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Obtain the expression of torque acting on electric dipole in uniform external electric field. |
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Answer» Solution :Consider a PERMANENT dipole of dipole moment `vecP`in a uniform external FIELD `vecE` as shown in figure. There is a force `-qvecE` on `-q` and a force `-qvecE` on - q. The net force on the dipole is zero, since `vecE`is uniform. However, the charge are separated, so the forces ACT at different points, resulting in a torque on the dipole. When the net force is zero, the torque (couple) is independent of the origin. Magnitude of torque =(Magnitude of each force) x (Perpendicular distance between the two forces) `=qE xx 2a SIN theta` `=2qaEsin theta` Its direction is NORMAL to the plane of the paper, coming out of it. `vectau = vecp xx vecE` `therefore tau = pEsin theta` This torque will tend to align the dipole with the field `vecE` . When `vecp`is aligned with `vecE`the torque is zero. |
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