Glossary term
Magnetic Flux
The surface integral of magnetic flux density through a defined area, linking magnetic fields to induction and magnetic circuits.
Definition
quantityMagnetic flux is the integral of magnetic flux density over a surface, representing how much magnetic field passes through that area.
Magnetic flux is calculated as the surface integral of magnetic flux density over an area. For a uniform field normal to a flat surface, Phi = B A. It is central to Faraday induction, transformers, inductors, electric machines, magnetic sensors, relays, eddy-current devices, and magnetic materials. Changing magnetic flux induces voltage in a circuit, while material properties and geometry determine how flux is guided or opposed.
Magnetic flux measures how much magnetic flux density passes through a surface:
For a uniform magnetic field perpendicular to a flat area:
where \Phi is magnetic flux, B is magnetic flux density, and A is area. The SI unit is the weber.
Connection to induction
Changing magnetic flux induces voltage. Faraday’s law states that the induced voltage in a coil is proportional to the rate of change of flux linkage:
where N is the number of turns. This is the basis of transformers, generators, inductors, electric motors, magnetic pickups, and many sensors. The negative sign expresses Lenz’s law: the induced voltage opposes the change in flux that caused it.
Magnetic circuits and materials
In magnetic cores, flux is guided by high-permeability materials, but real materials saturate. Once saturation occurs, a large increase in magnetizing current produces only a small increase in flux. Hysteresis and eddy currents create losses, especially under alternating fields. Air gaps, leakage flux, fringing, core geometry, temperature, and material grade all affect the useful flux in a device.
Common mistakes
A common mistake is confusing magnetic flux with magnetic flux density. Flux is an integral over area, measured in webers. Flux density is measured in tesla. Another mistake is assuming all generated flux links the intended winding or air gap. Leakage flux can reduce efficiency, create electromagnetic interference, induce unwanted currents, and heat nearby conductive parts. Good magnetic design states field path, core material, air gap, frequency, peak flux density, saturation margin, and loss model.