A circular loop of radius `R` carries current `I_2` in a clockwise direction as shown in figure. The centre of the loop is a distance `D` above a long, straight wire. What are the magnitude and direction of the current `I_1` in the wire if the magnetic field at the centre of loop is zero?

Explanation

At the centre of the circular loop the current `I_2` generates a mag-
netic field that is into the page. So the current `I_1` must point to
the right. For complete cancellation the two fields must have the
same magnitude
`(mu_0I_1)/(2piD)=(mu_0I_2)/(2R)`
Thus `I_1=(piD)/R I_2`