Magnetic Flux
Definition
How much magnetic field passes through a surface — the quantity whose change creates electric current (Faraday's law).
Symbol: · Unit: Wb (weber)
Technical definition
Φ_B = ∫B·dA; for a uniform field through a flat loop, Φ = BA·cosθ. Units: webers (Wb). Faraday's law: the induced EMF equals the negative rate of change of flux, ε = −dΦ/dt.
Example
Turning a coil in a magnetic field changes the flux through it — that is exactly how generators produce electricity from rotation.
Common confusion
Flux depends on orientation, not just field strength. A loop edge-on to the field (θ = 90°) has zero flux through it — maximum field, minimum flux. Angle matters as much as magnitude.