Two thin cocentric and coplanar spherical shells , of radii 'a' and 'b' ` (b gt a) `, carry charges q and Q respectively. Find the magnitude of the electric at a point distant x from their common centre for
`0 lt x lt a, `
(b) ` a le x le b,`
(c) ` b le x lt infty`

Explanation

We know that electric field at a point inside a charged spherical shell is zero and for calculating field at an outer point the whole charge of spherical shell can be considered to be situated at the centre of shell. Hence
Field at a point `P_1 , "where a" 0 lt x lt a , `
` E =E_1 +E_2 =0 +0 =0 `
` (##U_LIK_SP_PHY_XII_C01_E10_027_S01.png" width="80%">
(b)
Field at a point `P_2` , where a `le x le b, `
` E = E_1 +E_2 = (q)/( 4 pi in_0x^(2)) +0 =(q)/( 4 pi in _0 x^(2)) `
(iii) Field at a point `P_3 ` where `b le x le infty `
` E =E_1 +E_2 =(q)/( 4 pi in_0x^(2)) +(Q)/( 4 pi in _0 x ^(2)) =((q+Q))/( 4 pi in _0 x ^(2)) `