Where exactly is the energy of an inductor stored — in the magnetic core, the magnetic field, or the air gap?

The air gap is the “main concentration area” of magnetic field energy, but it is not the source of energy. In a magnetic core structure with an air gap, the magnetic permeability of the air gap is close to that of air (approximately 0), which is much lower than that of the magnetic core material. Due to the conservation of magnetic flux, the magnetic induction intensity B in the magnetic core and the air gap is roughly equal, and the energy density is inversely proportional to the magnetic permeability (ω = B² / 2μ). This makes the energy density of the air gap much higher than that of the magnetic core. For example, when the magnetic permeability of the magnetic core is μ = 1000, the energy density of the air gap is 1000 times that of the magnetic core. In practical applications, in inductors with air gaps, the energy stored in the air gap can account for more than 90%, making it the main concentration area of energy. However, the role of the air gap is to “undertake” energy rather than “generate” it — if there is no magnetic field passing through the air gap, even if the air gap exists, it cannot store energy.

Essentially, the magnetic field is the “direct carrier” of energy, the magnetic core is the “constrainer” of the magnetic field, and the air gap is the “main concentration area” of magnetic field energy. The three form an organic connection through the magnetic field. The current generates a magnetic field through the coil, and the magnetic field is concentrated in the magnetic core and the air gap under the constraint of the magnetic core. The air gap becomes the main energy storage area due to its advantage in energy density. This is like the water flow (current) pushing the waterwheel (coil) to form a water flow field (magnetic field), the dam (magnetic core) restricts the water flow in the river channel (magnetic circuit), and the drop (air gap) becomes the main concentration point of water energy — the essence of energy is the movement of the water flow (magnetic field), and other structures only affect its distribution state.

In summary, the energy of an inductor is neither simply stored in physical structures such as the magnetic core or air gap, nor abstractly attached to a certain concept. Instead, it exists in the space covered by the magnetic field in the form of magnetic field energy. The magnetic core reduces energy dispersion by constraining the magnetic field, and the air gap becomes the main concentration area of energy by virtue of its own characteristics, while the magnetic field is the fundamental carrier of energy. Understanding this logic can not only clarify the essence of energy storage but also grasp the core idea of “magnetic core constraining the magnetic field and air gap optimizing energy storage” in inductor design.

PS: The purpose of adding an air gap to the magnetic core is to prevent the magnetic core from saturating. If that’s the case, why can’t we choose a magnetic core with lower magnetic permeability from the beginning, so that there is no need to add an air gap? How to explain this?