Question

Two equal and opposite charges of $ 2\times {{10}^{-10}}C $ are placed at a distance of 1 cm forming a dipole and are placed in an electric field of $ 2\times {{10}^{5}} $ N/C. The maximum torque on dipole is:

• A. $ 2\sqrt{2}\times {{10}^{-6}}Nm $
• B. $ 8\times {{10}^{8}}Nm $
• C. $ 4\times {{10}^{-9}}Nm $
• D. $ 4\times {{10}^{-7}}Nm $

Answer 1

Answer: (D)

From the formula for torque $ \vec{\tau }=\vec{p}\times \vec{E}=pE\sin \theta $ For maximum value of torque $ \sin \theta ={{90}^{o}}=1 $ Hence, $ {{\vec{\tau }}_{\max }}=pEq.2l=(2\times {{10}^{-10}}\times 1\times {{10}^{-2}}) $ $ \times (2\times {{10}^{5}}\,N/C) $ $ =4\times {{10}^{-7}}\,Nm $