CBSE Class 12 · 2005 · ElectrostaticsShort Answer

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An electric dipole is held in a uniform electric field.
(i) Using a suitable diagram, show that it does not undergo any translatory motion, and
(ii) derive an expression for the torque acting on it and specify its direction.

Solution

(i) The charge

+ q

feels a force

+ q E

and

- q

feels

- q E

.
These are equal and opposite, so the net force is zero and the dipole does not translate.
(ii) The two forces form a couple; the perpendicular distance between their lines of action is

2 a sin θ

τ = (q E) (2 a sin θ) = pE sin θ, p = q (2 a)

τ = p \times E

perpendicular to the plane of

p

and

E

, tending to align the dipole with the field.

CBSE Class 12 Board · 2005 · Subjective

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