(a) State Gauss's law for magnetism. Explain its significance.
(b) Write the four important properties of the magnetic field lines due to a bar magnet.

Explanation

(a) According to Gauss. law for magnetism , "the net magnetic flux through may closed surface is zero" i.e.,
`phi_B = oint vecB .""vecd s = 0`
It signifies that in magnetism isolated monopoles do not exist. There are no sources/sinks of magnetic field `vecB`. The simplest magnetic element is a dipole or a current loop.
(b) Four important properties of the magnetic field lines due to a bar magnet are given below:
(1) Magnetic field lines are a visual and intuitive realisation of the magnetic field. A magnetic field line is a smooth curve in a magnetic field, tangent to which at any point gives the direction of magnetic field at that point.
(2) In free space around a magnetic dipole the magnetic field lines start from N-pole and end at S-pole. However, inside the magnet they travel from S-pole to N-pole. Thus, magnetic field lines of a magnet or a solenoid form continuous closed curves.
(3) The magnetic field lines do not intersect one another. It is so since the direction of the magnetic field would not be unique at the point of intersection.
(4) The larger the number of magnetic field lines crossing per unit normal area in a given region, the stronger is the magnetic field `vecB` there.