Question

how can direction of magnetic field line at a place be determined

Answer 1

The direction of the cross product may be found by application of the right hand rule as follows: The index finger points in the direction of the velocity vector v. The middle finger points in the direction of the magnetic field vector B. The thumb points in the direction of the cross product F.

Answer 2

By right hand thumb rule,


Keep your thumb in the direction of current, then the direction in which your fingers rotate is the direction of magnetic field