State Gauss' theorem. With the help of this theorem, find out the electrical intensity at any nearby point due to a uniformly charged thin and long straight wire.
Or, Define electrical dipole moment. An electrical dipole is placed within a uniform electric field (E) and is rotated to an angle `angle(theta) = 180^(@)`. find out the work done.

Explanation

Two equal and opposite charge`pm`q, separated by a distance vector `vec(l)` directed from -q to +q, from an electric dipole. Its dipole moment is `vec(p) = q vec(l)`.
Torque on an electri dipole in an electric field `vec(E)` is, `vec(tau) = vec(p) xx vec(E)`.
Its magnitude is, `tau = p E sin theta, ` where ` theta` = angle between `vec(p) and vec(E)`.
`therefore` Work done to rotate the dipole from `0^(@)` to `180^(@)` is,
W = `int dW = int_(0)^(180) tau d theta`
= `p E int_(0)^(180) sin theta d theta = p E [ -cos theta]_(0)^(180) = 2pE`