Difficulties with dipoles & point charges.... Help would be appreciated a lot with the following problems: ~ A positive point charge +Q is at the origin, and a dipole of moment p is a distance r away (r>>L) and in the radial direction as shown below: http://img462.imageshack.us/img462/1482/dipole6fp.th.gif [Broken] a) Show that the force exerted on the dipole by the point charge is attractive and has a magnitude ~ 2kQp/r3 (see previous problem). [the previous problem was: an electric dipole consists of two charges +q and -q separated by a very small distance 2a. Its center is on the x axis at x=x1 and it points along the x axis in the positive x direction. The dipole is in a nonuniform electric field, which is also in the x direction, given by E=Cxi where C is a constant. - Find the force on the positive charge and that on the negative charge and show that the net force on the dipole is Cpi - Show that, in general if a dipole of moment p lies along the x axis in an electric field in the x direction the net force on the dipole is given approximately by (dEx/dx)pi]. I know the force on the negative side of the dipole is -kQq/r2 and the force on the positve side of the dipole is kQq/(r+L)2, but this leads to nothing..... I can derive an expression for the electric field of the dipole as k2p/r3 and conclude the force of the dipole on the point charge= the force on the dipole of the point charge (Newtons 3rd law)= QE, therefore F= Qk2p/r3, but that would ruin the second question below (because I need to use the formula derived in this question in order to prove E= k2p/r3 for a dipole. So how should I tackle this problem? b) Now assume that the dipole is centered at the origin and that a point charge Q is a distance r away along the line of the dipole. Using Newton's third law and your result for part (a), show that at the location of the positive point charge the electric field E due to the dipole is toward the dipole and has a magnitude of ~k2p/r3. I think I can handle this one if I get part (a) correct ~ Two neutral polar molecules attract each other. Suppose that each molecule has a dipole moment p and that these dipoles are aligned along the x-axis and separated by a distance d. Derive an expression for the force of attraction in terms of p and d. I do need some help with this one. First of all, if a line like this - is a symbol for the dipole, are the two aligned one after another (like - -) or parallel to each other (like =)? Secondly, I don't really know how to treat a system of dipoles if the distance isn't big (in the previous question it was) so I would really appreciate a couple of hints to give me something to start off with.... I totally don't have a clue..... PS Sorry for the big text!