Question

An electric dipole will be in stable equilibrium if the angle between the axis of the dipole and the electric field is ________

(a) 0 degree

(b) 180 degree

(c) 90 degree

(d) 45 degree

This question was posed to me in my homework.

Origin of the question is Dipole in a Uniform External Field topic in section Charges and Fields of Physics – Class 12

Answer 1

The correct choice is (a) 0 degree

Easy explanation: Torque acting on a dipole is p*E*sinθ where E is the electric field. Now θ is 0 degree, so sinθ becomes 0 and hence no torque acts on the dipole. So in this case, no force or torque acts on the dipole. Therefore it will be the condition of stable equilibrium. In the case of θ=180 degree, sinθ is also 0 but the condition is known as unstable equilibrium i.e. if we rotate the dipole a bit, it will not come back to its initial position.