simple force question So given this diagram, I was supposed to find the magnitude and direction of the net force on the -10nC charge on the bottom right corner. I found the magnitude by myself using Coulomb's law for the two force components and then found the horizontally-pointing-left force, which was marked correct.
I am confused as to why the direction (which I put as 180 degrees) is wrong. It doesn't make sense. What am I missing?
(By the way, in this picture, I know it says -180 but I just put that after it marked 180 wrong. I wasn't sure what they wanted).

 A: The diagram you need to draw is this:

Now we can compute the forces 
$$F_1 = \frac{q\cdot q_2}{4\pi\epsilon_0 (a^2 + d^2)}$$
The horizontal component is given by 
$$F_{1h} = F_1 \cdot \frac{d}{h}$$
The vertical component is
$$F_{1v} = F_1 \cdot \frac{a}{h}$$
and finally, the force between the two green charges is
$$F_2 = \frac{q_1\cdot q_2}{4\pi\epsilon_0 (a^2)}$$
When I substitute in all the values given, I obtain the following:
$$\begin{align}F_1  &= 0.0012 N\\
F_{1v} &= 0.0004 N\\
F_{1h} &= -0.0012 N\\
F_2 &= 0.0045 N\\
\end{align}$$
In other words - my diagram is really quite badly drawn. Because $q_1$ and $q_2$ are so much closer, the force between them is much larger than the force between $q$ and $q_2$. That's the inverse square law for you.
Adding the horizontal and vertical components together, you obtain a net vector $F_n$ that is not pointing horizontally - not even close. The components are
$$\begin{align}F_{nv}  &= -0.0041 N\\
F_{nh} &= -0.0012 N\\
F_n &= 0.0043 N\\
\theta &= 254°\\
\end{align}$$
and the diagram should look more like this:

The python script I used for the calculations (makes it easy to see where I made a mistake, if any):
import math
e0=8.854e-12
pi=math.pi
q=13e-9
q1=5e-9
q2=10e-9
a=0.01
d=0.029
h=math.sqrt(a*a+d*d)
F1=q*q2/(4*pi*e0*h*h)
F1h=F1*d/h
F1v=F1*a/h
F2=q1*q2/(4*pi*e0*a*a)
Fnh=F1h
Fnv=F1v-F2
Fn = math.sqrt(Fnv*Fnv + Fnh*Fnh)
theta=math.atan2(Fnv,-Fnh)*180/pi+360
print "Net force: magnitude = %.4f N; direction = %.0f deg"%(Fn, theta)

A: If you only found the leftward force on the particle, technically your answer is wrong (or at least obtained by invalid methods). The vertical component may have been small enough that it didn't really affect the magnitude, but it would appear it certainly affects the direction. So what you'll need to do it calculate the vertical component of the force as well, and use some trigonometry to find the angle that the force makes with the horizontal.
