Magnetic Flux | CAIE 9702 Paper 4 Notes

Paper 4

Physics9702

Magnetic Flux

Magnetic flux measures the magnetic field passing through a chosen area. It combines field strength with the area presented to the field.

Field through one area

A loop facing the field presents its full area. Turning the loop edge-on reduces the perpendicular area to zero. Field lines in the figure are a model of direction and spacing; they are not physical threads.

Definition

2 marks

What is meant by magnetic flux?

Model answer: The product of magnetic flux density and the area perpendicular to the magnetic field.

Recent marking gave one mark for the product of flux density and area, and one mark for the perpendicular condition (9702/42/M/J/23 Q6(a)(i)).

\Phi=BA

Symbols

$\Phi$ = magnetic flux (Wb)
$B$ = magnetic flux density (T)
$A$ = area perpendicular to the field (m²)

Swipe left or right to see the whole diagram →

Always measure θ between the magnetic field and the normal to the area. Parallel B and normal gives maximum flux, BA. Tilting the normal gives BA cos θ; at 90°, the loop is edge-on and flux is zero.

For a tilted area, measure the angle from the normal, which is an imaginary line at 90° to the surface.

\Phi=BA\cos\theta

When $\theta$ is 0°, the field is parallel to the normal and flux is maximum.
When $\theta$ is 90°, the field is parallel to the surface and flux is zero.
Flux is measured in webers.
$1\ \mathrm{Wb}=1\ \mathrm{T\,m^2}$

Your turncalculation check

A 2.0 × 10⁻³ m² loop faces a 0.30 T magnetic field. Calculate the magnetic flux through the loop.

Show worked answer

\Phi=BA=(0.30)(2.0\times10^{-3})

Answer

\Phi=6.0\times10^{-4}\ \mathrm{Wb}