(23) Four equal point charges, \( q_{1}=q_{2}=q_{3}=q_{4}=+3 \mu \mathrm{C} \), are placed at the four corners of a square that has a side \( a=0.4 \mathrm{~m} \), see Fig. 19.22. (a) Find the force
19.4 Exercises
655
on \( q_{1} \). (b) Find the force exerted on a test charge of 1 C placed at the center \( P \) of the square.