How is magnetic field due to a straight current carrying wire affected if current in wire is (a) decreased, (b) reversed?

A straight wire lying in a horizontal plane carries a current from north to south. (a) What will be the direction of magnetic field at a point just underneath it? (b) Name the law used to arrive at the answer in part (a).

What is the direction of magnetic field at the centre of coil carrying current in (i) clockwise, (ii) anticlockwise direction?

How is the magnetic field due to a solenoid carrying current affected if a soft iron bar is introduced inside the solenoid?