Q. Two opposite and equal charges $4 \times 10^{- 8}\ coulomb$ when placed $2 \times 10^{- 2}cm$ away, form a dipole. If this dipole is placed in an external electric field $4 \times 10^{8}\ newton/coulomb,$ the value of maximum torque and the work done in rotating in through $180^{o}$ will be
A) $64 \times 10^{- 4}Nm$ and $64 \times 10^{- 4}\ J$
B) $32 \times 10^{- 4}Nm$ and $32 \times 10^{- 4}\ J$
C) $64 \times 10^{- 4}Nm$ and $32 \times 10^{- 4}\ J$
D) $32 \times 10^{- 4}Nm$ and $64 \times 10^{- 4}\ J$
Explanation
(d) Dipole moment $p = 4 \times 10^{- 8} \times 2 \times 10^{- 4} = 8 \times 10^{- 12}m$
Maximum torque $= pE = 8 \times 10^{- 12} \times 4 \times 10^{8}$
$= 32 \times 10^{- 4}Nm$
Work done in rotating through $180^{o} = 2pE$
$= 2 \times 32 \times 10^{- 4} = 64 \times 10^{- 4}J$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.