A molecule has a permanent electric dipole moment of $10^{-29} ext{ C m}$. A mole of this substance

Question

A molecule has a permanent electric dipole moment of $10^{-29}	ext{ C m}$. A mole of this substance is polarised by a strong electrostatic field of $10^6	ext{ V/m}$. If the field direction is suddenly changed by $60^\circ$, estimate the heat released by the substance. (Assume 100% polarisation)

Professor PC Thomas Classes · Answer

Total dipole moment $p = 6 	imes 10^{23} 	imes 10^{-29} = 6 	imes 10^{-6}	ext{ C m}$. Initial potential energy $U_i = -pE\cos(0^\circ) = -6 	imes 10^{-6} 	imes 10^6 = -6	ext{ J}$. Final potential energy $U_f = -pE\cos(60^\circ) = -6 	imes 0.5 = -3	ext{ J}$. Change in energy = $-3 - (-6) = 3	ext{ J}$. This loss in potential energy is released as heat.