swarley's problem

problem.txt

<swarley> Two point charges, q1 and q2 are placed 0.30m apart on the x-axis, as show in the figure above. Charge 
          q1 has a value of -3e-9 C. The net electric field at point P is zero. Calculate the magnitude and charge 
          of q2.
<masak> swarley: where's point P?
<swarley> 0.1m left of point q1

Formula:

|E| = 1/(4 pi e0) |q|/r**2

But we have two point charges, so we need two of those:

|E1| = 1/(4 pi e0) |q1|/r1**2
|E2| = 1/(4 pi e0) |q2|/r2**2

Besides, we know these:

E1 = -E2
q1 = -3e-9 C
q2 = <unknown>
r1 = 0.1 m
r2 = 0.2 m

Because r2 is twice as big as r1 and both are squared, for E1 and E2 to cancel out,
q2 needs to be 4 times as big as q1, and with opposite sign.