Identical thin rods of length 2$a$ carry equal charges +Q uniformly distributed along their lengths. The rods lie along the $x$ axis with their centers separated by a distance $b>2 a(\text { Fig. } \mathrm{P} 23.75)$ . Show that the magnitude of the force exerted by the left rod on the right one is
$$F=\left(\frac{k_{e} Q^{2}}{4 a^{2}}\right) \ln \left(\frac{b^{2}}{b^{2}-4 a^{2}}\right)$$