State that the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point

4.1 Simple Phenomena of Magnetism

Learning Objective

State that the direction of a magnetic field at a point is the direction of the force that would act on the north (N) pole of a magnet placed at that point.

Key Concepts

• A magnetic field is the region around a magnet (or a current‑carrying conductor) in which a magnetic pole experiences a force.
• Magnetic field lines are a convenient visual aid; they emerge from the north pole of a magnet and enter the south pole.
• The direction of the magnetic field at any point is defined by the direction a north‑pole would be pushed if it were placed there (the “north‑pole rule”).

Forces Involving Magnets

1. Between magnetic poles

• Like poles repel; unlike poles attract.
• Qualitatively, the force is directly proportional to the product of the pole strengths (m₁ m₂) and inversely proportional to the square of the separation (r²):
  

  F ∝ m₁ m₂ / r²

• In the IGCSE syllabus the proportionality constant is not required.

2. Between a magnet and magnetic materials

• Ferromagnetic materials (e.g., iron, nickel, cobalt) are attracted because the external field induces a temporary dipole within the material.
• Example: A paper‑clip is drawn toward a fridge magnet when the magnet’s field aligns the domains of the iron in the clip.

3. Induced magnetism

• When a piece of soft iron is placed in a magnetic field, the field aligns the atomic domains, turning the piece into a temporary magnet.
• The induced magnetism disappears as soon as the external field is removed.

4. Permanent vs. temporary magnets

5. Magnetic vs. non‑magnetic materials

• Magnetic (ferromagnetic): iron (Fe), nickel (Ni), cobalt (Co) and most of their alloys.
• Non‑magnetic (practically unaffected): plastic, wood, aluminium, copper, water, air, most other metals.

Defining the Direction of a Magnetic Field

Place a small test magnet (or a compass needle) at the point of interest. The direction in which the north pole of the test magnet is forced is taken as the direction of the magnetic field B at that location.

Drawing Magnetic Field‑Line Patterns

• Field lines start on the north pole and end on the south pole.
• The density of lines indicates field strength – the closer the lines, the stronger the field.
• At any point, the tangent to a field line gives the direction of the magnetic field.

Plotting Field Lines Experimentally

1. Place a bar magnet on a flat surface with its poles clearly marked.
2. Cover the magnet with a thin sheet of paper.
3. Scatter a fine layer of iron filings over the paper.
4. Tap the paper gently; the filings align along the field lines, revealing the pattern.
5. Alternatively, use a tiny freely rotating bar magnet or a compass needle; the needle points in the direction of the field at its position.

Experimental Determination of Field Direction (Compass Method)

1. Place a bar magnet on a flat surface.
2. Position a compass a short distance away, ensuring it does not touch the magnet.
3. Observe the direction in which the north end of the compass needle points.
4. Draw an arrow from the compass toward the magnet; this arrow represents the direction of the magnetic field at the compass’s location.

Mathematical Representation

The magnetic flux density (magnetic field strength) is denoted by \$B\$ and measured in tesla (T). The force \$F\$ on a magnetic pole of strength \$m\$ (in weber, Wb) placed in a magnetic field \$B\$ is

\$F = mB\$

Because the pole strength of a north pole is taken as positive, the force on a north pole points in the same direction as the field \$B\$.

Summary Table of Symbols

Key Points to Remember

• The magnetic field direction is the direction of the force on a north pole.
• Field lines exit the north pole and enter the south pole of a magnet.
• At any point, the tangent to a field line gives the direction of the magnetic field.
• Using a compass or iron‑filings provides a practical way to visualise field direction and pattern.
• Ferromagnetic materials are attracted to a magnet because the external field induces a temporary dipole.
• Permanent magnets retain magnetisation; soft‑iron magnets do not.

Common Misconceptions

• Misconception: The magnetic field points from south to north.
  

  Correction: Field lines are drawn from north to south; the direction of the field is the direction a north pole would move – i.e., from north toward south.

• Misconception: The field is strongest at the centre of a bar magnet.
  

  Correction: The field is strongest near the poles, where the lines are most densely packed.

• Misconception: All metals are attracted to magnets.
  

  Correction: Only ferromagnetic metals (Fe, Ni, Co) are strongly attracted; aluminium, copper, etc., are essentially non‑magnetic.

Practice Question

Question: A small compass is placed near the north pole of a bar magnet. The north end of the compass needle points directly toward the magnet’s north pole. What does this indicate about the direction of the magnetic field at the compass’s location?

Answer: The magnetic field at that point is directed toward the magnet’s north pole – the same direction that a north pole would experience a force.