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 10 ^{6} NC**

^{–1}**. The maximum torque exerted by the field on the dipole is**

**(a) 6×1****0**^{–3}** Nm (b) 3×1****0**^{–3}** Nm (c) 15×1****0**^{–3}** Nm**

**(d) 1.5×1****0**^{–3}** Nm (e) 9×1****0**^{–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 *pE*sinθ 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}) ×10^{6} = 1.5×10^{–3} Nm.