The PE of an electric dipole in an external E-field is U=-p.E where p is it's dipole moment. I was under the impression I could find U, and then easily determine the force on the dipole using F=-∇U, to obtain Fx=px∂Ex/∂x+py∂Ey/∂x+pz∂Ez/∂x Fy=px∂Ex/∂y+py∂Ey/∂y+pz∂Ez/∂y Fz=px∂Ex/∂z+py∂Ey/∂z+pz∂Ez/∂z however my book annoyingly states that these should be Fx=px∂Ex/∂x+py∂Ex/∂y+pz∂Ex/∂z Fy=px∂Ey/∂x+py∂Ey/∂y+pz∂Ey/∂z Fz=px∂Ez/∂x+py∂Ez/∂y+pz∂Ez/∂z which they give a derivation for in a different way. However they do go on to prove later that FL=-∂U/∂L with L the direction in question. I'm now very confused. What is wrong with my approach?