[填空题]
At each corner of a square of l/o(f/ 4p v dc,kkh2rnside l there are8ql;u vp((w h+4lo z31cvedsg7pz te4 point charges of magnitude Q, 2Q, 3Q, and 4Q (Fig. 16–52). Determine the ;sep ogwdqtc1vl p3784h(4uvlz+( z eforce on (a) the charge 2Q, and (b) the charge 3Q, due to the other three charges.
$F_{2Q}$ = $\frac{kQ^2}{l^2}$
$\theta_{2Q}$ = $^{\circ}$
$F_{3Q}$ = $\frac{kQ^2}{l^2}$
$\theta_{3Q}$ = $^{\circ}$
