First slide

Electric dipole

Question

Two opposite and equal charges 4 x l0^-8 coulomb when placed 2 x 10^-2 cm away, form a dipole. If this dipole is placed in an external electric field 4 x 10⁸ newton/ coulomb, the value of maximum torque and the work done in rotating it through 180 ° will be

Easy

A

64 x 10^-4 Nm and 64 x 10^-4 J
B

32 x 10^-4 Nm and 32 x 10^-4 J
C

64 x 10^-4 Nm and 32 x 10^-4 J
D

32 x 10^-4 Nm and 64 x 10^-4 J

Solution

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 ° = 2pE $= 2 \times 32 \times 10^{- 4} = 64 \times 10^{- 4} J$

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