The magnetic field. Quantities describing the magnetic field
A magnetic field exists around permanent magnets and current-carrying conductors and acts with a force on a magnetic needle or a current-carrying wire placed in it. Outside the magnet the field lines are closed curves directed from the north (N) pole to the south (S) pole, so the magnetic field is called a vortex field (electrostatic field lines, by contrast, start and end on charges). The force characteristic of the field is the vector magnetic induction B; its SI unit is the tesla (T), 1 T = 1 N/(A·m). B points along the tangent to a field line, and the denser the lines, the larger B. The magnetic flux through a flat surface of area S is Φ = B·S·cosα, where α is the angle between B and the normal to the surface; flux is a scalar, measured in webers (Wb), 1 Wb = 1 T·m². The flux is largest when the field is perpendicular to the surface (α = 0°) and zero when the lines run parallel to it (α = 90°).
Lay a sheet of paper on the desk and draw «field lines» on it with a pencil: first perpendicular to the sheet (dots), then parallel to it (arrows). For each case count how many lines cross the surface and decide when the flux is larger. Only paper and pencil are needed.