Question

Two point charges +4q and +q are kept at a distance of 30 cm from each other. At which point between them, the field intensity will be equal to zero?

(a) 15cm away from the +4q charge

(b) 20cm away from the +4q charge

(c) 7.5cm away from the +q charge

(d) 5cm away from the +q charge

I have been asked this question in an internship interview.

Asked question is from Electric Field in section Charges and Fields of Physics – Class 12

Answer 1

Right answer is (b) 20cm away from the +4q charge

Explanation: The electric field at a distance of r from a charge q is equal to \(\frac {q}{4\pi\varepsilon_or^2}\). Let the electric field intensity will be zero at a distance of x cm from +4q charge, so the fields due to the two charges will balance each other at that point. Therefore \(\frac {4q}{4\pi\varepsilon_ox^2}=\frac {q}{4\pi\varepsilon_o(30-x)^2}\). Solving this we get x=20cm. Therefore the point will be 20cm away from the +4q charge.