3 particles electric field?

Question

3 particles are located in the x-y plane. Particle 1 has a charge Q1=2.0uC and coordinates (0cm,0cm) particle 2 has a charge Q2=-2.0uC and coordinates (3cm,0cm) and particle 3 charge Q3=-3.0uC
and coordinates (0cm,3cm) find
a) magnitud
b) direction of the electric field at coordinates (1.5cm,1.5cm)
C)find the magnitud and d) direction of the force exerted on a fourth particle with charge Q4=3.0uC and coordinates (1.5cm,1.5cm)

Answer 1

F1,2=k(Q1Q2)/R^2

a) magnitude of force at every charge is
F1= F1,2 + F1,3 (Note this is an algebraic sum...)
F2= F2,1 + F2,3
F3= F3,1 + F3,2
Example R1,2=sqrt[(x2-x1)^2 + (y2-y2)^2)=
R1,2=sqrt( (3-0)^2 + (0 - 0)^2)]=3
F1,2 - will be toward spositive x axis.
F1,3 - will be toward spositive y axis and so on

b) Hint use a point charge q

F= kq( Q1/R1^2 + Q2/R2^2 +Q3/R3^2)
E=F/q =k( Q1/R1^2 + Q2/R2^2 +Q3/R3^2)

c) See b