The total number of magnetic field lines crossing through the surface measures the magnetic flux through any surface held in a magnetic field. It is a scalar quantity. Weber is the unit of magnetic flux.
Magnetic flux linesIf we consider a number of small compass needles around a magnet, each compass needle experiences a torque due to the field of the magnet. The torque acting on the compass needle aligns it in the direction of the magnetic field.
The path along which the compass needles are aligned is known as the magnetic flux lines.Properties of magnetic flux lines(i) The magnetic flux lines form closed continuous loops. We imagine magnetic field lines to be extending through the body of the magnet from south pole to north pole.(ii) Outside the body of the magnet, the direction of magnetic flux lines is from north pole to south pole. Inside the body of the magnet, the direction of the magnetic flux line is from south pole to north pole.(iii) At any given point, tangent to the magnetic flux line gives the direction of magnetic field at that point.(iv)
The magnitude of the magnetic field at any point is represented by the number of magnetic flux lines passing normally through unit area around that point. So, crowded magnetic flux lines represent a strong magnetic field and the distant magnetic flux lines represent the weak magnetic field.(v) No two magnetic flux lines can intersect each other. Because, if the two magnetic flux intersects then at a single point the magnetic field has two directions at the intersecting point, which is not possible.(vi) The magnetic flux lines have a tendency to contract longitudinally indicating attraction between unlike magnetic poles.
(vii) The magnetic flux lines have a tendency to dilate laterally indicating repulsion between the like poles of the magnet.Conclusion on magnetic flux lines:In any particular region, where there is no magnetic field, there is no any magnetic flux line. Magnetic flux lines exist inside every magnetized material. Magnetic flux lines are imaginary. Magnetic flux lines are closed continuous loops while electric flux lines are discontinuous loops.