • They emerge from the north pole of a magnet and enter the south pole.
  • The tangent to a field line at a point gives the direction of B there.
  • Lines never cross.

2. Forces Between Magnetic Poles

• Opposite poles (north ↔ south) attract; like poles (north ↔ north or south ↔ south) repel.
• The force is strongest when the poles are close together and weakens with increasing distance.
• The magnetic force on a pole arises from the interaction of its own field with the external magnetic field (superposition of fields).
• For a pole of strength p in a magnetic field B:

  \$F = B\,p\$

  (the force is directed along the field line for a north pole and opposite to it for a south pole).

• For a moving charge q with velocity v in a magnetic field:

  \$\mathbf{F}=q\,\mathbf{v}\times\mathbf{B}\$

  (the force is perpendicular to both v and B).

3. Permanent and Temporary Magnets – Uses

4. Induced (Temporary) Magnetism

• When a piece of soft iron is placed near a strong magnet, the external field aligns its domains, turning the iron into a temporary magnet.
• Demo: Hold a bar magnet close to a paper‑clip; the clip is attracted even though it is not permanently magnetic.
• Removing the magnet allows the domains to randomise, and the iron loses its magnetism.

5. Magnetic vs. Non‑magnetic Materials

• Magnetic (ferromagnetic) materials: iron (Fe), nickel (Ni), cobalt (Co) and many of their alloys – attracted strongly to a magnet.
• Non‑magnetic materials: wood, plastic, glass, rubber, aluminium, copper, etc. – show no attraction.

6. Magnetic Field Lines – What They Represent

• Direction: tangent to a line gives the direction of B.
• Relative strength: the density (spacing) of the lines.

  • Closer (denser) lines → stronger field.
  • Wider spaced lines → weaker field.

• Lines never intersect because a single point cannot have two different field directions.

7. Why Line Spacing Represents Strength (Quantitative Insight)

The magnetic force on a pole is proportional to the field strength (F = Bp). If the line density in one region is twice that in another, the field strength – and therefore the force on an identical pole – is roughly twice as large. This proportionality underpins the convention of drawing denser lines where B is larger.

8. Magnetic Field Patterns

8.1 Bar Magnet

• Lines emerge from the north pole, curve around the sides, and enter the south pole.
• Near the poles the lines are very close together (strong field); midway between the poles they spread out (weaker field).

8.2 Solenoid (long coil)

• Inside the coil the field lines are parallel, uniformly spaced and point from the south‑to‑north face – a strong, uniform field.
• Outside the coil the lines spread out and become increasingly spaced, showing the field weakens with distance.

8.3 Straight Current‑Carrying Wire (Supplementary)

• A current I produces concentric circular field lines centred on the wire.
• Direction is given by the right‑hand rule: grip the wire with the right hand so the thumb points in the direction of conventional current; the curled fingers show the direction of the magnetic field.
• This pattern is the basis for later topics (e.g., magnetic forces on a current‑carrying conductor).

9. Drawing Field‑Line Patterns – Practical Tip

1. Place a sheet of white paper over a bar magnet, the end of a solenoid, or a current‑carrying wire.
2. Sprinkle a fine, even layer of iron filings on the paper.
3. Tap gently to help the filings settle into the field pattern.
4. Observe the arrangement, then sketch the lines, keeping the spacing of your drawn lines proportional to the observed density.

10. Comparative Table of Field‑Line Spacing

11. Common Misconceptions

• Number of lines is a physical quantity: The number drawn is a convention; only the spacing conveys relative strength.
• Field lines are material objects: They are a representation, not something that exists physically.
• Field strength is uniform around a magnet: The varying spacing of lines shows the strength changes with position.
• All metals are magnetic: Only ferromagnetic materials (Fe, Ni, Co) are attracted; many metals are non‑magnetic.
• Magnetic force acts only between poles: It is actually an interaction between the magnetic field of one pole (or current) and the external field.

12. Quick‑Check Questions

1. If the field lines are drawn twice as close together in a region, how does the magnetic field strength compare to a region where the lines are spaced normally?
   Answer: Approximately twice as strong (since field strength is proportional to line density).
2. Describe what would happen to a compass needle placed in a region where the field lines are widely spaced.
3. Explain why the magnetic field inside a long solenoid appears uniform.
4. State the direction of the force on a north pole placed near the south pole of another magnet and give a real‑world example.
5. What happens to a piece of soft iron when it is moved away from a strong magnet after being attracted?
6. Using the right‑hand rule, predict the direction of the magnetic field at a point above a straight wire carrying current to the right.