How many external forces act on each dipole and why?

[Lee33]

Homework Statement

Two identical permanent dipoles, each consisting of charges $+q$ and $-q$ separated by a distance $s$, are aligned along the $x$ axis, a distance $r$ from each other, where $r >> s.$

a. How many external forces act on each dipole?

Homework Equations

Columb's law

The Attempt at a Solution

For a, I got for each dipole only two external forces acts on them from the dipole next to them. So for the net external forces it will be four. But its wrong, why is that?

 

[ehild]

How many external forces act on each dipole?

You have dipole A and dipole B. What external forces act on dipole A?

ehild

 

[Lee33]

Dipole B will be an external force on A.

 

[ehild]

Dipole B is a dipole, not a force. And you wrote previously that it acts by two forces on dipole A. What are they?
B exerts two forces on dipole A, and also dipole A exerts two forces on dipole B. The question was the number of forces acting on one dipole, and it is the same for both dipoles. You can not add forces acting on different bodies.
ehild

 

[Lee33]

So, the external force acting on dipole A will be the only the two charges making up dipole B?

 

[ehild]

So, the external force acting on dipole A will be the only the two charges making up dipole B?

The only external forces on dipole A will be the forces exerted on both its charges by the two charges, making up dipole B.
One dipole has its electric field http://en.wikibooks.org/wiki/Physics_Exercises/Electrostatics and you can determine the force on both charges of the other dipole in that field.

ehild