Magnetic flux measures the total field passing through an area

Magnetic Flux & Flux Linkage — AQA A-Level Physics

Key Definition

Magnetic flux (Φ) — The product of magnetic flux density and the cross-sectional area perpendicular to the field. Symbol: Φ. Unit: weber (Wb).

\Phi = BA

$Φ$: magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area. (Wb). (Wb)
$B$: magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³.The strength of a magnetic field. The force per unit length per unit currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). on a currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).-carrying conductor perpendicular to the field. Measured in teslaThe SI unit of magnetic flux density. One tesla is the flux density when a force of 1 N acts on a 1 m conductor carrying 1 A perpendicular to the field. (T). (T)
$A$: cross-sectional area perpendicular to the field (m²)

Flux densityMass per unit volume of a material. Measured in kg m⁻³. B tells you how strong the field is at a point. Flux Φ tells you how much total field passes through a given area. They answer different questions.
Think of it like rain. B is how hard it's raining (drops per square metre). Φ is how much rain actually lands in your bucket. A bigger bucket (larger A) catches more rain, even if the rainfall intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. (B) stays the same.
If you double the area, you double the flux. If you double the field strength, you double the flux. Flux is proportional to both.
$\Phi = BA$ only works when the field is perpendicular to the area. If the field hits the surface at an angle, you need the cos θ version (next card).
The concept of flux becomes essential in the next spec point (3.7.5.4 — Faraday's lawThe magnitude of the induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is proportional to the rate of change of magnetic flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns)..). A changing flux is what induces an EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. No change in flux, no EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V)..

Flux through a rectangular area

A uniform magnetic field (parallel arrows) passing through a rectangular area A oriented perpendicular to the field. Flux lines pass straight through the area. Labelled B (field), A (area), Φ = BA.

Examiner Tips and Tricks

$\Phi = BA$ is on the data sheet.
But be careful — this is the special case where the field is perpendicular to the area.
Most exam questions involve an angle, so you'll need $\Phi = BA \cos \theta$.