Explanation
(a)A magnetic field is a region in which a force of magnetic origin is experienced. It is usually due to the presence of magnetic flux which is a vector quantity.
(b)
A straight wire carrying current is filled through a hole in a cardboard as shown. Compasses are placed round the conductor or the cardboard. The needles of the compass will be noticed to deflect ina particular direction. They all conform to a circular pattern indicating that the magnetic field around the conductor carrying current is circular or concentric in shape. When the current is reversed, the direction of the compass needle reverses as well.
(c)
(d) Current in the same direction

The material is surrounded by a ring of iron. This shields the flux of the earth away from the material.


