The magnetic field lines are a visual and intuitive realisation of the magnetic field.
The field lines of (a) a bar magnet, (b) a current-carrying finite solenoid and (c) electric dipole. At large distances, the field lines are very similar. The curves labelled (i) and (ii) are closed Gaussian surfaces.
Their properties are:
(i) The magnetic field lines of a magnet (or a solenoid) form continuous closed loops. This is unlike the electric dipole where these field lines begin from a positive charge and end on the negative charge or escape to infinity.
(ii) The tangent to the field line at a given point represents the direction of the net magnetic field B at that point.
(iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B. In Fig.(a), B is larger around region (ii) than in region (i).
(iv) The magnetic field lines do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection.
