A long wire is bent into a circular loop of radius R and a current I is passed through it produces a magnetic field B at its centre. If the same wire is bent into two turns of smaller redius and the same current is passed through it, how does the magnetic field at the centre change?

Explanation

First case, `B=(mu_0I)/(2R)`
Second case, `B=(mu_0"2I")/(2R/2)=(4mu_0I)/(2R)=4B`