Question

2 point charges 4Q, Q are seaparated by 1m in air. At what point on the line joining the charges is the electric field intensity zero?

Answer 1

4Q ------------t ----------Q

two charge seperated 1 m distance ,
let at r distance from 4Q charge Electric field intensity is zero .

we know,
Electric field intensity = KQ/r²
so,
Electric field by 4Q charge =K4Q/r²
=4KQ²/r²

Electric field by Q charge =KQ/(1-r)²
=KQ/(1-r)²

because electric field intensity =0
so,
electric field by 4Q charge =electric field by Q charge

4KQ/r² = KQ/(1-r)²

4/1 = r²/(1-r)²

take square root both sides ,

2/1 = r/(1-r)

2(1-r) =r

2- 2r =r

r =2/3

hence, at 2/3 m from 4Q charge electric field intensity zero .