Describe the forces between magnetic poles and between magnets and magnetic materials, including the use of the terms north pole (N pole), south pole (S pole), attraction and repulsion, magnetised and unmagnetised

4.1 Simple Phenomena of Magnetism

Learning Objective

Describe the forces between magnetic poles and between magnets and magnetic materials, using the terms:

• North pole (N pole)
• South pole (S pole)
• Attraction
• Repulsion
• Magnetised
• Unmagnetised

Key Concepts

All permanent magnets have two distinct ends called magnetic poles. The end that points towards the Earth's geographic north is called the north pole (N); the opposite end is the south pole (S). The magnetic field lines emerge from the N pole and enter the S pole.

Forces Between Magnetic Poles

The interaction between two magnetic poles follows the same rule as electric charges:

• Like poles (N–N or S–S) repel each other.
• Unlike poles (N–S) attract each other.

The magnitude of the force \$F\$ between two isolated magnetic poles can be expressed by an analogue of Coulomb’s law:

\$F = \frac{\mu0 \, m1 \, m_2}{4\pi r^{2}}\$

where \$m1\$ and \$m2\$ are the pole strengths (in ampere‑metres), \$r\$ is the separation distance, and \$\mu_0 = 4\pi \times 10^{-7}\,\text{N·A}^{-2}\$ is the permeability of free space.

Forces Between Magnets and Magnetic Materials

Magnetic materials can be classified as:

• Ferromagnetic (e.g., iron, nickel, cobalt) – strongly attracted to magnets.
• Paramagnetic – weakly attracted.
• Diamagnetic – weakly repelled.

When a ferromagnetic material is placed near a magnet, the material becomes magnetised (its domains align) and experiences an attractive force toward the nearest pole, regardless of whether the pole is N or S. Unmagnetised ferromagnetic material shows no net attraction until the external field aligns its domains.

Summary Table

Practical Demonstrations

1. Place two bar magnets on a table. Observe that like poles push apart while opposite poles pull together.
2. Bring a small iron nail close to a magnet. Note that the nail is attracted to either pole, indicating that the nail becomes magnetised.
3. Use a compass near a magnet. The needle aligns with the magnetic field lines, pointing toward the magnet’s north pole.

Common Misconceptions

• “Magnetism only works between two magnets.” – Ferromagnetic materials become magnetised in the presence of a magnetic field and are attracted.
• “North and south are fixed labels on any object.” – Only a magnet has distinct poles; an unmagnetised piece of iron has no permanent poles.
• “All metals are attracted to magnets.” – Only ferromagnetic metals show strong attraction; others may be weakly attracted or repelled.