2 dipoles

vabamyyr

there are 2 dipoles. One with electrical moment 10pC*m and other 16pC*m. The distance between 2 dipoles is 20 mm. Dipole moments are located on the same line and are pointed in the same direction. The question is what is the force between 2 dipoles.

What i have achieved so far:

i think a way is to use Coulomb`s law. We look one dipole and add the forces that exist in the one-pole system.
the interaction between two dipoles as simply the sum of four pairwise terms which are dependent on the distances between the four charges of the dipoles (pos1-pos2, neg1-neg2, pos1-neg2 and neg1-pos2). i know that p=q*l
l is distance between +q and -q in one pole. in that summing equation i dont know l1, l2, q1, q2. There is also force of field E. When i have one dipole and whatever distance r from its center then E=k*p/r^3. But how to use this knowledeg remains yet a mystery for me.

Can someone help me?

ehild

You are on the right track. Derive the force on one dipole from the other in terms of the charges (q1, q2) and lengths (l1 and l2) of both dipoles and the distance R between their centers. Expand this formula in terms of l1/R and l2/R up to the non-vanishing second order terms in d1/R and d2/R. This epression will contain the product q1*l1 and q2*l2, which you can replace by p1 and p2.

ehild