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Ampère’s Theory of Magnetization

Ampère’s Theory of Magnetization, proposed by André-Marie Ampère, is a foundational concept in the field of electromagnetism that explains the magnetic properties of materials based on the presence of molecular currents. This theory helps in understanding how and why materials exhibit magnetic behavior.

Key Concepts

Basic Assumption

  • Molecular Currents: Ampère postulated that the magnetic properties of materials are due to tiny, closed-loop currents circulating within the molecules of the material. These currents are often referred to as “molecular currents” or “Ampèrian currents.”


  • Magnetization \((\mathbf{M})\): Magnetization is the vector field that expresses the density of magnetic dipole moments in a material. According to Ampère’s theory, this is a result of the alignment of molecular currents within the material.
  • Magnetic Dipoles: Each molecule with a circulating current behaves like a tiny magnetic dipole, with a north and south pole.

Theory Explanation

  1. Circulating Currents: Ampère suggested that even in non-magnetized materials, molecules possess circulating currents. These currents create tiny magnetic fields.
  2. Alignment of Dipoles: In an unmagnetized state, these molecular magnetic dipoles are randomly oriented, and their magnetic fields cancel each other out.
  3. External Magnetic Field: When an external magnetic field is applied, it influences these molecular currents, causing the magnetic dipoles to align in the direction of the applied field.
  4. Resulting Magnetization: This alignment of dipoles enhances the overall magnetic field within the material, resulting in magnetization.

Mathematical Representation

The magnetization \((\mathbf{M})\) in a material can be described as:
\mathbf{M} = \frac{1}{V} \sum_i \mathbf{m}_i


  • \(V\) is the volume of the material.
  • \(\mathbf{m}_i\) is the magnetic dipole moment of the \(i\)-th molecule.

Applications and Implications

Understanding Ferromagnetism

  • Ferromagnetic Materials: In ferromagnetic materials (like iron), the molecular currents are naturally aligned in regions called domains. When these domains align due to an external magnetic field, the material exhibits strong magnetic properties.

Paramagnetism and Diamagnetism

  • Paramagnetic Materials: In paramagnetic materials, the alignment of molecular currents is weak and only occurs in the presence of an external magnetic field.
  • Diamagnetic Materials: In diamagnetic materials, the induced molecular currents oppose the applied magnetic field, resulting in weak, opposite magnetization.

Ampère’s Theory of Magnetization provides a microscopic explanation for the magnetic properties of materials by attributing these properties to molecular currents. This theory has significant implications in understanding various types of magnetism, including ferromagnetism, paramagnetism, and diamagnetism. By considering the behavior of molecular currents, Ampère’s theory offers a fundamental perspective on how magnetic fields originate and how materials respond to them.

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