Hello, I am reading the volume 2 of the Feynman's Lectures on Physics, and something is bothering me when he calculates the dipole moment of a single atom induced by an extern field : https://books.google.co.uk/books?id...=onepage&q=feynman dipole single atom&f=false Indeed, he states that : " p = qex " But why ? I would use in general : " p = Zqex " where Z is the number of electrons in the atom. x is the displacement of the center of charges of the electrons, and thus x is also the displacement of each electron. Could you explain his reasoning ? It is not the first time he uses a single electron charge instead of Z in his calculations, and I do not understand. Thanks. PS : First, I thought that was because the square of the natural pulsation ω0 depended on Z, which means that ω²0(Z) = Zω²0 (Z=1), which would simplify the Z replacing ω²0(Z) by Zω²0 (Z=1) ; but Feynman seems to use ω0 = ω0(Z) and not ω0 (Z=1) everywhere, so it does not matter.