Two concentric rings, one of radius a and the other of radius b, have the charges +q, and `-(2//5)^(-3//2)q`, respectively as shown in fig.

Find the rario `b//a` if a charge particle palaced on the axis at z =a is i equilibrium.

Answer

`2`

Explanation

The charge at 'P' is in equilibrium
Hence `E_(P)=0, vec(E)_(P)=vec(E)_(Pa)+vec(E)_(Pb)=vec(0)`
`(Kqa)/((a^(2)+a^(2))^(3//2))-(Kq(2/5)^(-3//2))/((a^(2)+b^(2))^(3//2))=0rArr b/a=2`