# Cant get y-component of point charges on test charge

1. Oct 3, 2007

1. The problem statement, all variables and given/known data
Six point charges, four positive (+q) and two negative (-q), are fixed at thepositions shown (i attached a .gif file --pls tell me if this isnt visable) in the x-y plane; q = 2.1 × 10-6C and a = 0.93 m. A test charge Q = 0.6 × 10-6C is at the origin.

(a) Find the x- and y-components of the total force on Q:

2. Relevant equations
i successfully calcd the x-component (-.017016N) but cant get the y component

3. The attempt at a solution

heres what i did

1. labelled the charges A thru F starting with the charge on the neg y-axis (A) and the labelled in cw direction.

2. so for y component everything either cancels or has no y-component, EXCEPT for the 2 charges on the y-axis. These are both pos charges.
so heres the calc
Ay = (9*10^9)(2.1*10^-6)(.6*10-6) / (.93^2) = .01311N (pos charge)
Dy = (9*10^9)(2.1*10^-6)(.6*10-6) / (.93^2) = .01311N (also pos charge)
so 2(.01311) = .02622N
this is wrong!
thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Oct 3, 2007

### learningphysics

No. this is wrong. The two charges on the y-axis exert equal but opposite forces on Q, hence they cancel.

Which are the two charges you need to consider?