Two identical circular loops, P and Q, each of radius r and carrying equal currents arekept in the parallel planes having a common axis passing through O. The direction of current in P is clockwise and in Q is anti-clockwise as seen from O which is equidistant from the loops P and Q. Find the magnitude of the net magnetic field at O.



The standard formula for field at an axial point is given as 
(see attachment)

Now, as the current flowing in loop P is clockwise by using right hand thumb’s rule the direction of the magnetic field will be towards left and as the current in loop Q is clockwise then the direction of magnetic field is towards left. So the net magnetic field at point O will be the sum of the magnetic fields due to loops P and Q.Also, as the fields produced are at an equal distance to O, BP = BQ,So, net field