The following MCQ which appeared in Kerala Medical Entrance 2007 question paper is straight forward, requiring your knowledge of the expression for torque on a dipole:
An electric dipole consists of two opposite charges each 0.05 μC separated by 30 mm. The dipole is placed in an uniform external electric field of 106 NC–1. The maximum torque exerted by the field on the dipole is
(a) 6×10–3 Nm (b) 3×10–3 Nm (c) 15×10–3 Nm
(d) 1.5×10–3 Nm (e) 9×10–3 Nm
The torque on a dipole of moment p in an electric field E is the vector product p×E. The magnitude of the torque (τ) is pEsinθ where θ is the angle between dipole moment vector and the electric field vector. The maximum value of the torque is pE (when θ = 90º).
Note that p = q×2a where ‘q’ is the charge and 2a is the separation between the charges.
Therefore, maximum torque = (0.05×10–6×30×10–3) ×106 = 1.5×10–3 Nm.
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