The direction of magnetic field lines can be obtained by Right Hand Thumb Rule
Right Hand Thumb Rule
If we hold a current carrying conductor in the grip of the right hand in such a way that thumb points in the direction of current , then curling of fingers represents the direction of magnetic field lines .
Magnetic Field on the Axis of a Current Carrying Circular Coil
Magnetic field at axis at a distance x from centre O .
where , r = radius of the coil , n = number of turns in the coil and I = current , At centre of the coil , B = μ o nl / 2 r
If we look at one face of the coil and the direction of current flowing through the coil is clockwise , then that face has south polarity and if direction of current is anti-clockwise , then that face has north polarity .