# E-Fields Question

1. Jan 14, 2010

### apphysicsgirl

1. The problem statement, all variables and given/known data
Two charges, charge one is +Q and charge two +2Q, are fixed in a plane along the y axis of an x-y coordinate plane. Charge one is at point (0,a) and Charge two is at the point (0,2a).
(a) Is there any portion of the y-axis in which the total electric field is zero? If so, where?
(b) If a small negative charge, -q, of mass m were placed at the origin, determine its initial acceleration (magnitude and direction).

2. Relevant equations

E1=E2
F=ma
E=F/q
E=1/4pi(epsilon not)xQ/r^2

3. The attempt at a solution

For (a), there will always be an e-field along the y-axis except for a point in between the two charges. I am having trouble finding that point, though. I set the two e-fields equal to each other, but was unsure of what to put as the radius for each.

For (b) I used F=ma and then plugged in F=Eq to get a=Eq/m. My final answer was (-1/4pi(epsilon not)x(Q/a^2+Q/2a^2)xq)/m.