In Section 22.3 we claimed that a charged object exerts a net attractive force on an electric dipole. Let€™s investigate this. FIGURE CP22.77 shows a permanent electric dipole consisting of charges +q and -q separated by the fixed distance s. Charge +Q is distance r from the center of the dipole. We€™ll assume, as is usually the case in practice, that s << r.

a. Write an expression for the net force exerted on the dipole by charge +Q.
b. Is this force toward +Q or away from +Q? Explain.
c. Use the binomial approximation (1 + x)^-n ‰ˆ 1 - nx if x << 1 to show that your expression from part a can be written F_net = 2KqQs/r³.
d. How can an electric force have an inverse-cube dependence?
Doesn€™t Coulomb€™s law say that the electric force depends on the inverse square of the distance? Explain.

+) q -9 FIGURE CP22.77