Two point like charges  q1=4.0  nC and q2=1.0  nC  occupy fixed positions in free space. If at every point on a curve surrounding the charges as shown in figure, net electrostatic potential (V) created by these charges is 900 V,  the separation between these charges in cm is  ----------

⇒  The electric field at the point where equipotential ends itself is zero. It means that at line joining the charges the point where field is zero the potential is 900 V.kq1r12=kq2r22       .....(1)4r1 2=2r 22⇒r1=2 r2.........2kq1r1+q2r2=900   .......(3)9×1094×10-9r1+1×10-9r2=9004 r2 +r1=100 r1r2......44r2+2r2=100×2 r2×r2⇒r2=3×10-2r1=2r2=6×10-2r1+r2=9×10-2=9 cm