What are magnetic field lines? List three characteristics of these lines. Describe in brief an activity to study the magnetic field lines due to a current carrying circular coil.

Magnetic field lines: These are the imaginary close curves which are used to represent the magnetic field around the magnet.
The properties of the magnetic field lines are listed below:
(a) Magnetic field lines start at the north pole and end at the south pole.
(b) Magnetic field lines do not intersect each other, because there can't be two directions of the magnetic field at any one point.
(c) The degree of closeness of the field lines depends upon the strength of the magnetic field. Stronger the field, closer are the field lines.
In order to find the magnetic field due to a coil, it is held in a vertical plane and is made to pass through a smooth cardboard in such a way that the centre ( $O$ ) of the coil lies at the cardboard. A current is passed through the coil and iron filings are sprinkled on the cardboard. These iron filings arrange themselves in a pattern similar to one shown in the figure. This pattern represents the magnetic field lines due to the coil.
In order to find the direction of magnetic field lines, we plot the magnetic field with the help of a compass needle. The pattern of magnetic field lines so obtained is shown in figure (b). From this pattern, the following important conclusion have been drawn.

(i) The magnetic field lines near the coil are nearly circular and concentric. This is due to the reason that the segments of the coil in contact with the board at the points $A$ and $B$ are almost like straight conductors. The direction of the field lines can also be found by applying right-hand thumb rule.
(ii) The field lines are in the same direction in the space enclosed by the coil.
(iii) Near the centre of the coil, the field lines are nearly straight and parallel. As such the magnetic field at the centre of the coil can be taken to be uniform.
(iv) The direction of the magnetic field at the centre is perpendicular to the plane of the coil.
(v) As we move towards the centre of the coil, the strength of magnetic field increases. Magnetic field is maximum at its centre. This is due to the reason that the two magnetic field (one due to the semicircular segment of the coil through $A$ and the other due to the semicircular segment through B) assist each other.
The magnitude of the magnetic field at the centre of the coil is directly proportional to the current flowing through it and total number of turns and inversely proportional to the radius of the coil.
This is due to the reason that the current in all the circular turns of the coil is in the same direction. As such, the resultant magnetic field due to the coil is equal to the sum of the field due to all these turns.