Magnetic flux (Φ)
Electromagnetism - OCR A-Level Physics
Key Definition
Magnetic flux (Φ)
The product of the magnetic flux density and the area perpendicular to the field. Measured in weber (Wb). 1 Wb = 1 T m².
The product of the magnetic flux density and the area perpendicular to the field. Measured in weber (Wb). 1 Wb = 1 T m².
$$\Phi = BA\cos\theta$$
- Maximum flux when the field is perpendicular to the plane of the coil (θ = 0°, the normal is parallel to B).
- Zero flux when the field is parallel to the plane of the coil (θ = 90°, the normal is perpendicular to B).
- Flux linkageThe product of 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). 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). = NΦ, where N is the number of turns. Measured in Wb turns or simply Wb.
- For a coil rotating in a uniform field: NΦ = $BAN\cos\theta$, where θ changes with time.
Common Mistake
MEDIUM
Wrong: θ in $\Phi = BA \cos \theta$ is the angle between B and the plane of the coil.
Right: θ is the angle between B and the NORMAL to the coil. When B is perpendicular to the coil plane, the normal is parallel to B, so θ = 0° and $\Phi = BA (maximum)$.
Right: θ is the angle between B and the NORMAL to the coil. When B is perpendicular to the coil plane, the normal is parallel to B, so θ = 0° and $\Phi = BA (maximum)$.