components of an electric field / charges

scholio

1. The problem statement, all variables and given/known data
four point charges are arranged at the corners of a square, as shown. a = 0.1m, q = 10^-3 coulombs

+1q
*
(+6q)* *(-3q)<-----------3a------------->P
*
+1q

an * indicates a point charge
the distance between each vertice is equal to 'a = 0.1m'
the y axis is vertical, the x axis is oriented horizontal

a) what is the y component of the electric field E at point P?

b) which charge, the +6q or the -3q gives the larger contribution to E at point P?

c) what is the force on a -12microcoulomb charge placed at point P?


2. Relevant equations

not sure with this one -- do i use pythagorean theorem?

E = kq/r^2 where E is electric field, k is constant 9*10^9, q is the charge, r is the distance



3. The attempt at a solution

for a) i think i find the net electric field of all four point charges using E = E1 + E2 + E3 + E4, but am not sure how to derive the y component, do i use the pythag theorem, do i use a/2 as the height, and (3a + a/2) for the base

i am confused now with how to use that to find the y component

any help with working throught this problem is greatly appreciated, asap please...