There is a +4 microCoulombs (ìC) charge at each of 3 corners of a square (each side 58. mm long). What is the force on +10 microCoulombs (ìC) placed at the center of the square?
Let the charge q1 be in left bottom corner, then clockwise q2 and q3, and q4 =10microC in the center.
The forces F14=F34.
The force
F24=1/(4πε0)•(q2q4)/r2,
where r=a/sqroot2=58mm/ 1.41=38.9 mm=0.0389 m.
F24=(9•10^9)•(4•10^-6•10•10^-6)/(0.0389)^2=142.7N