1.

A charge q is placed at the centre of the line joining two equal charges Q. The system of the three charges will be in equilibrium , if q is equal to

Answer»

`-(Q)/(2) `
` -(Q)/(4) `
` +(Q)/(4) `
` +(Q)/(2) `

Solution :For equlibrium of system the net force on any charges must be ZERO. Considering net force on A, we have `oversetto(F_(AC))+oversetto(F_(AB))=0 `
` THEREFORE ""(1)/(4 pi in _0) .(Qq)/((®/(2))^(2)) +(1)/(4 pi in _0 ).(Q^(2))/(r^(2)) =0 rArr(1)/(4 pi in _0) .(Q)/(2) [4Q +Q] =0 `
or`""4q +Q=0 or q=-(Q)/(4) `


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