Question

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 flowing in a circular coil.

Answer 1

Representation of the magnetic field. Path along which an imaginary free north pole would tend to move. The tangent at any point on the magnetic field line gives the direction of the magnetic field at that point.

(a) Emerge at north pole and merge at south pole. Inside the magnet, the direction of field lines is
from south pole of magnet to its north pole, are closed curves.

(b) At the points where the magnetic field is stronger, field lines are crowded and vice-versa.

(c) No two magnetic field lines can intersect each other.

Answer 2

The magnetic field direction is the same direction a compass needle points, which is tangent to the magnetic field line at any given point.

Magnetic lines of force have a number of important properties, which include:*They seek the path of least resistance between opposite magnetic poles.
*They never cross one another.
*They all have the same strength.

Magnetic field produced by a current carrying circular coil.  

(1) A rectangular cardboard having holes is taken and a circular coil having large number of turns is inserted in the cardboard. The coil should be inserted in the cardboard in such a manner that the coil is normal to the plane of the cardboard. 

(2) The ends of the coil are connected in series with a battery, a key and a rheostat. 

(3) Then iron filings are sprinkled uniformly on the cardboard. 

(4) Now the key is plugged. 

(5) On tapping the cardboard gently, the circular magnetic field  lines near the current carrying loop are observed. On moving away from the circular loop, the concentric circles representing magnetic field lines become bigger and bigger. At the centre of the circular loop, the magnetic field lines are straight.