Question

Find the angle at which the potential due a dipole is measured, when the distance from one charge is 12cm and that due to other is 11cm, separated to each other by a distance of 2cm.

(a) 15

(b) 30

(c) 45

(d) 60

I had been asked this question in a national level competition.

Enquiry is from Electric Dipole in chapter Electrostatic Fields of Electromagnetic Theory

Answer 1

Right answer is (d) 60

The explanation: Here, the two charges are separated by d = 2cm.

The distance from one charge (say Q1) will be R1 = 11cm. The distance from another charge (say Q2) will be R2 = 12cm. If R1 and R2 is assumed to be parallel, then R2 – R1 = d cos θ. We get 1 = 2cos θ and cos θ = 0.5. Then θ =

cos^-1(0.5) = 60.