A molecule of a substance has a permanent electric dipole moment of magnitude `10^(-30)` cm. A mole of this substance is polarised by applying a strong electrostatic field of magnitude `10^(7) V m^(-1).` The direction of field is charged by an angle `60^(@).` The heat released by the substance in aligning its dipole along the new direction of the field is

Explanation

here dipole moment of each molecule = `10^(-29)` C m. As 1 mole of the subtance contains ` 6 xx 10^(23)` molecules , total diploe moment of all the molecules,
` p = 6xx 10^(23) xx 10^(-29)` C m
` = 6 xx 10^(-6) ` C m
Initial potential energy, `U_(i) = -" pE cos"theta = - 6 xx 10^(-6) xx 10^(6) cos 0^(@)` = - 6 J
Final potential energy (when `theta = 60^(@), U_(f) = - 6 xx 10^(-6) xx 10^(6) cos 60^(@) = - 3 j`
Change in potential energy = -3 j - (- 6 J) = 3 J
So, there is loss in potential energy, this must be the energy released by the substance in the form of heat in aligning its dipoles.