StellarMagnetic flux measures how much magnetic field passes through a surface. It is defined by
For a uniform magnetic field through a flat surface, this becomes
where θ is the angle between the magnetic field and the area vector. This means magnetic flux depends not only on the strength of the field, but also on the area of the surface and the orientation of that surface. If the field is parallel to the area vector, the flux is maximum. If the field is perpendicular to the area vector, the flux is zero.
This is conceptually similar to electric flux, but here the source is magnetic field rather than electric field. Magnetic flux is important because changes in flux produce induced emf in the next topic. The key idea is that flux is a scalar quantity built from a vector field and an oriented surface. In more advanced situations, the field can vary over the surface, which is why the integral form matters.