Magnetic Field Strength

The Source of Magnetism: Understanding Magnetic Field Strength (H-field)

In electromagnetism, there are two distinct ways to quantify a magnetic field: the magnetic flux density (B-field) and the magnetic field strength (H-field). The H-field, also known as magnetic field intensity or magnetizing field, represents the 'cause' of a magnetic field. It is a measure of the strength of the magnetic field produced by external electric currents in a vacuum, independent of the material placed within the field. In essence, it is the raw magnetizing force generated by moving charges.

The distinction between the H-field and the B-field is crucial when dealing with magnetic materials. The H-field is produced by the external currents (like the current in a coil of wire). When a material is placed in this H-field, its internal magnetic dipoles may align, creating their own internal magnetic field. The B-field (magnetic flux density) represents the *total* resulting magnetic field—the sum of the external H-field and the material's internal response. In a vacuum, the B and H fields are simply proportional (B = μ₀H), but in materials, the relationship is more complex (B = μH). The SI unit for the H-field is Amperes per meter (A/m). This quantity is fundamental in the design of electromagnets, transformers, and magnetic recording heads, as it directly relates the applied current to the resulting magnetizing force.

Relevant Formulas in Science and Mathematics

• Ampere's Law (in terms of H-field): A fundamental law of magnetism, one of Maxwell's equations, states that the line integral of the H-field around a closed loop is equal to the free electric current passing through the loop: ∮ H ⋅ dl = I_free.
• H-field of a Solenoid: For a long, ideal solenoid, the H-field inside is uniform and given by H = nI, where 'n' is the number of turns of wire per unit length and 'I' is the current. This shows how the magnetizing force is directly created by the current in the wires.
• Relationship between B and H fields: The general relationship is B = μH = μ₀(H + M), where 'μ' is the permeability of the material, 'μ₀' is the permeability of free space, and 'M' is the magnetization of the material (its internal magnetic response).

A Deep Dive into Common Magnetic Field Strength Units

• Ampere per meter (A/m): This is the standard SI unit for magnetic field strength (H-field). It is derived directly from the definition in Ampere's law, representing the current per unit length.
• Oersted (Oe): The Oersted is the unit of magnetic field strength in the older CGS (centimeter-gram-second) system of units. It is named after the Danish physicist Hans Christian Ørsted, who discovered the connection between electricity and magnetism. One Oersted is defined as a dyne per unit pole. It is still sometimes used in the field of magnetism, particularly in the United States. 1 A/m is equal to 4π/1000 Oersteds.