The first dipole remains perpendicular to the separation and is now also perpendicular to the second dipole. Both terms in the point-dipole interaction vanish, giving \b[$U=0$]. This is the electrostatic point-dipole contribution, not a claim that real finite water molecules have no other intermolecular interaction.


Solution

Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / A10 / b / Solution

 Solution (source code)