Page "Magnetic flux" Paragraph 2

The magnetic interaction is described in terms of a vector field, where each point in space ( and time ) is associated with a vector that determines what force a moving charge would experience at that point ( see Lorentz force ).

Since a vector field is quite difficult to visualize at first, in elementary physics one may instead visualize this field with field lines.

The magnetic flux through some surface, in this simplified picture, is proportional to the number of field lines passing through that surface ( in some contexts, the flux may be defined to be precisely the number of field lines passing through that surface ; although technically misleading, this distinction is not important ).

Note that the magnetic flux is the net number of field lines passing through that surface ; that is, the number passing through in one direction minus the number passing through in the other direction ( see below for deciding in which direction the field lines carry a positive sign and in which they carry a negative sign ).