why does field lines always emerge from north pole and merge at south pole , not vice-versa?
 
 
 
 
 
 
 
 
 
 
 
 
 

 

the magnetic field of a magnet is represented by a series of lines around the magnet. when a small north magnetic pole is placed in the magnetic field it will experience a force.these lines around the magnet represent the path of an imaginary independent north pole if it were allowed to move freely in the direction of the magnetic force.
hence we can say that the path traced by a north magnetic pole free to move under the influence of a magnetic field is called a magnetic field line.
 
magnetic lines of force or magnetic field lines shows the direction and strength of a magnetic field.
the direction of the magnetic field at any particular place is the direction in which the north pole of a magnetic compass needle points if placed at that point.since the direction of magnetic field line is the direction of force on a north pole, the magnetic field lines always begin from the north pole of a magnet and end on the south pole of the magnet.