1 \begin{problem*}{20.69}
2 The $x$ axis is the symmetry axis of a stationary, uniformly charged
3 ring of radius $R$ and charge $Q$ (Fig.~P20.69). A particle with
4 charge $Q$ and mass $M$ is located at the center of the ring. When it
5 is displaced slightly, the point charge accelerates along the $x$ axis
6 to infinity. Show that the ultimate speed of the point charge is
8 v = \left(\frac{2 k_e Q^2}{MR}\right)^{1/2}
10 \end{problem*} % problem 20.69
13 Conserving energy, the inital energy is entirely electric,
15 E_i = U_e = k_e \frac{Q^2}{R}
17 because all the ring charge is a distance $R$ from the particle.
19 The final energy is entirely kinetic
21 E_f = K = \frac{1}{2} M v^2
26 k_e \frac{Q^2}{R} = E_i &= E_f = \frac{1}{2} M v^2 \\
27 v &= \ans{\sqrt{\frac{2 k_e Q^2}{M R}}}